The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Modiolus modiolus is a boreal species that reaches its southern limit in UK waters and forms beds of large individuals only in the north of Britain and Ireland (Hiscock et al. 2004). The depth range of Modiolus modiolus increases at higher latitudes with intertidal specimens more common on northern Norwegian shores where air temperatures are lower (Davenport & Kjørsvik, 1982). Little direct information on temperature tolerance in Modiolus modiolus was found, however, its upper lethal temperature is lower than that for Mytilus edulis (Bayne, 1976) by about 4°C (Henderson, 1929, cited in Davenport & Kjørsvik, 1982). Observations on Modiolus modiolus exposed to high temperatures suggest that this species is restricted by upper limiting sea water temperatures of 23ºC (Read & Cummings 1967). Although individuals may survive short-term exposure to higher temperatures as Read (1967) found that in intertidal pools most Modiolus modiolus survived (for at least a week) following exposure of temperatures that rose from 19º to 32.5ºC over 5.5 hours. 

Subtidal populations are protected from major, short-term changes in temperature by their depth. However, Holt et al. (1998) suggested that because Modiolus modiolus reaches its southern limit in British waters it may be susceptible to long-term increases in summer water temperatures. Hiscock et al. (2004) suggest that warmer seas may prevent recovery of damaged beds and recruitment to undamaged beds so that decline in the occurrence of beds can be expected at least in the south of their range. Declines of horse mussel beds in Strangford Lough (Magorrian, 1995) may be linked to increased water temperatures but other factors such as trawling have also contributed to changes (Strain et al., 2012).

Sensitivity assessment. Modiolus modiolus is a boreal species, and the fact that dense aggregations seem to reach their southerly limit around British shores suggests this species would be sensitive to long-term increases in temperature. Adult populations may be unaffected at the pressure benchmark and, in such long-lived species, an unfavourable recruitment may be compensated for in the following year. Resistance to an acute and chronic change in temperature at the pressure benchmark is therefore assessed as ‘High’ and recovery as ‘High’ (by default) and the biotope is considered ‘Not Sensitive’.  It should be noted that the timing of acute changes may lead to greater impacts, temperature increases in the warmest months may exceed thermal tolerances whilst changes in colder periods may stress individuals acclimated to the lower temperatures. Sensitivity (particularly of southernmost populations)  to longer-term, broad-scale perturbations such as increased temperatures from climate change would, however, be likely to be greater, based on the extent of the impact. 

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Modiolus modiolus is a boreal species that reaches its southern limit in UK waters and forms beds of large individuals only in the north of Britain and Ireland (Hiscock et al., 2004). Davenport and Kjørsvik (1982) suggested that its inability to tolerate temperature change was a factor preventing the horse mussel from colonising the intertidal in the UK. Intertidal specimens were more common on northern Norwegian shores (Davenport & Kjørsvik, 1982). Subtidal populations are protected from major, short-term changes in temperature by their depth. 

Observations on a shallow (10m depth), Modiolus modiolus population from the Gulf of Maine, found that individuals undergo seasonal thermal compensation, altering enzyme concentrations to maintain growth and reproduction as temperatures decrease (Lesser & Kruse, 2004). The study does not, however, indicate responses to rapid temperature decreases at the pressure benchmark and mussels were kept at temperatures they would typically experience, rather than temperatures outside the usual annual range.

Sensitivity assessment. Modiolus modiolus is a boreal species, with beds in higher latitudes exposed to colder temperatures than experienced at the southern limit of its range in the UK. Beds of Modiolus modiolus are therefore considered to have 'High' resistance to decreased temperatures at the benchmark.  Resilience is assessed as 'High' (by default) and this biotope is considered to be 'Not sensitive'. 

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Modiolus modiolus is an osmoconformer.  In short-term fluctuating salinities, valve closure limits exposure to salinity changes in the surrounding waters, although slow diffusion through the byssal aperture means that the osmolarity of fluids will eventually increase ( Shumway, 1977; Davenport & Kjørsvik, 1982). Experimental evidence for short-term tolerances of M. modiolus to increased salinities is provided by Pierce (1970). Modiolus modiolus was exposed to a range of salinities between 1.5 and 54 psu:  at 27 to 41 psu Modiolus modiolus survived for 21 days (the duration of the experiment) (Pierce, 1970). 

Sensitivity assessment. The only evidence to support this assessment is provided by short-term experiments. As this biotope has only been recorded from areas of full salinity (Connor et al., 2004)  a change at the pressure benchmark refers to an increase to hypersalinity (>40 ppt). No direct evidence was available to support this assessment but over the course of a year, an increase in salinity may lead to mortality of Modiolus modiolus. Biotope resistance is, therefore, assessed as 'Low' and resilience as 'Low' so that sensitivity is assessed as 'High'.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Local populations may be acclimated to the prevailing salinity regime and may, therefore, exhibit different tolerances to other populations subject to different salinity conditions and therefore, caution should be used when inferring tolerances from laboratory experiments and from populations in different regions. The sensitivity of  Modiolus modiolus to changes in salinity at the benchmark can be inferred from distribution information and from laboratory experiments that have exposed individuals to decreased salinities.

Some populations of Modiolus modiolus are present in areas where salinities are lower than typical, fully marine conditions. From the Baltic Sea distribution pattern, Dinesen & Morton, (2014 and references therein) inferred that the lower, long-term salinity tolerance of adult Modiolus modiolus is likely to be ∼26. This is supported by observations of Davenport & Kjørsvik (1982) who reported the presence of large horse mussels in rock pools at 16 psu in Norway, subject to freshwater inflow and noted that they were probably exposed to lower salinities.  By keeping the shell valves closed, the fluid in the mantle cavity of two individuals was found to be at a salinity of 28–29 despite some hours of exposure (Davenport & Kjørsvik, 1982).  Short-term tolerances to a salinity of 15 were similarly identified for Modiolus modiolus from the White Sea, north west Russia (where salinity is typically 25), whereas salinity levels of between 30 and 35 appeared optimal.    However, after a winter and spring of extremely high rainfall, populations of Modiolus modiolus at the entrance to Loch Leven (near Fort William) were found dead, almost certainly due to low salinity outflow (K. Hiscock, pers. comm). Holt et al. (1998) reported that dense populations of very young Modiolus modiolus do occasionally seem to occur subtidally in estuaries, but the species is more poorly adapted to fluctuating salinity than many other mussel species (Bayne, 1976) and dense populations of adults are not found in low salinity areas. The biotope records suggest that this biotope only occurs in the UK in full salinity (30-40 ppt) habitats (Connor et al., 2004).

Laboratory experiments exposing Modiolus modiolus to reduced salinity water have demonstrated short-term effects.  Pierce (1970) exposed Modiolus sp. to a range of salinities between 1.5 and 54 psu and reported that Modiolus modiolus survived for 21 days (the duration of the experiment) between 27 and 41 psu. Shumway (1977) exposed individual Modiolus modiolus to simulated tidal, (sinusoidal)  fluctuations between full seawater (salinity 32 ‰) and 50% freshwater and to more abrupt changes in salinity in laboratory experiments. Individual Modiolus modiolus that was able to close their valves survived 10 days exposure to salinity changes compared with individuals which had their shells wedged open that survived for 3 days of the experiment only. Exposure to reduced salinities has been observed to lead to reduced ctenidial ciliary stroke, (after 3 days at a salinity of 15 and 10°C, Schlieper et al., 1958) and increased intracellular liquid/water (Gainey, 1994).

Sensitivity assessment. The available evidence indicates that Modiolus modiolus is an osmoconformer able to tolerate decreases in salinity for a short period. However, a decrease in salinity at the pressure benchmark (a decrease in one MNCR unit) from full salinity to variable (18-35 ppt) would be considered to result in the mortality of all adults within the biotope over the course of a year. This assessment is supported by observed distribution across different salinity regimes (Connor et al., 2004, Dineson & Morton, 2014) and laboratory experiments (Shumway, 1977, Pierce, 1970) which suggest that a change at the pressure benchmark would exceed the lower threshold tolerance of adults over the course of a year). 

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Holt et al. (1998) suggested water movement was important in the development of dense reefs and beds of Modiolus modiolus.  It is likely therefore that there is an optimum range of water flows, currently unknown, which are strong enough to disperse larvae and supply food in suspension, but that are not so strong that the current removes the bed, prevents settlement of larvae within beds (which is key for self-recruiting populations) or prevents the extension of feeding siphons. Conversely, decreased flow rates may inhibit larval settlement and the supply of suspended food and allow greater siltation on beds. This biotope occurs where tidal streams are moderately strong, from 0.5-1.5 m/s (JNCC, 2015).

Adult Modiolus  modiolus occur commonly in areas with moderate to high water exchange in Nova Scotia (Wildish & Peer, 1983; Wildish & Kristmanson, 1985, 1994; Wildish & Fader, 1998; Wildish et al.,1998), and low field densities have been correlated with low current regimes and reduced food availability.   Densities of up to 220 individuals/m² have been recorded from the Faroese shelf (Dinesen, 1999) where maximal tidal current speed has been estimated to be between 0.79 and 0.98 m/s at two Modiolus modiolus sites (Nørrevang et al., 1994: BIOFAR Stn. 661 & 662, cited from Dinesen & Morton, 2014). Mair et al. (2000) also observed that in Scottish sites with Modiolus modiolus beds, densities were greater where there were high tidal currents.

Comely (1978) suggested that areas exposed to strong currents required an increase in byssus production, at energetic cost, and resulted in lower growth rates.  At water velocities exceeding 16 cm/s in a flume tank, Carrington et al., (2008) observed that Modiolus modiolus individuals could not extend the foot beyond the shell to form and attach byssus threads.  However, the mussel bed reduces water flow rates by increasing drag through friction. Carrington et al., (2008) observed that mussel beds of Mytilus trossulus and Mytilus galloprovincialis in laboratory and field studies were able to reduce flow rates between 0.1 and 10% of free-stream velocity.  This modification of flow may enhance suspension feeding in areas of high current flow and allow byssus production to continue (Carrington et al., 2008).

