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

Lagis koreni is widely distributed, found from the Artic and north-east Europe to the Adriatic. Phaxas pellucidus is widespread, often abundant around Britain and distributed from Norway to north-west Africa (Hayward & Ryland, 1995b).

Feeding rates of Lagis koreni have been reported to be influenced by temperature (Nicolaidou, 1983, 1988). However, the range of temperatures investigated in Nicolaidou (1988) varied from 7-15°C, and are not conclusive regarding the possible behaviour of Lagis koreni in response to increases in temperature. Furthermore, Schückel et al. (2010) investigated the temporal variability of macrofauna communities in the northern North Sea in relation to changes in temperature and/or changes in hydrography. The authors observed a significant negative correlation between total macrofaunal abundance, including of Lagis koreni, and mean surface temperature (SST). Mean surface temperatures recorded varied between 13.4-16.6°C. Mean bottom temperatures remained more stable and varied between 6.7-7.6°C. The authors observed a strong decrease in mean abundance as a result of increased SST because increased SST mainly enhanced stratification and contributed to a decrease in food availability for the benthic community at the site. 

Populations of the razor shell Ensis siliqua in the warmer waters of Portugal spawn several months earlier in the year than UK populations and are sexually mature at only one year old (Gaspar & Monteiro, 1998; Henderson & Richardson, 1994) compared to three in the UK (Hill, 2006). Recruitment of Phaxas pellucidus may therefore be affected by an increase in temperature.

Some protection may be afforded by the infaunal position of the characterizing species, as these are not subjected to large temperature variations and may be less resistant to changes, leading to a decline in species diversity. The temperature may also affect microbial activity within the sediment which could alter the depth at which the anoxic layer appears.

Sensitivity assessment. No direct evidence concerning temperature tolerances of the biotope and the characterizing species was found. The characterizing species of the biotope are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4-19°C (Huthnance, 2010), and maximum sea surface temperatures rarely exceed 20°C (Hiscock, 1998). Elevated temperatures may affect the growth of some of the characterizing species but no mortality is expected. It is therefore likely that Lagis koreni and Phaxas pellucidus are able to resist a long-term increase in temperature of 2°C and potentially benefit from increased feeding activity and recruitment opportunities. However, based on Schückel et al. (2010), an acute 5°C increase for one month period may result in some mortality, so resistance is therefore assessed as Medium (loss <25%). Resilience is likely to be High so the biotope is considered to have Low sensitivity to an increase in temperature at the pressure benchmark level.

Temperature decrease (local)

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

Evidence

Lagis koreni is widely distributed, found from the Artic and north-east Europe to the Adriatic. Phaxas pellucidus is widespread, often abundant around Britain and distributed from Norway to north-west Africa (Hayward & Ryland, 1995b).

Growth, survivorship, reproduction and productivity of a Lagis koreni (studied as Pectinaria (Lagis) koreni) population was studied in Colwyn Bay, Wales. Growth was fast initially but it ceased completely during the winter, probably due to low temperatures (Nicolaidou, 1983). Studies by Irlinger et al. (1991) of the Bay of Seine macrobenthic community, France, equally reported Lagis koreni densities to vary throughout the year, with a severe decrease in abundance at the end of winter from populations with more than 2000 individuals /m² after spring recruitment. Both these reports agree with Arntz & Romohr (1986), who reported Baltic populations of the species were sensitive to extremely low bottom temperatures. Furthermore, feeding rates of Lagis koreni were reported to be influenced by temperature (Nicolaidou, 1983, 1988). Nicolaidou (1988) found that the animals were most active at higher temperatures. At 7°C the rate of sediment reworking was significantly less than that at 10 and 15°C, with some worms completely ceasing their reworking activity at 7°C (Nicolaidou, 1988).

Reiss et al. (2006) studied cold winter effects on benthic communities in the North Sea.  In 1995/1996 mean sea surface temperature was more than 2°C below average. The authors observed that nearshore communities changed dramatically while offshore benthic communities changed more gradually. Lagis koreni seemed to have been one of the species to have benefited from the adverse conditions as, probably due to its opportunistic lifestyle, the species was able to very quickly become one of the dominant species at sites affected by cold winters (Reiss et al., 2006). These results agree with those of Kröncke et al. (2013a), who also reported Lagis koreni as dominating macrofauna communities following mortality of macrofauna species during and after the cold winters of 1978/1979 and 1995/96 that affected species biomass and abundance in the study area (the southern North Sea), including Ensis spp., which are related to characterizing species in this biotope. However, no direct evidence of tolerance of this Phaxas pellucidus was found.