Wildish et al., (2000) examined suspension feeding in Modiolus modiolus in a flume tank and noted that individuals kept the exhalant and inhalant siphons open over the range of flow rates studied, from 0.12-0.63 m/s. However, the inhalant siphon closed by about 20% in currents above 0.5m/s. Although partial closure of the inhalant siphon may reduce food intake this may be compensated by the greater abundance of food supply in higher currents. Widdows et al., (2002) found that there was also a slight decline in feeding rate of Mytilus edulis at current velocities below 0.05 m s/s, which was probably due to algal cell depletion and greater recirculation of near bed water by the group of mussels.

Fouling by epifauna and algae in the infralittoral may also decrease the population’s resistance to increased water flow. Witman (1984, cited in Suchanek, 1985) found that over 11 months in New England, 84% of fouled mussels were dislodged in comparison with 0% of unfouled individuals. As the beds described by the biotope group occur in the circalittoral , algal overgrowth is not relevant but the presence of high levels of epifauna on circalittoral beds may similarly increase dislodgement. Conversely, attached epifauna may reduce turbulence and flow, which could be beneficial.

Changes in water flow may also be a spawning cue, although the available evidence does not strongly support this hypothesis. Schweinitz & Lutz, (1976) observed spontaneous, spawning in a group of Modiolus modiolus individuals kept in a tank when the water flow stopped while previous attempts to induce spawning by various methods had failed. However, subsequent attempts to induce spawning by stopping the water flow failed (De Schweinitz & Lutz, 1976). A similar spawning response in Mytilus edulis to the cessation of flow (Williamson, 1997) was cited (De Schweinitz & Lutz, 1976).

The density of Modiolus modiolus and the character of the substratum will influence the level of sediment erosion following increases in water flow rates. Widdows et al (2002) conducted a series of experiments in a flume on sediment erodibility in relation to the density of Mytilus edulis and substratum type. In sand sediments, sediment erosion was greater where mussel coverage was between 25% and 50% due to scouring around clumps. Bare sediments (no mussels) and sediments with full coverage had lower rates of erosion.

Sensitivity assessment.  Flow rates are an important factor for Modiolus modiolus and influence food transport, feeding rate and sediment erosion and transport which may reduce feeding success where high rates of inorganic particles are present in the water column. Modiolus modiolus may be sensitive to both increases and decreases in flow. Direct evidence is not available to identify the optimal range and increases may be moderated by the bed structure and density which will depend on the degree of recession into sediments and the size and type of associated epifauna (if any).  Adult Modiolus modiolus may have ‘High’ resistance to changes in water flow rates at the pressure benchmark based on the occurrence of this biotope in moderately strong tidal streams. resilience is assessed as 'High' (by default) and the biotope is assessed as 'Not sensitive'. Changes in flow rates greater than the pressure benchmark, that alter feeding success through changes in clearance rates and food supply and larval recruitment, however, may lead to the presence of beds, composed of ageing adults, that are not sustainable in the long-term. 

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

The majority of populations are subtidal, however, intertidal populations in rock pools or shallow subtidal populations occasionally exposed at extreme low water may occur and be vulnerable to increased emergence and hence, exposure to desiccation and temperature extremes. Modiolus modiolus has a lower tolerance of exposure than other Mytilidae such as Mytilus edulis, although whether this is due to the effects of desiccation, fluctuating temperatures or reduced feeding times is not clear (Dinesen & Morton, 2014). Gillmor (1982) showed that growth rate of juvenile  Modiolus modiolus (SL < 20 mm), was zero at aerial exposure times of 20–25. Gillmor (1982) also recorded 100% mortality of Modiolus modiolus at 40% aerial exposure.

Sensitivity assessment. Modiolus modiolus is considered to be ‘Not sensitive’ to a decrease in emergence or an increase in sea level, which would, theoretically, increase habitat suitability for this species. The available evidence indicates that Modiolus modiolus would be sensitive to increased emergence and decreases in sea level where exposed, but these pressures are considered to be not relevant to this biotope group which is restricted to circalittoral subtidal habitats. 

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

The majority of Modiolus modiolus populations are subtidal and unlikely to be affected by wave action directly. However, increased wave action results in increased water flow in the shallow subtidal. Wave mediated water flow tends to be oscillatory, i.e. move back and forth (Hiscock, 1983), and may result in dislodgement or removal of individuals. This biotope is found in areas that are sheltered from wave action but where tidal flows are moderately strong.

The mussels, Mytilus edulis, Perna perna and Mytilus galloprovincialis have been shown to increase byssus production in response to agitation and wave action (Young, 1985, Zardi et al., 2007) and Modiolus modiolus may respond similarly. However, horse mussels attached to hard substrata are probably more intolerant of wave action than Mytilus edulis due to their larger size and hence increased drag. The intolerance of semi-infaunal or infaunal populations probably owes more to the nature of the substratum rather than their attachment. Populations on mobile sediment may be removed by strong wave action due to removal or changes in the substratum or be buried by bed-load transport of material. Where increased wave action results in sediment re-suspension this may reduce feeding rates through increased non-organic particulates and reduced food production in the more turbid waters.

No information concerning storm damage and wave tolerances was found. Shallow, nearshore subtidal populations in Strangford Lough were exposed to wave mediated flows of 0.1 m/s (Elsäßer et al., 2013).   

Decreased wave action may allow horse mussel beds to extend into shallower depths, however, the rates of increase in bed size are likely to be slow, probably much longer than the benchmark level.

Sensitivity assessment. No direct evidence was found to assess sensitivity to changes at the pressure benchmark. Circalittoral beds in mixed sediment were assessed as ‘High’ resistance and ‘High’ resilience at the pressure benchmark to increases and decreases in wave height and are, therefore, assessed as ‘Not Sensitive’.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Little direct evidence of the effects of Hydrocarbons and PAHs on Modiolus spp. was found. The following assessments incorporate evidence from a larger evidence base on Mytilus spp. (see Mytilus evidence review).

Modiolus sp. was reported to accumulate heavy metals in its soft tissues without adverse effects. For example, Julshamn & Andersen (1983) examined the digestive tissue distribution of cadmium (Cd), zinc (Zn), copper (Cu), magnesium (Mg), manganese (Mn), iron (Fe) and lead (Pb) in Modiolus modiolus. They reported the presence of Cd-binding proteins and the accumulation of the above heavy metals but did not document any adverse effects. Richardson et al. (2001) examined the presence of Cu, Pb and Zn in the shells of Modiolus modiolus from a relatively un-contaminated site and a site affected by sewage sludge dumping. The persistence of a population of horse mussels at the sewage sludge dumping site suggested tolerance to heavy metal contamination levels at that site. Holt et al. (1998) reported that long-term changes in contaminant loads associated with spoil dumping were detectable in the shells of horse mussels in a bed off the Humber estuary. This observation showed the survival of horse mussels in the vicinity of a spoil dumping ground but no information on their condition was available (Holt et al., 1998). Klumpp & Burdon-Jones (1982) also reported that Modiolus auriculatus accumulated Pb, Cd, Cu, Zn, cobalt (Co), nickel (Ni) and silver (Ag) from their environment but did not report any adverse effects. Similarly, Shulkin et al. (2002) reported that Modiolus kurilensis from near Vladivostok (northwest of the Sea of Japan) accumulated Pb, Cu, Cd, Zn and Ag in their tissues but did not report any adverse effects. Chou et al. (2003) reported that Mn and Zn accumulated in the digestive glands of Modiolus modiolus from three sites (the Bay of Fundy, St Croix, New Brunswick and Jeddore) in Nova Scotia. 

However, Hilmy et al. (1981) examined the effects of mercury poisoning on Modiolus modiolus in the Red Sea. They reported a 24-hour LC50 of 0.90 mg/l Hg, a 48-hour LC50 of 0.56 mg/l Hg, a 72-hour LC50 of 0.35 mg/l Hg and a 96-hour LC50 of 0.23 mg/l Hg in Modiolus modiolus.  Similarly, Ramakritinan et al. (2011) examined the acute toxic effects of heavy metals (Cu, Pb, Cd, Zn, and Hg) on the bivalve, Modiolus philippinarum, using static renewal bioassay tests. The mortality of Modiolus philippinarum positively correlated with the increasing concentration of metals and an increasing exposure period. The 96-hour LC50 values for Modiolus philippinarum were 0.023, 0.221, 2.876, 2.337 and 0.007 mg/l for the Cu, Pb, Cd, Zn, and Hg respectively. Ramakritinan et al. (2011) determined that the order of increasing toxicity of metals in Modiolus philippinarum was Hg > Cu > Cd > Zn > Pb. They suggested that safe concentrations for Modiolus philippinarum were 0.2, 2.2, 28.8, 23.4 and 0.07 µg/l for Cu, Pb, Cd, Zn, and Hg respectively. Wahbeh & Zughul (1995, cited by ECOTOX, Olker et al., 2022) reported a 9-day LC50 of 2 mg/l Zn, a 96-hour LC50 of 0.5 mg/l Cu, and a 9-day LC50 of 2 mg/l Cd in Modiolus auriculatus. 

Pikula et al. (2020) examined the cytotoxicity of several nanoparticulates (NPs) in the haemocytes of Modiolus modiolus, Crenomytilus grayanus and Arca boucardi.  Metalbased nanoparticulates (i.e. cadmium and zinc sulphides, gold nanoparticulates and titanium oxide) had the highest cytotoxicity (based on EC50s) compared to carbon nanotubes, carbon nanofibres and silicon nanotubes. The haemocytes of Modiolus modiolus were the most sensitive to all the NPs tested. Pikula et al. (2020) concluded that molluscs were good indicators of nanoparticulate toxicity. However, the study was conducted on harvested haemocytes rather than the whole organism. 

Modiolus spp. may show a similar tolerance to heavy metals as Mytilus edulis. In general, the Mytilus spp. evidence (see Mytilus evidence review) suggested that longer exposure times were required to understand the true impacts of metal exposure on Mytilus (and by inference other mussels such as Modiolus spp.), as mussels can close their shells for days. Hence, short-term exposures (e.g. <48 hrs) may underestimate sensitivity. This agrees with Widdows & Donkin (1992) who suggested that LC50 values in Mytilus gave a false impression of high tolerance because adult bivalves were able to close their valves and isolate themselves from extreme (potentially lethal) conditions for long periods (i.e. days).  Different life stages had different sensitivities.  This also agrees with Widdows & Donkin (1992) who noted that adults were >10-fold more sensitive than larvae to copper (Cu), petroleum hydrocarbons and sewage sludge.