The temperature may also affect microbial activity within the sediment which could alter the depth at which the anoxic layer appears.

Sensitivity assessment. The important characterizing species of the biotope are widely distributed and likely to occur both north and south of the British Isles, where typical surface water temperatures vary seasonally from 4-19°C (Huthnance, 2010). However, reduced temperatures may affect growth and result in some mortality of the characterizing species, based on Kröncke et al. (2013a) who reported mortality at 2°C anomalies below normal in sea surface temperature. Resistance is therefore assessed as Low (loss 25-75%). Resilience is likely to be 'High', so the biotope is considered to have 'Low' sensitivity to a decrease in temperature at the pressure benchmark level. 

Salinity increase (local)

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

Evidence

The biotope is only recorded in fully marine conditions (JNCC, 2022) and no records of the biotope or characterizing species occurring in hypersaline conditions were found. Furthermore, salt is used as a method of dislodging Ensis spp. from their burrows (Hill, 2006), suggesting that the species is sensitive to perturbations such as increased salinity. It is therefore likely that Phaxas pellucidus would also be sensitive.

Sensitivity assessment. No direct evidence was found on the resistance of the characterizing species to increased salinity. Long-term changes in salinity are likely to result in the loss of some species, and also result in a decrease in species richness. For example, in a review of the impacts of desalination plant discharges on the marine environment, Roberts et al. (2010b) reported changes in infaunal community structures (including reduced abundance and diversity of polychaetes and molluscs) within close proximity from discharge areas, with aggravated impacts in poorly flushed sites. Resistance of the biotope is assessed as 'Low' and resilience as 'High'. The biotope is therefore considered to have 'Low' sensitivity to increases in salinity at the pressure benchmark level.

Salinity decrease (local)

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

Evidence

Darr et al. (2014) investigated the functional changes in benthic communities along a salinity gradient in the Baltic Sea, where mean salinities were 22, 12 and 8 psu at each of the study sites. Lagis koreni was reported to be amongst the ten most abundant species at the two sites experiencing higher salinities, but not at the site with the lowest salinity. It is not clear whether the species was totally absent from the site where the mean salinity of 8 psu occurred, but it is possible to conclude that the species is likely to have experienced a major decline as a result of decreased salinity. These results agree with those of Gogina et al. (2010) and Gogina et al. (2014) who reported Lagis koreni occurring in the Baltic Sea at near-bottom salinities of 15-20 and 13-22 psu, respectively.

No direct evidence was found to assess the resistance of Phaxas pellucidus to changes in salinity. However, Ensis ensis, also a species of razor shell, does not occur in water of reduced salinity, although its absence from estuaries may sometimes be due to the lack of sediments of suitable grade (Holme, 1954). Furthermore, Ensis ensis concentrates K and Ca (Kinne, 1971b) and may be able to resist a degree of salinity reduction given that it may be subject to periodic precipitation in the intertidal. However, Darriba & Miranda (2005) concluded that salinity decreases interrupted gonadal development in the razor clam Ensis magnus.

Rees et al. (1992) reported that the biotopes SS.SSa.CMuSa.AalbNuc and SS.SMu.CSaMu.LkorPpel displayed cyclical behaviour in the Liverpool Bay area, with the community periodically switching from one biotope to the other in areas where variable salinities may occur.

Sensitivity assessment. The evidence suggests that Lagis koreni is likely to survive reduced salinities (18-30 psu), but is not clear regarding Phaxas pellucidus. However, the biotope is only recorded in fully marine conditions (JNCC, 2022), so the species are unlikely to experience hyposaline conditions and hence unlikely to resist reduced salinity. A reduction in salinity from full to reduced will probably reduce species diversity of result in a change in the community and, hence, the biotope. Resistance of the biotope is therefore assessed as 'Low' but with low confidence. Resilience is likely to be 'High', and the biotope is considered to have 'Low' sensitivity to decreases in salinity at the pressure benchmark level.

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

The hydrographic regime, including flow rates, is an important structuring factor in sedimentary habitats. For example, Irlinger et al. (1991) noted that the strong tidal currents that occur in the English Channel limit the occurrence of muddy fine sands, and consequently Lagis koreni communities to sheltered areas, which suggests this species distribution is limited to areas where substrata present are suitable to support tube building activities. The most damaging effect of increased flow rate would be the erosion of the substratum as this could eventually lead to loss of the habitat. Increased water flow rates are likely to change the sediment characteristics in which the species live, primarily by resuspending and preventing deposition of finer particles (Hiscock, 1983). This may be particularly relevant for tube building species occurring in the biotope such as characterizing species Lagis koreni. Furthermore, increased water flow rate may prevent settlement of larvae and therefore reduce recruitment. Olivier et al. (1996) noted that Lagis koreni (studied as Pectinaria koreni) purposefully selected habitats for settlement with lower densities of conspecific adults, relying on currents to reposition onto suitable substrata. Mature adults buried at depth are likely to be unaffected as muddy substrata are particularly cohesive.

Decreased water movement would result in increased deposition of suspended sediment (Hiscock, 1983). An increased rate of siltation resulting from a decrease in water flow may result in an increase in food availability for the characterizing species and therefore growth and reproduction may be enhanced, but only if food was previously limiting.   Phaxas pellucidus is recorded from moderately strong to very weak (negligible - 1.5 m/s) tidal streams (Connor et al., 2004).

Sensitivity assessment. Sand particles are most easily eroded and likely to be eroded at about 0.20 m/s (based on Hjulström-Sundborg diagram, Sundborg, 1956). Furthermore, a change in water flow could potentially change the sediment type given that the cohesive nature of muddy sediments is likely to be lessened in this biotope due to its high sand content (approx. 50%, Connor et al., 2004) and due to the instability resulting from feeding and sediment reworking activities of Lagis koreni (Eagle, 1975; Rees et al., 1975, both cited in Rees & Dare, 1993). However, the biotope occurs in sandy muds in strong, moderately strong, weak and very weak tidal steams (Connor et al., 2004), and a change at the benchmark level of 0.1-0.2 m/s is likely to fall within the range experienced by the biotope. Resistance and resilience are therefore considered to be High and the biotope is assessed as Not Sensitive to a change in water flow rate at the pressure benchmark level.

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

SS.SMu.CSaMu.LkorPpel is a circalittoral biotope (Connor et al., 2004). Changes in emergence are Not Relevant to biotopes which are restricted to fully subtidal/circalittoral conditions. The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

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

Potentially, the most damaging effect of increased wave heights would be the erosion of the fine sediment substratum as this could eventually lead to loss of the habitat that characterizes the biotope. Decreased exposure will probably lead to increased siltation and reduced grain size (to muddy sediment). Changes in wave exposure may therefore influence the supply of particulate matter for tube building and feeding activities of the characterizing species. Food supplies may also be reduced affecting growth and fecundity of the species. Strong wave action may cause damage or withdrawal of the siphons and delicate feeding structures, resulting in loss of feeding opportunities and compromised growth for the characterizing species. Additionally, individuals may be dislodged by scouring from sand and gravel mobilized by increased wave action.

In a study of the life history of Lagis koreni (studied as Pectinaria koreni) in Colwyn Bay, the author reported using samples of a nearby population which had supposedly settled at the same time, with reduced wave action being suggested as a possible reason to explain the length of survival difference between the two communities (Nicolaidou, 1983). Furthermore, the author reported that growth of the species ceased completely during the winter, probably due to disturbance by storms, as well as temperature (Nicolaidou, 1983). Similarly, the species has been considered vulnerable to storm-induced sediment disturbance (Rees & Dare, 1993), and strandings have been reported following storms (Rees et al., 1977; Fish & Fish, 1996)

Razor shells seem to be absent on exposed beaches where the sand is continually churned by waves. Rees et al. (1976) reported that wave scour caused by winter gales may have caused some individuals of Ensis ensis to be washed out along the north Wales coast. Therefore, an increase in wave exposure may remove some individuals of Phaxas pellucidus in a population.

Sensitivity assessment. Records indicate that the biotope occurs in a range of wave exposures, including exposed and moderately wave exposed conditions (Connor et al., 2004). However, wave action reduces with depth, and the biotope occurs below 10 m where wave-mediated flow will be reduced. Although the evidence suggests that the characterizing species are excluded from areas of intense disturbance and are likely to be dislodged by increased disturbance, a change in wave height at the pressure benchmark (change in nearshore significant wave height >3% but <5%) is unlikely to be significant. Resistance and resilience are therefore assessed as High, and the biotope is considered Not Sensitive at the benchmark level.