The majority of the evidence on Mytilus spp. examined Cu, followed by Cd, Zn, Ag, and Hg (see Mytilus evidence review; Figure 1.3; Table 1.3). However, it is also clear that there is considerable variation in response to metal exposure, due in part to the variation in the experimental studies, and especially the concentration and exposure duration used. The evidence suggests that Mytilus adults and juveniles have a ‘High’ sensitivity to copper, cadmium, mercury and silver and a ‘Medium’ sensitivity to iron, lead, methylmercury and neodymium. The confidence in those assessments is probably ‘Medium’ due to the volume of evidence examined. 

Less evidence for the remaining metals, especially the organometals and nanoparticulate metals was found in Mytilus spp. In some cases, the sensitivity assessment is based on one or two papers (e.g. nanoparticulate Zinc, or tributyltin oxide).  While the articles presented were all ‘High’ to ‘Medium’ quality and directly applicable, it may be prudent to treat these assessments with more caution and assess their confidence as ‘Low’.  No information was found on the effects of organometals on Modiolus spp. 

The number of articles that reported the effects of metals on Mytilus spp. larvae and embryos alone were also dominated by studies on the effects of Cu (Mytilus evidence review; Table 1.4).  The evidence suggests that Mytilus larvae and embryos are highly sensitive to Cu, Pb, and Zn, plus molybdenum (Mo) and Mn although the last two are based on single papers. There is also evidence that organotins result in severe mortality in larvae and embryos. 

Sensitivity assessment. The evidence of the effects of transitional metals on Modiolus spp. suggests that several metals have been reported to cause significant mortality (Low resistance) under laboratory conditions, depending on concentration and duration of exposure.  Similarly, there is evidence that several metals have been reported to cause severe (>75%) mortalities in adult and juvenile Mytilus spp. (resistance is ‘None’) based on a larger sample of evidence. Therefore, the resistance of Modiolus spp. to transitional metals is assessed as ‘Low ’ based on the direct evidence presented above. Hence, resilience is probably ‘Low’ and sensitivity is assessed as ‘High’ but with Medium confidence due to the similarities with Mytilus spp. 

No information was found on the effects of organometals or nanoparticulate metals on Modiolus spp. The evidence review on Mytilus spp. (Mytilus evidence review; Tables 1.3 and 1.4) report significant or severe mortality due to organotin exposure and significant mortality due to exposure to one nanoparticulate zinc in adults and juveniles but severe mortality in larvae and embryos. The larvae and embryos of Modiolus spp. may share this sensitivity.  Therefore, the resistance of Modiolus spp. to organometals and nanoparticulate metals is assessed as Low as a precaution but with Low confidence due to the lack of direct evidence. Hence, resilience is probably ‘Low’ and sensitivity is assessed as ‘High’.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Little direct evidence of the effects of Hydrocarbons and PAHs on Modiolus spp. was found. The following assessments incorporate evidence from a larger evidence base on Mytilus spp. (see Mytilus evidence review).

Oil spills. Scarratt & Zitko (1972) reported that Modiolus modiolus accumulated oil residues in their tissues after a Bunker C oil spill in Chedabucto Bay, Nova Scotia. However, there was no evidence that the Bunker C oil contributed to the deaths of Modiolus modiolus.  Hampson & Moul (1978) examined the effects of a fuel oil spill in October 1974 on the affected saltmarsh community in Buzzards Bay, Massachusetts. They reported that visible mortality of freshly gaping ribbed mussels Geukensia demissa (studied as syn. Modiolus demissus) was seen at a depth of 2.0 to 2.5 m in 1974. Hampson & Moul (1978) suggested that Geukensia demissa was an opportunistic species as they repopulated an area when Spartina roots were absent. Overall, Hampson & Moul (1978) suggested that Geukensia demissa could recover successfully after an oil spill, but no mortality rates or exposure concentrations were given.

Little evidence on the direct physical effects of oil (smothering, or clogging) on Mytilus spp., was found and few studies examined blue mussel beds, except in Babcock et al. (1998) and Rostron & Bunker (1997).  The evidence suggests that Mytilus spp. can be relatively tolerant of direct oiling (in the absence of dispersants or other cleaning treatments) and survived oil spilt by the Torrey Canyon and Sea Empress.  In particular, blue mussel beds in Prince William Sound (Babcock et al., 1998) survived direct oiling after the Exxon Valdez spill and continued exposure to oil retained in the sediment underneath the mussel beds for 3-4 years, although their condition was impaired.  However, Mytilus trossulus abundance in other intertidal habitats was reduced significantly after the Exxon Valdez spill (Highsmith et al., 1996).  In addition, a significant reduction in Mytilus galloprovincialis abundance was also noted after the Hebei Spirit spill in Korea (Jung et al., 2015).  

Modiolus modiolus beds may be protected from the direct effects of oiling due to their subtidal habitat, depending on mixing due to wave action and depth. Only sublethal effects were reported in Modiolus spp. but the evidence of the effects of oil spills on Mytilus spp. and blue mussel beds suggests that the effects depend on the type of oil spilt, the local habitat, and wave conditions at the time of the spill.   Therefore, the worst-case resistance of mytilid beds to oiling is assessed as ‘Low’ to represent the potential for mortality   Resilience is probably ‘Low’ so sensitivity to oil spills is assessed as ‘High’ but with ‘Low’ confidence due to the lack of direct evidence on Modiolus spp.

Petroleum hydrocarbons (oils). Bakhmet et al. (2021) reported that crude oil and seawater mixtures caused significant variations in heart rate in Modiolus modiolus and even acute cardiac arrest at high (450 mg/l) or medium concentrations (45 mg/) but returned to normal within two or 10 days depending on the concentration. Crude oil emulsion exposure also altered lipid metabolism depending on the concentration and duration of exposure. However, Bakhmet et al. (2021) did not report any resultant mortality.  Al-Sabagh et al. (2013) examined the toxicity of seven non-ionic and anionic surfactants used as oil spill dispersants prepared from locally raw materials (Dasic, Crude Oil and linear alkyl benzene) on Modiolus adriaticus collected from the intertidal rocky shores of the El-Montaza region to the east of Alexandria, Egypt. The mortality rate increased with exposure time. They reported 120-hour LC50s were D1 1,200 ppm, D2 500 ppm, D3 1,400 ppm, D4 1,200 ppm, D5 800 ppm, D6 700 ppm, D7 1,700 ppm and the Dasic was 1,300 ppm. In conclusion, the toxicity of these dispersants was ranked as D2 > D6 > D5 > D4 > D1 > Dasic > D4 > D7 (Al-Sabagh et al., 2013). 

Refined oils (e.g. lubricant and fuel oils) were reported to be more toxic than crude oils in Mytilus spp.  Widdows et al. (1982) also noted that the 30-36 µg/l WAF concentrations used in their experiments were comparable to levels found in the environment (e.g. the Thames in 1980) but that very high concentrations (5-1,000 mg/l) were required to elicit a lethal response in Mytilus edulis (see Craddock, 1977).  Overall, the evidence suggests (10% of articles on the effects of oils) that exposure to oils or their water-saturated (WSF) or water-accommodated fraction (WAF) can result in severe mortality (>75%) while another 30% of the articles report significant (25-75%) mortality depending on the type of oil and its concentration. 

The direct evidence on Modiolus spp. suggested that exposure to crude oil emulsions could result in sublethal effects, while dispersants are potentially toxic and result in significant mortality, but based on only two articles. The evidence from Mytilus spp. reported that oil emulsions (WAF or WSF) could result in severe or significant mortality. Therefore, the worst-case resistance of Modiolus spp. to oil emulsions is assessed as ‘Low’ based on direct evidence and evidence from Mytlius spp. Resilience is probably ‘Low’ so sensitivity to petroleum hydrocarbons is assessed as ‘High’ but with ‘Low’ confidence due to the limited direct evidence on Modiolus spp.

Sensitivity to ‘Hydrocarbons and PAH’ contamination. In their review, Widdows & Donkin (1992) note that (one reason) mussels are good sentinels for pollution is because they are relatively tolerant of, but not insensitive, to a range of environmental conditions and contaminants. Furthermore, they noted that adults were >10-fold more sensitive than larvae to copper (Cu), petroleum hydrocarbons and sewage sludge. Widdows & Donkin (1992) noted that lethal responses give a false impression of high tolerance since the adults can close their valves and isolate themselves from the environment for days. They suggested that sub-lethal effects e.g., shell growth and scope for growth (SFG), were more sensitive indicators of the effects of contaminants.

The Mytlilus spp. evidence review suggests that exposure to hydrocarbon contamination can cause mortality in Mytilus spp., which is significant or even severe in some cases. The degree of mortality, or absence of mortality, depends on the type of hydrocarbon (crude or refined oils, oil-saturated water fractions, PAHs, or refined products) to which the species is exposed, how they are exposed (through oil spills, effluents, the sediment, or food supply e.g. algae), the concentration of the contaminant and the duration of exposure, as well as seasonal influences on the species’ condition, especially spawning and reproduction. However, Modiolus spp. beds may derive protection from their subtidal habitats. 

Therefore, the ‘worst case’ resistance of Modiolus spp. to ‘Hydrocarbons and PAHs’ is assessed as ‘Low’. Resilience is probably ‘Low’ so sensitivity to hydrocarbon is assessed as ‘High’ but with ‘Low’ confidence due to the limited direct evidence on Modiolus spp. However, it should be noted that the Mytilus spp. evidence reviewed also documented several occasions in which blue mussels and blue mussel beds had survived significant oiling and most evidence (70% of the articles examined) of exposure to hydrocarbons was reported to result in sub-lethal effects, although it was not clear how detrimental sub-lethal effects or stress is to the species survival. 

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Little direct evidence of the effects of ‘Synthetics’ on Modiolus spp. was found. The following assessments incorporate evidence from a larger evidence base on Mytilus spp. (see Mytilus evidence review).