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

This pressure is Not assessed but evidence is presented where available.

There is little or no information on the resistance of the key species in the biotope. Experimental studies with various species suggest that polychaete worms are quite tolerant of heavy metals, whereas metal-contaminated sediments can exert a toxic effect on burrowing bivalves and echinoderms, especially at larval stages (Bryan, 1984). Possible suggested effects of this toxicity were reduced growth, abundance and abnormalities in areas of heavy pollution. Kanakaraju et al. (2008) found that, of the heavy metal contents analysed (Pb, Fe, Zn, Cu, Cd and Mn), razor clams of the Solen spp. in Malasya concentrated higher levels of Fe and Zn in their tissues, and Pb and Mn in their shells. These results suggest that razor clams potentially are likely to bioaccumulate toxic heavy-metals from their environment, but no biological effects of this bioaccumulation were suggested.

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

This pressure is Not assessed but evidence is presented where available.

Invertebrate communities respond to severe chronic oil pollution in much the same way. Initial massive mortality and lowered community diversity is followed by extreme fluctuations in populations of opportunistic mobile and sessile fauna (Suchanek, 1993). Oscillations in population numbers slowly dampen over time and diversity slowly increases to original levels. Infaunal communities, such as those characterizing this biotope are highly likely to be adversely affected by an event of oil pollution, but the biological effects of accumulation of PAHs are likely to depend on the length of time exposed (Viñas et al., 2009). Sub-lethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates may increase at low concentrations and decrease at high concentrations.

Ensis ensis suffered mass mortality after the Torrey Canyon and Amoco Cadiz oil spills (Southward & Southward, 1978; Southward, 1978; Cabioch et al., 1978). Ensis ensis is also reported to bioconcentrate aromatics and is highly likely to be sensitive to hydrocarbons. Four days after the Sea Empress oil spill, moribund razor shells (mostly Ensis siliqua) were the first organisms observed to have been affected (SEEEC, 1998). Glegg & Rowland (1996) observed dead razor shells washed up on the shore a few days after the final break-up of the Braer wreck. These reports suggest razor shells, such as Phaxas pellucidus have low resistance to oil pollution.

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

This pressure is Not assessed but evidence is presented where available.

High levels of mortality of Ensis spp. resulted from the use of dispersants following the Torrey Canyon oil spill (Smith, 1968). Almost complete mortality of razor shells was found at stations more than a kilometre from the shore at a depth of about 20 m, suggesting Phaxas pellucidus may be vulnerable. Polychaete worms, such as Lagis koreni are generally more resistant of a range of marine pollutants so a change in the faunal composition may be expected if chemical pollution increases. Polluted areas would be characterized by biotopes with lower species diversity and a higher abundance and density of pollution resistant species such as polychaetes.

Radionuclide contamination

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

Evidence

No Evidence is available on which to assess this pressure.

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

This pressure is Not assessed.

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

A number of animals have behavioural strategies to survive periodic events of reduced dissolved oxygen. These include shell closure and reduced metabolic rate in bivalve molluscs and either decreased burrowing depth or emergence from burrows for sediment-dwelling crustaceans, molluscs and annelids. Furthermore, oxygen-deficient marine areas are characterized by a decline in the number and diversity of species.

In experiments with natural sediments, many infaunal species leave the sediment at oxygen concentrations below ca 1 mg/l (12% saturation). After several days at that concentration, the polychaete Lagis koreni (studied as Pectinaria koreni) emerged from the sediment with its tube. Later the tube fell over and the worm died (Diaz & Rosenberg, 1995). These results agree with reports of high mortality associated with periodic oxygen deficiency of bottom waters of Kiel Bay (e.g. Nivhols, 1977); the mortality at some, but not all, stations in the German Bight following a period of hypoxia (that lasted over than one month and oxygen concentrations were as low as 1 mg/l) (Niermann et al., 1990); and reports that the species may favour organic enrichment but is displaced in anoxic sediments (e.g. Pearson & Rosenberg, 1978). Finally, a review of the thresholds of hypoxia for marine biodiversity by Vaquer-Sunyer & Duarte (2008) supports the evidence that the sub-lethal oxygen threshold for Lagis koreni (studies as Pectinaria koreni) was 1 mg/l (Nilsson & Rosenberg, 1994 cited in Vaquer-Sunyer & Duarte, 2008).