Bourdelin (1996) examined the effect of the organochloride pesticide Lindane on oxygen consumption and feeding (clearance) rates in Modiolus auriculatus in French Polynesia. Short-term laboratory experiments compared the effects of both clean and lindane-exposed water, after a 7-day incubation period in seawater and an initial lindane concentration of 10 µg/l. Oxygen consumption and feeding (clearance) rates were lower in the lindane-exposed water compared to the clean water. For example, in a solution of Lindane 10 µg/l the oxygen consumption rate was reduced by 8.38% (+/- 3.41%) for intertidal mussels and by 9.29% (+/- 3.84%) for subtidal mussels. The clearance rate was reduced by 48% to 0.6% for intertidal mussels, and by 51% to 4.3% for subtidal mussels dependent on their weight as the filtration rate increases with increasing body weight. Bourdelin (1996) concluded that the presence of Lindane in seawater significantly affected the pumping activity of Modiolus auriculatus under laboratory conditions.

Edwards & Davis (1975) examined the effects of single and repeated exposure of the herbicide Ansar 529 (34% Monosodium Methanearsonate (MSMA) and 16.1% Arsenic) on the ribbed mussel Geukensia demissa (studied as syn. Modiolus demissus) in salt march ecosystems on Sapelo Island, Georgia. Specimens were exposed to Ansar 529 in the field in metal cylinder containers which were sunk into the soil, filled with seawater treated with herbicide for two hours, pumped out and repeated twice a day for five consecutive days in three consecutive months. All 16 specimens were feeding when filled with seawater.  After the second 5th day of treatment, two individuals died. Another four died between the second and third treatments but no more deaths occurred after this. However, Everest (1978; cited in ECOTOX, Olker et al.,2022) reported no mortality (NOEC) in Geukensia demissa (studied as syn. Modiolus demissus) exposed to 100 ppm of the herbicide Fluometuron. 

Lauth et al. (1996) examined the effect of the organophosphate insecticide Azinphosmethyl (AZM) on the ecology of replicated estuarine systems (mesocosms) in South Carolina. Ten ribbed mussels Geukensia demissa (studied as syn. Modiolus demissus) were exposed to 8 µg/l and 2 µg/l AZM in the mesocosm system and were monitored for 120 hours. The high exposures had a 100% mortality rate in Geukensia demissa and the low dose test had a 100% survival rate.

In general, the Mytilus spp. evidence review suggested that longer exposure times were required to understand the effects of exposure to synthetic contaminants on Mytilus, as mussels could close their shells for days.  Hence, short-term exposures (e.g. < 48 hours) may underestimate sensitivity.  This agrees with Widdows & Donkin (1992) who suggested that LC50 values in Mytilus gave a false impression of high tolerance because adult bivalves were able to close their valves and isolate themselves from extreme (potentially lethal) conditions for long periods (i.e. days).

Nevertheless, the majority of articles reported a lethal response to exposure to synthetic compounds in Mytilus spp.  A total of 57% of ranked mortalities reported in the evidence review were lethal (Severe, Significant or Some), while 27% reported no mortality (‘None’) and 16% reported sub-lethal effects. Most of the articles examined pesticides/biocides and pharmaceuticals (Mytilus evidence review; Figure 1.10).  A total of 15 (56%) of the 27 articles that examined pesticides reported lethal effects.  The majority of the evidence suggested that pesticides resulted in lethal effects in adults and juvenile Mytilus spp. but that larval and embryos were probably more sensitive.  Therefore, we can suggest that Mytilus spp. probably has a ‘High’ sensitivity to pesticide exposure, with a few exceptions. The confidence in the assessment is ‘Medium’ because of the number of articles examined and the consistency in the response.

However, 19 (70%) of the articles that examined pharmaceuticals reported lethal effects.  The larvae and embryos showed the most lethal responses rather than adults and juveniles.  Therefore, we can suggest that Mytilus spp. probably has a ‘High’ sensitivity to the pharmaceuticals examined, especially in the larvae and developmental stages.  The confidence in the assessment is assessed as ‘Medium’ because of the number of articles examined and the consistency in the response.

Nevertheless, the results (Mytilus evidence review; Table 1.7 & 1.8) suggest that Mytilus spp. are probably sensitive to a number of synthetic compounds, especially in early development or as larvae. Pesticides and pharmaceuticals interact with the biochemical and endocrine pathways of species. The taxonomic similarities of mytilids (e.g. Mytilus and Modiolus) suggest that their response to these synthetic compounds is likely similar. 

Therefore, the worst-case resistance of Modiolus sp to ‘Synthetic compounds’ is assessed as ‘None’ as a precaution based on the limited direct evidence for Modiolus spp. (above) and the extensive evidence base for Mytilus spp., especially in larvae and developmental stages. Hence, resilience is probably ‘Very low’ and sensitivity is assessed as ‘High’. The confidence in the assessment is probably ‘Low’ because of the limited direct evidence on Modiolus spp. 

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

The periostracum of Mytilus edulis was reported to concentrate uranium (Widdows & Donkin, 1992). Mussels have also been reported to bioaccumulate ¹⁰⁶Ru, ⁹⁵Zr, ⁹⁵Nb, ¹³⁷Cs and ⁹⁰Sr (Cole et al., 1999). While the above data demonstrates that Mytilus edulis can accumulate radionuclides, little information concerning the effects of radionuclides on marine organisms was found. Sensitivity to this pressure is therefore not assessed based on lack of evidence. 

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

Little direct evidence of the effects of ‘Other substances’ on Modiolus spp. was found. Rajagopal et al. (2003) examined the lethal and sublethal effects of different chlorine concentrations on different mussels including Modiolus philippinarum, Perna spp. and Brachidontes spp. The mussels were collected from Madras Atomic Power Station (MAPS) in Kalpakkam, to the south of Madras on the east coast of India. They were split into three size groups (1, 6 and 12 months) and then tested in the laboratory at seven different chlorine concentrations (1, 2, 3, 5, 8, 10 and 15 mg/l). Mortality rates were higher in smaller size classes of the mussels tested than in larger size classes. However, between 8 and 15 mg/l residual chlorine, all size groups of all the mussels tested took the same time to reach 100% mortality (ca <0.5 hours, extracted from Figure 1). Modiolus philippinarum reached 50% mortality (LT50) after 296 hours at 1 mg/l chlorine. Rajagopal et al. (2003) suggested that the short-term survival of Modiolus in unfavourable conditions depended on their ability to close their valves and isolate themselves from the external medium. In conclusion, the data from this study showed that all mussel species could be killed with exposure to chlorine of concentrations 10 to 15 mg/l when exposed for 48 hours.

Sensitivity assessment. Not surprisingly, high chlorine concentrations (used to disinfect power plant cooling water intake pipes) were reported to cause severe mortality in Modiolus spp. and other mussels. Therefore, the worst-case resistance of Modiolus spp. to chlorination is assessed as ‘None’, resilience as ‘Very low’ and sensitivity as ‘High’. However, confidence is assessed as ‘Low’ due to the limited evidence.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Theede et al. (1969) examined the relative tolerance of gill tissue from several species of bivalve to exposure to 0.21 mg/l O₂ with or without 6.67 mg of sulphide (at 10°C and 30 psu). Modiolus modiolus tissue was found to be the most resistant of the species studied, retaining some ciliary activity after 120 hours compared with 48hrs for Mytilus edulis.

Sensitivity assessment. While it is difficult to extrapolate from tissue resistance to whole animal resistance (taking into account behavioural adaptations such as valve closure) the evidence suggests that horse mussels are more, or at least similarly, tolerant of hypoxia and hydrogen sulphide than the common mussel. In addition, most bivalve molluscs exhibit anaerobic metabolism to some degree. Therefore, a resistance of 'High' has been recorded at the benchmark level and resilience is assessed as 'High' (based on no effect to recover from). Modiolus beds are therefore considered to be 'Not sensitive' at the pressure  benchmark. Resistance is likely to be influenced by temperature. An oxygen debt may induce wide valve gape and potentially increase susceptibility to predation. Wide valve gape is noted in Hutchison et al., (2016) as a response after unburial and is suggested to be due to an oxygen debt.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Modiolus modiolus is not considered sensitive at the pressure benchmark that assumes compliance with good status as defined by the WFD.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Little direct evidence was available to support the assessment of this pressure, which is largely based on expert judgement. In areas of strong tidal flow where some Modiolus modiolus beds are found, deposits of organic matter may be removed fairly rapidly mitigating the impact, although some deposits will be trapped within crevices and spaces where they may be utilised by the infaunal deposit feeding community. Where currents are weaker, as in some of the sheltered lochs and similar areas where beds occur, organic deposits may be removed more slowly and impacts may be greater. The persistence of a Modiolus modiolus population in the vicinity of a sewage sludge dumping site (Richardson et al., 2001) suggests that the species is tolerant of high levels of organic matter. Beds of Modiolus modiolus enrich the surrounding sediment via faeces and pseudofaeces so that the bed accumulates deposits rich in organic matter and increases in height compared to the surrounding seabed.  At the pressure benchmark, which refers to enrichment rather than gross organic pollution (Tillin & Tyler-Walters, 2014), the extra rate of organic matter accumulation may not far exceed the natural background level, particularly in sheltered areas.

Sensitivity assessment. At the pressure benchmark, which refers to enrichment rather than gross organic pollution, Modiolus modiolus is considered to have 'High' resistance and hence, 'High' resilience. This biotope group is therefore considered to be 'Not Sensitive'.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is, therefore ‘High’.  Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.  

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

The introduction of artificial hard substratum is considered at the pressure benchmark level and it is noted that Modiolus modiolus can colonise bedrock and artificial structures.  On Georges Bank in the Northwestern Atlantic, Modiolus modiolus larvae recruited onto test panels within two years (Collie et al. 2009). Anwar et al. (1990) reported a substantial population on the legs of an oil rig, 10 years after installation. It was suggested that growth was enhanced in this situation due to a lack of predation (OSPAR, 2009). The results suggest that on suitable surfaces, recruitment may be relatively rapid where there is a supply of larvae. However, the results refer to a dense settlement of juveniles rather than the development of reefs and the examples cited were habitats that were not suitable for the long-term establishment of a natural bed.  Modiolus modiolus is also found on natural bedrock but again, a hard substrate is not suitable for the establishment of a natural bed, equivalent to the biotope description.