Phaxas pellucidus was absent during the period of stagnation influenced by the severe oxygen depletion in the water column in the northern Adriatic Sea (Nerlović et al., 2011). Rosenberg et al. (1991) exposed benthic species from the NE Atlantic to oxygen concentrations of around 1 mg/l for several weeks, including species of small bivalves. After 11 days in hypoxic conditions, bivalve individuals were still alive, although individuals stretched their syphons out of the sediment.

Sensitivity assessment. Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l. Different species in the biotope will have varying responses to deoxygenation. Based on the evidence presented, a decrease in oxygenation at the pressure benchmark level is likely to result in significant (25-75%) mortality of the characterizing species of this biotope. With the loss of these species, the biotope would likely be lost. Community composition would likely become dominated by fewer species that are resistant to hypoxic conditions, such as some polychaete worms so the overall species richness would decline significantly. Resistance is therefore assessed as 'Low' and resilience as 'High', and sensitivity is assessed as 'Low'.

Nutrient enrichment

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

Evidence

Increased nutrients are most likely to affect the abundance of phytoplankton, which may include toxic algae (OSPAR, 2009). This primary effect resulting from elevated nutrients will impact upon other biological elements or features (e.g. toxins produced by phytoplankton blooms or deoxygenation of sediments) and may lead to ‘undesirable disturbance’ to the structure and functioning of the ecosystem. With enhanced primary productivity in the water column, organic detritus that falls to the seabed may also be enhanced, which may be utilized by the deposit feeders in the community.

Sensitivity assessment. The community, and hence the biotope, may change to one dominated by nutrient enrichment-resistant species, in particular polychaete worms. Kröncke (1990) postulated that the increase in opportunistic species, on the Dogger Bank between 1951 and 1987 may be due to eutrophication. However, there is not enough evidence, at present, to assess the sensitivity of this biotope at the benchmark level. 

Organic enrichment

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

Evidence

Dense populations of Lagis koreni characteristically occur in organically-rich inshore sediments, e.g. off estuary mouths, suggesting that the species may favour organic enrichment (Pearson & Rosenberg, 1978, cited in Rees & Dare, 1993). However, Gordon (1966, cited in Nicolaidou, 1988) found that sediment reworking was inversely related to the organic carbon concentration of the sediment.

Borja et al. (2000) assigned Lagis koreni and Phaxas pellucidus to their Group I – ‘species very sensitive to organic enrichment and present under unpolluted conditions (initial state)’, whereas Gittenberger & Van Loon (2011) assigned Lagis koreni to Ecological Group III ‘species tolerant to excess organic matter enrichment. These species may occur under normal conditions, but their populations are stimulated by organic enrichment (slight unbalance situations)’, but gave no information concerning Phaxas pellucidus.  Although no further specific information regarding the response of Phaxas pellucidus to changes in nutrient levels was found, as filter feeders, the species is likely to benefit from some organic enrichment.

Sensitivity assessment. No direct evidence of the characterizing species’ specific tolerances to organic enrichment was found. It is likely that the characterizing species in the biotope are able to utilize additional organic load as food. However, it is possible that the characterizing species experience decreases in abundance as a result of organic enrichment, which can lead to shifts in community composition towards one dominated by tolerant species, such as polychaete worms (Pearson & Rosenberg, 1978). In addition, Forrest et al. (2009) identified that the recovery of muddy sediments beneath fish farms from enrichment can be highly variable and may be many years at poorly flushed sites, suggesting that the low energy environments that characterize the biotope may allow for prolonged periods of organic load sedimentations enhancing the adverse effects to sensitive characterizing species. Resistance is therefore assessed as 'Low' (loss 25-75%), but with low confidence. Resilience is likely to the 'Medium' and the overall sensitivity of the biotope is assessed as 'Medium'.

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

If the sediment that characterizes the biotope were replaced with rock substrata, this would represent a fundamental change to the physical character of the biotope. The characterizing species would no longer be supported and the biotope would be lost and/or reclassified.

Sensitivity assessment. Resistance to the pressure is considered None, and resilience Very Low, given the permanent nature of this pressure. Sensitivity has been assessed as 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 sediment type)

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

Evidence

Records indicate that SS.SMu.CSaMu.LkorPpel occurs in sandy muds (Connor et al., 2004), with the characterizing species within the biotope likely to occur within due to their narrow sediment preferences, as Lagis koreni typically occurs in sandy muds and muddy sands (Mayhew, 2007), and Phaxas pellucidus seems to have preferences for fine mixed sands (Neish, 2008). Furthermore, Lambert (1991, cited in Olivier et al., 1996) demonstrated that Lagis koreni (studied as Pectinaria koreni) post-larvae efficiently select muddy and muddy-fine sand over clean fine-sand sediment.