Sensitivity assessment.   The resistance of the biotope to a change to artifical or natural hard is assessed as ‘None’ (loss of >75% of extent) as these surfaces are not suitable for natural beds. Resilience (following habitat recovery) is assessed as ‘Very Low’ (at least 25 years, or negligible recovery).  Sensitivity, based on combined resistance and resilience is assessed as ‘High’. The more precautionary assessment for the biotope, rather than the species, is presented in the table as it is considered that any change to a sedimentary habitat from a rock reef habitat would alter the biotope classification and hence the more sensitive assessment is appropriate.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

The change in one Folk class is considered to relate to a change in classification to adjacent categories in the modified Folk triangle (Long, 2006).  For the mixed sediments that characterize this biotope the sediment changes considered may be to coarser or finer sediments.  Modiolus modiolus is found on and in a variety of substrata ranging from fine mud with shells and gravel to bedrock. Comely (1978) found Modiolus modiolus in different types of sediment at varying densities, with low densities (mean 4 individuals/m²) in clean gravel, stones and small boulders and at higher densities (mean 10 individuals/m² in fine muddy sand and silty sand with coarse gravel overlain by clean coarse sand with boulders).  Differences in shell morphology between habitat types, has been observed in response to currents, sediment type and density (Fariñas-Franco et al., 2016). Changes in habitat may, therefore, result in individuals being less suited to the changed conditions, with potential effects on fitness, condition and survivability.

Based on ROV and SCUBA survey in Strangford Lough, Elsässer et al. (2013) modelled suitable habitat and found that substratum type was a key predictor of distribution of beds. The occurrence of the remaining beds was strongly linked to the presence of finer substrata, such as sand and mud, and negatively correlated with coarser substratum types such as bedrock, boulders and cobbles. Given the correlation between substratum type, current speed and sedimentation, Roberts et al., (2011) suggest that the presence of beds of Modiolus modiolus in Strangford Lough with areas of finer sediment may be related to habitats where current speeds allow larvae to settle. These findings suggest that changes in seabed type that are the result of changes in water and sediment transport are likely to alter habitat suitability for beds (and lead to biotope reclassification where the biotope description is substrate specific).

Sensitivity assessment. Given the wide range of substratum types occupied by this species, a change in sediment type is not considered to negatively impact habitat suitability at the level of the individual, however, as older horse mussels may be adapted, through shell morphology, to specific habitats and changes in sediment type may alter fitness and survivorship. However, the biotope group refers specifically to beds of Modiolus modiolus occurring in mixed sediments, rather than individuals. Based on the available evidence a change to coarse sediments may be more detrimental to the biotope than a change to finer sediments.  As the biotope classification refers specifically to mixed sediments, an increase in fine or coarse sediments to the degree that sediments are re-classified would severely reduce habitat suitability.  Resistance is therefore assessed as ‘None’ (loss of >75% of extent), resilience (following habitat recovery) is assessed as ‘Very Low’ (10 -25 years) as a change at the pressure benchmark is permanent.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

Modiolus modiolus is found on and in a variety of substrata ranging from fine mud with shells and gravel to bedrock. The process of extraction is considered to remove sediment to 30cm depth and the horse mussel bed and associated biota, as beds of Modiolus modiolus are sessile and occur either on or within the sediment. No direct evidence for resistance and recovery to this pressure was found and the sensitivity assessment is therefore based on expert judgement and species traits.

Sensitivity assessment. The process of extraction is considered to remove all members of the biotope group as Modiolus modiolus are sessile. Resistance is therefore assessed as ‘None’, based on expert judgment but supported by the literature relating to the position of these species, on or within the seabed. At the pressure benchmark, the exposed sediments are considered to be suitable for recolonization almost immediately following extraction. Recovery will be mediated by the scale of the disturbance and the suitability of the sedimentary habitat. Local migration of adults could re-populate very small defaunated patches and passive transport of adults via water movements may occur around the disturbed edges of beds. Where larger areas have been affected by extraction, recovery is most likely to occur via larval recolonisation but the removal of adults is likely to reduce the chances of successful settlement (see recovery section). Resilience is considered to be ‘Very low', for the bed of Modiolus modiolus (25 or more years or negligible recovery). Sensitivity based on resistance and resilience is therefore categorised as ‘High’.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

As Modiolus modiolus are large, sessile and present on sediment surfaces or shallowly buried, individuals will be exposed to abrasion of the surface of the seabed. Abrasion from towed fishing gear (e.g. scallop dredges) is known to flatten clumps and aggregations and may break off sections of raised reefs and probably damages individual mussels (Holt et al., 1998), as described below in the ‘penetration and or disturbance’ pressure’ which assesses the impacts of both abrasion and sub-surface damage. Older individuals can be very brittle due to infestations of the boring sponge Cliona celata (Comely 1978). Abrasion will also damage or remove associated biota.

Sensitivity assessment.  Abrasion at the surface only is considered likely to flatten clumps and dislodge and break individuals. Resistance is assessed as ‘Low’ (damage or loss to 25-75% of the population), although the significance of the impact for the bed will depend on the spatial scale of the pressure footprint. Resilience is assessed as ‘Low’ (10-25 years), and sensitivity is assessed as ‘High’. Epifauna associated with the bed is also likely to be damaged and removed.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

As Modiolus modiolus are large, sessile and shallowly buried, individuals are unable to escape from penetration and disturbance of the substratum and clear evidence exists for declines in the extent and density of beds exposed to activities that lead to this pressure.   The associated attached epifauna and infauna are also likely to be damaged and removed by this pressure.

Evidence for long-term declines in response to abrasion and sub-surface penetration pressures, resulting from mobile gears has been found from surveys and monitoring in areas where beds have been impacted. Horse mussel beds in Strangford Lough in Northern Ireland have suffered notable declines in extent.  Magorrian & Service (1998) reported that queen scallop trawling resulted in flattening of horse mussel beds and disruption of clumps of horse mussels and removal of emergent epifauna in Strangford Lough. They suggested that the emergent epifauna were more intolerant than the horse mussels themselves but were able to identify different levels of impact, from impacted but largely intact to few Modiolus modiolus intact with lots of shell debris (Service & Magorrian 1997; Magorrian & Service 1998).

Comparisons of dive survey data sets collected in Strangford Lough in 1975-1983 and 2005-2007, demonstrated further declines in Modiolus modiolus, the bivalves Aequipecten irregularis and Chlamys varia and some erect sessile fauna between the survey periods (Strain et al., 2012). Strain et al. (2012) concluded that the epifaunal assemblage in Strangford Lough had shifted due to the period of intensive fishing for the queen scallop (Aequipecten irregularis) between 1985 and 1995. Strain et al. (2012) noted that although all mobile fishing gear was banned in 2004, there were no detectable differences that indicate recovery of epifaunal communities, including Modiolus modiolus beds between 2003 and 2007 surveys, seven years after the period of intensive fishing for queen scallops. 

Cook et al. (2013) were able to examine the effects of a single pass by a scallop dredge on Modiolus modiolus beds off the Lleyn Peninsula and an otter trawl on the northeast of the Isle of Man. The tracks from the mobile gears were observed during routine bed monitoring and the observations are based on normal fishing activities rather than designed experiments. The trawl resulted in a 90% reduction in the number of epifauna while the scallop dredge resulted in a 59% reduction. At both sites mean Modiolus modiolus abundance declined, with visible flattening of clumps in response to dredging.  No evidence of recovery was recorded at the Isle of Man site a year after impact was first recorded. Mean abundance of Modiolus modiolus within quadrats was 63.8 (SD 20.7) within the unimpacted area and 40.7 (SD 15.4) within the scallop dredge impact at the Lleyn peninsula. 

Kenchington et al. (2006) examined the effects of multiple passes of an otter trawl on benthic communities on the Western Bank on Canada’s Scotian shelf in the northwest Atlantic. The community was dominated (76%) by Modiolus modiolus attached to rocks, embedded in the seabed or in small groups but was not considered to represent a Modiolus reef habitat. The transect was trawled 12-14 times, on three occasions over a 20 month period. As a result, the epifauna was reduced (from 90% to 77% contribution to the community). The most marked decline was in Modiolus modiolus abundance, which declined by approximately 80% to 60% of the community, (a reduction in biomass from approximately 2753 g before trawling in 1997 to 987 g after trawling in 1999) due to direct damage from the trawl and subsequent consumption by predators and scavengers.

Sensitivity assessment.  Based on the available evidence, resistance to a single instance of penetration and disturbance at the pressure benchmark is assessed as ‘Low’ (loss of 25-75%), and resilience is assessed as ‘Low’ (> 25 years). Sensitivity is, therefore assessed as ‘High’. Due to the low levels of recovery, repeated impacts are likely to result in the loss of reefs.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Changes in light penetration or attenuation associated with this pressure are not relevant to Modiolus modiolus biotopes, however, alterations in the availability of food or the energetic costs in obtaining food or changes in scour could either increase or decrease habitat suitability for Modiolus modiolus beds. Horse mussels are selective feeders and can reject inorganic particles or larger, less nutrient rich phytoplankton in the form of pseudofaeces (Navarro &  Thompson, 1997). Modiolus modiolus is found in a variety of turbid and clear water conditions (Holt et al., 1998).  Muschenheim & Milligan (1998) noted that the height of the horse mussel beds in the Bay of Fundy positioned them within the region of high quality seston while avoiding high levels of re-suspended inorganic particulates (2.5-1500 mg/l) at the benthic boundary layer.  An increase in suspended inorganic solids may reduce feeding efficiency and where the concentration exceeds tolerances, individuals may close valves and cease feeding.

Decreases in turbidity may increase phytoplankton productivity and potentially increase food availability. Therefore, horse mussel beds may benefit from reduced turbidity.

Sensitivity assessment.  No directly relevant empirical evidence was found to assess this pressure.  Resistance to this pressure is assessed as 'High' as an increase in turbidity may impact feeding and growth rates but not result in mortality of adults. Resilience is assessed as 'High' (by default) and the biotope is assessed as 'Not Sensitive' to changes in turbidity at the benchmark level.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Modiolus modiolus are unable to actively emerge from sediments if buried. In areas of strong tidal flow where some Modiolus modiolus beds are found, deposits of silt may be removed fairly rapidly, although some silts will be trapped within crevices and spaces. Where currents are weaker, as in some of the sheltered lochs and similar areas where beds occur, deposits may be removed more slowly and impacts may be greater.  Mass Accumulation Rates of  0.63±0.09 g cm² year-1 following bottom trawling in Strangford Lough were suggested to act as a driver for potential negative effects on the physiological condition of remnant populations of Modiolus modiolus by Strong & Service (2008). In a series of burial experiments, Hutchison et al. (2016) tested the response of individuals to burial under three depths of sediment (2, 5 and 7cm), three sediment fractions (coarse-1-2mm; medium-fine-0.25-0.95 mm and fine-0.1-0.25 mm) and five burial durations (2, 4, 8, 16, 32 days). Modiolus modiolus could not re-emerge from sediments and mortality increased with duration of smothering and the proportion of fine particles in the smothering material, the depth of burial did not alter mortality rates. Buried individuals survived for 8 days without apparent mortality but by 16 days cumulative mortality was greater than 50% (Hutchison et al., 2016).

Sensitivity assessment. The experiments by Hutchison et al., (2016) show that duration of burial is a key factor determining survival, burial under even small amounts of fine sediment (2 cm) for longer than 8 days could lead to significant mortality. Site-specific hydrodynamics that influence the mobility of deposited sediments will mediate resistance. As this biotope may occur in sheltered conditions, resistance is assessed as 'Low' as some mussels may be smothered for longer than a week and begin to die before the overburden is removed. Resilience is assessed as 'Low' and sensitivity is, therefore, categorised as 'High'.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Modiolus modiolus are unable to actively emerge from sediments if buried. In areas of strong tidal flow where some Modiolus modiolus beds are found, deposits of silt may be removed fairly rapidly, although some silts will be trapped within crevices and spaces. Where currents are weaker, as in some of the sheltered lochs and similar areas where beds occur, deposits may be removed more slowly and impacts may be greater.  Mass Accumulation Rates of  0.63±0.09 g cm² year-1) following bottom trawling in Strangford Lough were suggested to act as a driver for potential negative effects on the physiological condition of remnant populations of Modiolus modiolus by Strong & Service (2008). In a series of burial experiments, Hutchison et al. (2016) tested the response of individuals to burial under three depths of sediment (2, 5 and 7cm), three sediment fractions (coarse-1-2mm; medium-fine-0.25-0.95 mm and fine-0.1-0.25 mm) and five burial durations (2,4,8,16,32 days). Modiolus modiolus could not re-emerge from sediments and mortality increased with duration of smothering and the proportion of fine particles in the smothering material, the depth of burial did not alter mortality rates. Buried individuals survived for 8 days without apparent mortality but by 16 days cumulative mortality was greater than 50% (Hutchison et al., 2016).

Sensitivity assessment. The experiments by Hutchison et al., (2016) show that duration of burial is a key factor determining survival, burial under even small amounts of fine sediment (2 cm) for longer than 8 days could lead to significant mortality. Site-specific hydrodynamics that influence the mobility of deposited sediments will mediate resistance, as the deposit at the pressure benchmark is substantial (30 cm) it is likely that the deposit will persist for some time (particularly if it consists of cohesive, fine particles) before removal. As this biotope may occur in sheltered conditions, resistance is assessed as 'Low' as some mussels may be smothered for longer than a week and begin to die before the overburden is removed. Resilience is assessed as 'Low' and sensitivity is therefore categorised as 'High'.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

No evidence.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

No evidence was found to assess the sensitivity of Modiolus modiolus to this pressure. However, experiments have demonstrated that Mytilus edulis show sensitivity to substrate-borne vibration, and it is possible that beds of Modiolus modiolus will also be affected by underwater noise. Behavioural changes (valve closure), in Mytilus edulis,  were observed in experiments in in response to vibration stimulus (Roberts et al., 2015). Thresholds were shown to be within the range of vibrations measured in the vicinity of anthropogenic operations such as pile driving and blasting. The responses show that vibration is likely to impact the overall fitness of both individuals and mussel beds of Mytilus edulis due to disruption of natural valve periodicity, which may have ecosystem and commercial implications (Roberts et al., 2015).

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

No evidence.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Not relevant.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

Not relevant’ to seabed habitats.  NB. Collision by grounding vessels is addressed under ‘surface abrasion’.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Not relevant.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Habitat restoration projects may translocate stock to repopulate areas of suitable habitat (Elsässer et al., 2013). No evidence was found for detrimental, genetic effects arising from this practice, although there is potential also for the movement of pathogens and non-indigenous, invasive species. In Strangford Lough, restoration efforts translocated Modiolus modiolus clumps within the Lough as it was considered that individuals from outside populations would be less suitable (Fariñas-Franco et al., 2013, 2016).  Translocation of individuals was demonstrated to support larval settlement on artificial reefs, when measured against cultch alone and is a useful technique to support habitat restoration (Fariñas-Franco et al., 2016). 

Gormley et al., (2015) developed biophysical models for larval dispersal in the Irish Sea validated by DNA studies indicate that populations of Modiolus modiolus in the North Irish Sea are connected. Genetic analysis was consistent with those of the biophysical models and indicated moderately significant differentiation between the Northern Ireland populations and those in the Isle of Man and Wales. Simulations of larval dispersal over a 30 day pelagic larval duration (PLD) suggest that connectivity over a spatial scale of 150km is possible between some source and sink populations. However, it appears unlikely that larvae from Northern Ireland will connect directly with sites on the Llŷn or Isle of Man. It also appears unlikely that larvae from the Llŷn connect directly to any of the other sites (Gormley et al., 2015). Inter-site differences in shell morphology, reflecting phenotypic differences have been observed between populations that relate to adaptation to local environmental conditions. Translocating individuals with ecophenotypes that are different to local populations may impact on the success of translocation may result in negative impacts on local populations through gene flow.

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009). Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024). In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Michin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Although a widespread invader, Didemnum vexillum has a limited ability for natural dispersal since the pelagic larvae remain in the water column for a short time (up to 36 hours). Therefore, it has a short dispersal phase that can allow the species to build localized populations (Herborg et al., 2009; Vercaemer et al., 2015; Holt, 2024). However, Bullard et al. (2007) suggested that Didemnum vexillum can form new colonies asexually by fragmentation. Colonies can produce long tendrils from an encrusting colony, which can fragment, disperse and settle, attaching to suitable hard substrata elsewhere (Bullard et al., 2007; Lambert, 2009; Stefaniak & Whitlatch, 2014). A fragmented colony can spread naturally for up to three weeks transported by ocean currents, attached to floating seaweed, seagrass or other floating biota, or as free-floating spherical colonies (Bullard et al., 2007; Lengyel et al., 2009; Stefaniak & Whitlatch, 2014; Holt, 2024). Fragments can reattach to suitable substrata within six hours of contact. Fragments have the potential to disperse around 20 km before reattachment (Lengyel et al., 2009). Valentine et al. (2007a) reported that colonies of Didemnum vexillum enlarged by 6 to 11 times by asexual budding after 15 days and enlarged 11 to 19 times after 30 days. Valentine et al. (2007a) concluded fragments could successfully grow, survive, and help to spread Didemnum vexillum.

While natural fragmentation of tendrils is thought to allow Didemnum vexillum to invade longer distances and increase its dispersal potential, Stefaniak & Whitlatch (2014) found that only a one tendril out of 80 reattached to the flat, bare substrata used in their study, because tendrils required an extensive (at least eight hour) period of contact to reattach. Stefaniak & Whitlatch (2014) suggested that once fragmented from a colony, the success of tendril reattachment was limited, and reattachment was not a major contributor to the invasive success of Didemnum vexillum. However, Stefaniak & Whitlatch (2014) also found that larvae-packed tendril fragments may increase natural dispersal distance, reproduction, and invasive success of Didemnum vexillum, and increase the distance larvae can travel. Not all colonies produce tendrils at all locations.

Human-meditated transport via aquaculture facilities, boat hulls, commercial fishing vessels, and ballast water is probably the most important vector that has aided the long-distance dispersal of Didemnum vexillum and explains its prevalence in harbours and marinas (Bullard et al., 2007; Dijkstra et al., 2007; Griffith et al., 2009; Herborg et al., 2009). Fragmentation of colonies during transport or human disturbance (such as trawling or dredging) could indirectly disperse the species and enable it to find suitable conditions for establishment (Herborg et al., 2009). For example, in oyster farms in British Columbia, large fragments of Didemnum sp. come off oyster strings when they are pulled out of water and other fragments can be pulled off oysters and mussels and thrown back into the water, which is likely to aid dispersal of the invasive species (Bullard et al., 2007). Dijkstra et al. (2007) hypothesised that Didemnum sp. was introduced to the Gulf of Maine with oyster aquaculture in the Damariscotta River and transported via Pacific oysters.

Didemnum vexillum was likely introduced into the UK from northern Europe or Ireland via poorly maintained or not antifouled vessels, movement of contaminated shellfish stock and aquaculture equipment, or via marine industries such as oil, gas, renewables, and dredging (Holt, 2024). Recent evidence from genetic material suggests that human-mediated dispersal, between marinas and shellfish culture sites, is the most likely pathway for connectivity of Didemnum vexillum populations throughout Ireland and Britain (Prentice et al., 2021; Holt, 2024). Didemnum vexillum can disperse away from artificial substrata, invading and colonizing natural substrata in surrounding areas (Tillin et al., 2020). Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. The current evidence of Didemnum vexillum’s ability to spread on natural habitats in this area is sparse and often conflicting, complicated by genetics and its apparent variable habitat preferences and tolerances and its variable ability to adapt to ‘new’ conditions (Holt 2024).