Sensitivity assessment. The characterizing species of SS.SMu.CSaMu.LkorPpel are likely to be resistant to a change in one Folk class from, for example, muddy sand to sandy mud (based on the Long, 2006 simplification). However, this would probably represent a fundamental change in the character of the biotope, and a change in the abundance of the characteristic species, resulting in the loss and/or reclassification of the biotope. Resistance is therefore assessed as None and resilience as Very Low, given the permanent nature of this pressure. The biotope is therefore considered to have High sensitivity to a change in seabed type by one Folk class.

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

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation, expose underlying sediment, which may be anoxic and/or of a different character, and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment / water interface, may be exposed to unsuitable conditions. Newell et al. (1998) state that the removal of 0.5 m depth of sediment is likely to eliminate benthos from the affected area. Some epifaunal and swimming species may be able to avoid this pressure. Removal of 30 cm of sediment is considered to remove species that occur at the surface and within the upper layers of sediment, such as the characterizing species of this biotope. For example, Lagis koreni inhabits the top 10 cm of the sediment (Mayhew, 2007) and were incapable of reconstructing their delicate sand tubes once removed from them, resulting in mortality (Schäfer, 1972). No evidence was found on the depth of burial for Phaxas pellucidus. Although razor clams are able to burrow rapidly into sediments making them difficult to capture, their short siphons indicate that their usual position in the sediment is close to the surface. However, even with this mobility, it is assumed that this species is unlikely to escape extraction of substratum to 30 cm. This environmental position, together with shell fragility, is likely to render the species vulnerable and result in a small proportion of the population would be damaged and killed.

Recovery of the sedimentary habitat would occur via infilling, although some recovery of the biological assemblage may take place before the original topography is restored if the exposed, underlying sediments are similar to those that were removed. Newell et al. (1998) indicate that local hydrodynamics (currents and wave action) and sediment characteristics (mobility and supply) strongly influence the recovery of soft sediment habitats.

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotope so resistance is assessed as Low. SS.SMu.CSaMu.LkorPpel occurs in low-energy environments, so resilience is therefore judged as 'Medium' (see resilience section) based on the time taken for the sediment to recover. Hence, sensitivity has been assessed as 'Medium'.

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

The characterizing species of SS.SMu.CSaMu.LkorPpel are infaunal and hence have some protection against surface abrasion only. However, they require contact with the surface for respiration and feeding, so siphons and delicate polychaete feeding structures may be damaged or withdrawn as a result of surface disturbance, resulting in loss of feeding opportunities and compromised growth.

As a deposit-feeder, Lagis koreni is motile while feeding within the surface layer of sediments. But adults are incapable of reconstructing (delicate) sand tubes once removed from them. Hence, mortality following any damage from trawl/tickler chain disturbance is likely to be very high (Schäfer, 1972, cited in Rees & Dare, 1993). It is likely the species is more vulnerable after dark, as the species is more active feeding on the sediment surface in darkness (Nicolaidou, 1988).

Reiss et al. (2009) found that higher intensities of trawling were related to lower levels of production of the larger infauna, while there was no significant relationship of production with fishing intensity for the smallest size fraction. However, r-selected species, such as characterizing polychaete Lagis koreni, occurred in highest abundances in the heavily trawled areas. These opportunistic species may indeed benefit from fishing disturbance due to their ability to respond to favourable conditions created by disturbance events by quickly using additional resources such as space and food (Pearson & Rosenberg, 1978; Warwick, 1986).

No evidence was found on the depth of burial for Phaxas pellucidus. Razor clams are able to burrow rapidly into sediments making them difficult to capture, although their short siphons indicate that their usual position in the sediment is close to the surface. Due to this mobility, it is assumed that this species could escape from surface abrasion. Bivalves such as Phaxas pellucidus, together with starfish have been reported to be relatively resistant to trawling (Bergman & Van Santbrink, 2000).