Didemnum vexillum has a seasonal growth cycle that is influenced by temperature (Valentine et al., 2007a). In warmer months (June and July) colonies may be large and well-developed encrusting mats. Populations experience more rapid growth from July to September sometimes continuing into December. Colonies begin to decline in health and ‘die-off’ when temperatures drop below 5°C during winter months from around October to April (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). Cold water months cause colonies to regress and reduce in size, yet they often regenerate as temperatures warm (Griffith et al., 2009; Kleeman, 2009, Mercer et al., 2009), although some populations may not survive winter at all (Dijkstra et al., 2007). The early growth phase, from May to July, is initiated by smaller colonies developing from remnants of colonies that survived the cold water (Valentine et al., 2007a). The seasonal growth cycle is also likely influenced by location. For example, the Didemnum sp. growth cycle for colonies in Sandwich tide pool (temperature range from -1 °C to 24 °C, with daily fluctuations), probably does not occur in deep offshore subtidal habitats in Georges Bank (annual temperature range from 4 °C to 15°C, and daily fluctuations are minimal) (Valentine et al., 2007a).  Larval release and recruitment typically occur between 14 to 20°C and slow or cease below 9 to 11°C as summer ends (Griffith et al., 2009; Mckenzie et al., 2017). In New Zealand, recruitment occurs from November to July, where the highest average temperatures were recorded in February (18 to 22°C) and the lowest average temperatures were recorded in July (9 to 10°C) (Fletcher et al., 2013a). In this New Zealand study, higher water temperatures were associated with a higher level of recruitment (Fletcher et al., 2013a).

Didemnum vexillum requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems because of its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock gravel, pebble, cobble, or boulders or artificial substrata such as a variety of maritime structures such as pontoons, docks, wood and metal pilings, chains, ropes and moorings, plastic and ship hulls and at aquaculture facilities (Valentine et al., 2007 a&b; Bullard et al., 2007; Griffith et al., 2009; Lambert, 2009; Tagliapietra et al., 2012; Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent, and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus  and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Groholz, 2015; Vercaemer et al., 2015).

There are few observations of Didemnum vexillum on soft bottom habitats as evidence suggests it is unable to establish or grow easily on mud, mobile sand or other unstable substrata, and it is vulnerable to smothering by fine sediment (Bullard et al., 2007; Valentine et al., 2007a; Griffith et al., 2009). The species is usually found established in areas where the colony is protected from sedimentation and wave action (Valentine et al., 2007b; Mckenzie et al., 2017; Tillin et al., 2020). For example, at Georges Bank, USA the Didemnum vexillum mats were limited to gravelly areas and unable to colonize the sand ridges that bounded the site, which have a mobile surface that is moved daily by the strong tidal currents (Valentine et al., 2007b). In addition, evidence found the species can also not survive being buried or smothered by coarse or fine grained sediment. Furthermore, in Holyhead marina, Didemnum vexillum colonies were contained in the harbour and established on artificial pontoons, and they were not present on the natural seabed under the pontoon, which is composed of silty mud or on deeper sections of mooring chains that are immersed in mud at low spring tides (Griffith et al., 2009).

However, some studies on Georges Bank, USA and Sandwich, Massachusetts observed colonies were able to survive partial covering by sand (Bullard et al., 2007; Valentine et al., 2007a). Gittenberger et al. (2015) reported that Didemnum vexillum was able to overgrow sandy bottom (cited Gittenberger, 2007). In northern Kent, Didemnum vexillum has been recorded covering London clay boulders on Whitestable Flats, West Beach, north Kent, covering tabulate sandstone boulders (0.5 to 2 m across) on the mid-shore and colonizing sediment mounds on the low shore characterized by larger areas of sand, mud and low-lying sediment at Reculver and Bishopstone, north Kent (Hitchin, 2012). It was also recorded from muddy substrata at that site. Hitchin (2012) noted that the site was exposed to enough waves and currents to cause sedimentation. However, Didemnum vexillum grew hanging from on the underside of sandstone boulders nestled on sediment, on consolidated sediment mounds and firm clays, hence burial may prevent colonization and its survival rather than sedimentation alone.

In contrast, Didemnum vexillum’s preference for sheltered conditions, established colonies observed in Georges Bank and Long Island Sound were exposed to moderately strong tidal currents (1 to 2 knots; ca 0.5 to 1 m/s recorded at both sites) that may mobilise sediment (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020). However, Valentine et al. (2007b) describe the substratum as immobile, presumably consolidated, gravel, cobbles, and pebbles. Kleeman (2009), stated that the presence of a consistent mild wave action or ‘swash zone’ appears to favour Didemnum sp. establishment in the intertidal. Although some evidence suggests that waves and currents can facilitate the fragmentation and spread of Didemnum vexillum (Mckenzie et al., 2017), the tidal current velocities at some sites where Didemnum vexillum has been reported (for example, New England, where current velocities reach up to around 3 m/s) is lower than the current velocity required for the dislodgement of Didemnum vexillum fragments (around 7.6 m/s) (Reinhardt et al., 2012). This suggests that not all tidal currents are likely to dislodge Didemnum vexillum fragments. When on boat hulls the species can experience higher current velocities which is enough to cause dislodgement (Reinhardt et al., 2012).  

Didemnum vexillum can overgrow bivalve species, such as oysters, scallops, and mussels, as the hard shells can provide suitable hard substrata for settlement. It has been described as a ‘shellfish pest’ by the aquaculture industry because it is likely to completely encapsulate bivalves and smother them resulting in death or partially encapsulate and partially smother them resulting in reduced bivalve growth (Auker, 2010; Bullard et al., 2007; Coutts & Forrest, 2007, Valentine et al., 2007a; Carman et al., 2009; Kleeman, 2009; Fletcher et al., 2013b; Tillin et al., 2020). Didemnum vexillum has been recorded overgrowing mussels in Strangford Lough, Northern Ireland (Minchin & Nunn, 2013) and recorded forming large mats over Blue Mussel beds in the Gulf of Maine, completely covering individuals (Auker et al., 2014). Didemnum vexillum fouling on aquaculture equipment and bivalve species causes great economic impacts, as Didemnum vexillum removal methods are expensive, labour-intensive, and not always effective (Coutts & Forrest, 2007; Carman et al., 2009; Kleeman, 2009; Fletcher et al., 2013b; Tillin et al., 2020; Holt, 2024). The fouling on aquaculture nets and bags can restrict water flow and food availability for shellfish and smothering on mussel farms may result in crop losses (Coutts & Forrest, 2007; Carver et al., 2003 cited by Carman et al., 2009; Fletcher et al., 2013b; Holt, 2024). Effects on mussels are likely to become more prominent as Didemnum vexillum becomes more abundant (Auker, 2010).

The epibiotic relationship between Didemnum vexillum and Mytilus edulis negatively impacts mussel growth (Auker, 2010). Clean control mussels with no Didemnum vexillum overgrowth had thicker shells, a significantly thicker lip, and a greater tissue index, compared to mussels overgrown by Didemnum vexillum (Auker, 2010). The clean mussels’ average length ranged from 3.2 cm to 5.37 cm, and had significantly greater shell lengths than overgrown mussels, which had an average length from 3.4 cm to 4.86 cm (Auker, 2010). Mortality of both control and overgrown mussels was relatively low over the one-year study period, but higher mortality was seen in overgrown mussels (6.7% died) compared to the clean control mussels (1.1% died) (Auker, 2010). Food is an important factor contributing to the decrease in mussel growth (Auker, 2010). Auker (2010) also found that Didemnum vexillum affected reproduction and recruitment of Mytilus edulis as the invasive species grew over gamete release point (siphons) or inhibited settlement of recruits, but this varied seasonally.

In contrast, the overgrowth of mussels by Didemnum vexillum has reduced the predation risk on mussels (Auker, 2010; Auker et al., 2014, Lyu et al., 2020). The Didemnum vexillum mats act as refuges for blue mussels (Lyu et al., 2020). Evidence has suggested that the relationship between Didemnum vexillum and Mytilus edulis reduces the predation by the green crab as Didemnum vexillum deters predator attacks (Auker et al., 2014). It was suggested that the negative impacts of Didemnum vexillum overgrowth on mussel growth, resulting in smaller-sized blue mussels, may protect smaller blue mussels from predation as these are preferred over larger blue mussels by predators (Auker et al., 2014). In Auker’s (2010) study, Carcinus maenas consumed fewer mussels that were overgrown by Didemnum vexillum. The toxic chemical defences of Didemnum vexillum and the release of secondary metabolites and sulfuric acid may deter crab predators (Lyu et al., 2020). The protection from predators provided by Didemnum vexillum may vary seasonally due to evidence suggesting the invasive ascidians deteriorate during the winter months, potentially reducing predation protection for mussels during this time (Auker et al., 2014).

However, Fletcher et al. (2013b) reported that smaller-sized Perna canaliculus mussels (20-40 mm) were significantly affected by fouling of Didemnum vexillum on cultured mussel ropes. The cultured ropes included ambient fouling (ropes left to be naturally colonized by Didemnum vexillum and other species), enhanced fouling (ambient fouling with ropes that were artificially inoculated with Didemnum vexillum) and a control (small levels of fouling maintained by the removal of Didemnum vexillum). The average mussel density and average mussel weight of smaller mussels were higher in the control than were in the treatments fouled by Didemnum vexillum. After 15 months, the smaller mussels were significantly smaller than the medium (40-60 mm) and large (60-70 mm)-sized mussels, which remained a similar size by the end of the experiment. They estimated there was a 40% reduction in small-sized mussel density per kilogram of Didemnum vexillum, indicating a negative relationship between small-sized mussel density and increasing Didemnum vexillum.

Small-sized mussels had a significant difference in mussel loss than the larger mussels, with greater loss of the smaller mussels seen in the ambient and enhanced fouling treatments. The small mussels were displaced and overgrown by Didemnum vexillum. Displacement was also evident to a lesser extent in the medium mussels but was less of a threat to larger mussels. However, the fouling treatments alone did not have a significant overall effect on mussel loss. It was suggested that high levels of fouling on the ropes may have resulted in small mussel loss as the mussels carry out a process of self-thinning, but high levels of fouling did not appear to affect individual mussel size or condition directly (Fletcher et al., 2013b). Fletcher et al. (2013b) also noted that Didemnum vexillum clogged cages and mesh used to house shellfish (e.g. mussels and oysters), which could reduce shellfish growth rates. Fletcher et al. (2013b) concluded that there were no direct effects of Didemnum vexillum fouling on mussel size and condition, but did indicate negative effects on small-sized mussels. However, in their study, Didemnum vexillum was only one of the fouling species contributing to fouling effects (Fletcher et al., 2013b).

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Hinz et al., 2011; Helmer et al., 2019; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). Crepidula fornicata is reported to settle and establish amongst mussel beds (Blanchard, 1997; Thieltges, 2005; Rayment, 2007).  If Crepidula fornicata becomes established in a bed it is likely to alter the bed structure particularly if it is on coarse sand or hard substrata. Crepidula fornicata has high fecundity and can disperse its larvae over large areas making mussel beds highly vulnerable if Crepidula fornicata is introduced even large distances away.  The larvae of Crepidula fornicata can survive transport in ballast water for a number of days allowing it to travel large distances before needing to settle in the areas where the ballast water is released (Blanchard, 1997).  Crepidula can colonize a wide range of substrata. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). It is recorded from the lower intertidal to ca 160 m in depth but it is most common in the shallow subtidal and low water springs (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020). Hinz et al. (2011) recorded Crepidula off the Isle of Wight in the English Channel at ca 60 m on rough ground in high tidal flow areas. Tillin et al. (2020) suggested that the effect of oscillatory wave meditated flow might have a greater effect on Crepidula than tidal flow, presumably due to mobilization of the substratum.  

Thieltges et al. (2003) reported that Crepidula fornicata was abundant on mussel beds in the intertidal to subtidal transition zone, in the northern Wadden Sea in the year 2000. Crepidula had increased in abundance since 1948 and had expanded its range from the extinct oyster beds to mussel beds where live mussels were its main substratum. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Thieltges et al. (2003) noted that Crepidula abundance at the intertidal to subtidal transition zone (ca 21 /m²) was significantly higher than in the upper, mid, and lower intertidal (ca <3 /m²). Thieltges (2005) reported a 28-30% mortality of Mytilus edulis when Crepidula fornicata was introduced to the beds in experimental studies. He also found that mussel shell growth was reduced by 3 to 5 times in comparison to unfouled mussels and that extra energy was probably expended on byssus production.  The most significant cause of mortality was increased drag on mussels due to the growth of stacks of Crepidula fornicata on the shells of the mussels, rather than competition for food. He concluded that Crepidula fornicata is potentially an important mortality factor for Mytilus edulis (Thieltges, 2005).  Thieltges (2005) also observed mussel beds in the shallow subtidal infested with high abundances of Crepidula fornicata with almost no living mussels, along the shore of the List tidal basin, northern Wadden Sea.  

Bohn et al. (2013a) reported that mussel shells provided a more suitable settlement substratum for Crepidula larvae than bare panels in larval settlement experiments. However, the presence of live Mytilus edulis did not increase colonization of the site by Crepidula in the Milford Harbour Waterway, e.g., no Crepidula were found on mussels at a site with 23% cover of mussels (Bohn et al., 2015). Bohn et al. (2015) suggested that its prevalence on mussels in the Wadden Sea was due to a lack of alternative substratum, together with the cold weather mortalities. 

Crepidula fornicata is likely to alter water flow over mussel beds. They form stacks of individuals that change water flow across the sediment surface.  When these stacks occur on the shells of Mytilus edulis they increase the drag on the mussel, increase the demands on the mussel’s energy reserves for attachment (e.g. byssus formation) and, hence, affect fecundity and survival (Thieltges, 2005; Sewell et al., 2008).  The increased drag may also result in clumps of mussels being removed by water flow (Thieltges, 2005).  Competition for suspended organic matter and space is also increased.  Space for the settlement of macrobenthic organisms (Blanchard, 1997) including mussels is particularly reduced.  In addition to the reduced space for settlement, larvae of macrobenthic organisms are consumed by the slipper limpet and may affect recruitment to an area. 

Sensitivity assessment. Crepidula fornicata has the potential to colonize this biotope due to the presence of mixed substrata and shell provided by the horse mussel Modiolus modiolus and the lack of wave action. Crepidula is reported to be most abundant in the shallow sublittoral, while this biotope occurs at 50-100 m. However, Hinz et al. (2011) recorded Crepidula at 60 m and it has been recorded at 160 m (OBIS, 2023), although its abundance was unclear. The evidence above suggests that Crepidula can colonize and damage blue mussel beds, so it may be able to colonize horse mussel beds. However, Crepidula has not been recorded from horse mussel beds, and no evidence of the effect of Crepidula on horse mussel beds has yet been reported. Therefore, a precautionary resistance of ‘Low’ is suggested because once established Crepidula has the potential to damage the horse mussel bed and smother the surrounding sediment. Resilience is likely to be ‘Very Low’ as the slipper limpet population would need to be removed for recovery to occur. Therefore, sensitivity to invasion by Crepidula is assessed as ‘High’ but with 'Low' confidence due to the lack of evidence relevant to this habitat. 

No evidence of the effects of Didemnum vexillum on Modiolus modiolus beds was found. Nor were records of Didemnum sp. on Modiolus modiolus beds found. Nevertheless, Didemnum vexillum has been recorded in the sublittoral to depths of 81 m in Georges Bank and 30 m in Long Island, USA (Bullard et al., 2007; Valentine et al., 2007b; Mercer et al., 2009). The horse mussel biotopes occur on mixed sediments that could provide suitable hard substratum for colonization by Didemnum sp. However, the horse mussels themselves provide a suitable substratum. Didemnum vexillum has also been recorded in moderately strong currents (Valentine et al., 2007b; Mercer et al., 2009; Tillin et al., 2020) and predicted to survive stronger currents, as the current velocity which will dislodge Didemnum vexillum is around 7.6 m/s (Reinhardt et al., 2012). Therefore, Didemnum vexillum could colonize examples of these biotopes that occur in tide-swept conditions. The smothering of the horse mussel beds by Didemnum vexillum may result in loss of condition, and possibly some mortality in smaller horse mussels based on its effect on mussel beds (Auker, 2010; Auker et al., 2014). Didemnum vexillum regresses as temperatures decline in winter so shallow examples may be able to recover their condition in winter (Gittenberger, 2007; Valentine et al., 2007a; Herborg et al., 2009). However, deeper examples may not experience enough temperature change to trigger the decline in Didemnum vexillum (Valentine et al., 2007a). The smothering by Didemnum sp. has been shown to kill a variety of epifauna, including hydroids and ascidians, so the biodiversity of the beds would probably decline (Gittenberger, 2007). Smothering may also reduce recruitment of Modiolus modious and could result in a long-term decline in the population, although horse mussels are long-lived. Holt (2024) noted that Didemnum vexillum had not spread as far as feared in the UK since it was first recorded. Therefore, a resistance of 'Medium' (some, <25% mortality) is suggested as a precaution in case Didemnum vexillum is able to colonize the biotope, but with 'Low' confidence due to the lack of direct evidence. Resilience is assessed as 'Very low' as recovery would require the physical removal of Didemnum sp., so sensitivity is assessed as 'Medium'. 

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Brown & Seed (1977) reported a low level of infestation (ca 2%) with pea crabs Pinnotheres sp. in Port Erin, Isle of Man and Strangford Lough. Comely (1978) reported that ca 20% of older specimens, in an ageing population, were damaged or shells malformed by the boring sponge Cliona celata. Infestation by the boring sponge reduces the strength of the shell and may render the population more intolerant of physical disturbance (see above). However, little other information concerning the effects of parasites or disease on the condition of horse mussels was found.

Shumway (1990) reviewed the effects of algal blooms on shellfish and reported that a bloom of Gonyaulax tamarensis(Protogonyaulax) was highly toxic to Modiolus modiolus. Shumway (1990) also noted that both Mytilus spp. and Modiolus spp. accumulated paralytic shellfish poisoning (PSP) toxins faster than most other species of shellfish, e.g. horse mussels retained Gonyaulax tamarensis toxins for up to 60 days (depending on the initial level of contamination). Landsberg (1996) also suggested that there was a correlation between the incidence of neoplasia or tumours in bivalves and outbreaks of paralytic shellfish poisoning in which bivalves accumulate toxins from algal blooms, although a direct causal effect required further research.

The parasites Martelia refringens or other Marteilia sp. can cause significant bivalve infections. Although these have been reported to infect Modiolus modiolus (Bower et al. 2004), no evidence was available to assess the scale of impact and therefore there is not enough evidence to assess sensitivity. 

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Artisanal fisheries have targeted Modiolus modiolus as bait for the long-line fishery (Jeffreys 1863; Wiborg 1946) and, more locally, for human consumption around the British Isles (Jeffreys 1863; Holt et al. 1998) and the Faroe Islands (Dinesen & Ockelmann 2005). Modiolus modiolus is not currently directly targeted in the UK and hence this pressure is considered to be ‘Not relevant’. While removal of commercially targeted species that are supported by Modiolus beds, such as scallops (Aequipecten opercularis), whelks (Buccinum undatum) and spider crabs (Maja brachydactyla) (Kent et al., 2017), will reduce species richness, the loss of targeted species is unlikely to adversely affect the Modiolus modiolus bed through biological effects. The physical effects of dredging for scallops and other targeted species are discussed through the abrasion and penetration pressures. The removal of target species that predate on Modiolus modiolus would potentially be beneficial allowing the recruitment of juveniles to the adult population. However, such effects are not directly documented and could not be included in the assessment.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Removal of Modiolus modiolus within this biotope, as by-catch, will alter the physical structure of the biotope and reduce habitat complexity: these are considered ecological impacts and hence this biotope group is considered to be sensitive to this pressure, at the pressure benchmark. Epifauna associated with the bed is also likely to be damaged and removed as bycatch (Magorrian & Service, 1998) altering the structural complexity of the bed. A study by Garcia et al., (2006) found that by-catch constituted 28% (by weight) of the total catch of a dredge fishery for the scallop Chlamys islandica.  Modiolus modiolus constituted 32% of the by-catch by biomass.

Sensitivity assessment. Resistance is assessed as ‘Low’ (damage or loss to 25-75% of the population), although the significance of the impact for the bed will depend on the spatial scale of the pressure footprint. Resilience is assessed as ‘Low’ (2-10 years) and sensitivity is assessed as ‘High’. Epifauna associated with the bed is also likely to be damaged and removed.