Hiddink et al. (2006) reported direct mortality of up to 31% of Lagis koreni (studied as Pectinaria koreni) caused by a single passage of a 4 m and 12 m trawl on sandy and silty sediments, and for Phaxas pellucidus of 27% (12 m beam trawl with ticklers), 29% and 33% (4 m beam trawl fitted with ticklers in silty and sandy sediments respectively), and 32% (otter trawl). The authors also noted that higher mortality occurs in silty areas compared to sandy areas, reflecting a deeper penetration of beam trawls into a softer seabed. Additionally, Ball et al. (2000b) found that Phaxas pellucidus was present at a wreck site that prevented fishing disturbance but was absent from adjacent Nephrops trawling grounds, indicating that this species may be sensitive to fishing impacts. Duineveld et al. (2007) also found abundances of Phaxas pellucidus and other fragile bivalves were higher in areas where fishing was excluded. Examination of historical and recent samples suggest that the spatial presence of  Phaxas pellucidus in the North Sea has more than halved in comparison with the number of ICES rectangles in which they were sampled at the beginning of the century, apparently in response to fishing effort (Callaway et al., 2007).

In the event of damage caused to the characterizing species as a result of this pressure, damaged or undamaged animals are likely to experience increased predation pressure, particularly Lagis koreni, which is a significant food source for commercially important demersal fish, especially dab and plaice (Macer; 1967; Lockwood, 1980; Basimi & Grove, 1985). Peer (1970) estimated that about 80% of mortality of the related species Pectinaria hyperborean was due to predation in Canadian waters.

Furthermore, SS.SMu.CSaMu.LkorPpel occurs in sandy muds (Connor et al., 2004). Abrasion events caused by passing fishing gear, or scour by objects on the seabed surface are likely to have marked impacts on the substratum and cause turbulent resuspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards from digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see 'change in suspended solids' and 'smothering' pressures). The effects of trawling on infauna are greater in areas with low levels of natural disturbance compared to areas of high natural disturbance (e.g. Hiddink et al., 2006). In a meta-analysis of the impacts of different fishing activities on the benthic biota of different habitats, muddy sands were found to be vulnerable to the impacts of fishing activities, with recovery times predicted to take years (Kaiser et al., 2006). The long recovery time for muddy sands is due to the fact that these habitats are mediated by a combination of physical, chemical and biological processes (compared to sand habitats which are dominated by physical processes and recovery time takes days to months).

Sensitivity assessment. The characterizing species live infaunally and are considered to have some protection against surface disturbance. However, the evidence presented suggests that soft-bodied organisms and fragile shells are likely to be damaged and removed by abrasion. Resistance to abrasion is therefore considered Low. The resilience of the biotope is likely to be 'High'. The biotope is therefore considered to have 'Low' sensitivity to abrasion or disturbance of the surface of the seabed.

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

Activities that disturb the surface and penetrate below the surface would remove/damage infaunal species such as the characterizing species within the direct area of impact. The footprint of the impact will depend on the type of gear used (Hall et al., 2008).

Lagis koreni adults are incapable of reconstructing (delicate) sand tubes once removed from them, hence mortality following any damage from trawl/tickler chain disturbance is likely to be very high (Schäfer, 1972, cited in Rees & Dare, 1993). Hiddink et al. (2006) reported direct mortality of up to 31% of Lagis koreni (studied as Pectinaria koreni) caused by a single passage of a 4 m and 12 m trawl on sandy and silty sediments, and for Phaxas pellucidus of 27% (12 m beam trawl with ticklers), 29% and 33% (4 m beam trawl fitted with ticklers in silty and sandy sediments respectively), and 32% (otter trawl). The authors also noted that higher mortality occurs in silty areas compared to sandy areas, reflecting a deeper penetration of beam trawls into a softer seabed. Additionally, Ball et al. (2000b) found that Phaxas pellucidus was present at a wreck site that prevented fishing disturbance but was absent from adjacent Nephrops trawling grounds, indicating that this species may be sensitive to fishing impacts. Duineveld et al. (2007) also found abundances of Phaxas pellucidus and other fragile bivalves were higher in areas where fishing was excluded. Examination of historical and recent samples suggest that the spatial presence of  Phaxas pellucidus in the North Sea has more than halved in comparison with the number of ICES rectangles in which they were sampled at the beginning of the century, apparently in response to fishing effort (Callaway et al., 2007). Eleftheriou & Robertson (1992) observed large numbers of Ensis ensis killed or damaged by dredging operations and Gaspar et al. (1998) reported high levels of damage in Ensis siliqua from fishing.

Furthermore, penetrative events caused by passing fishing gear are also likely to have marked impacts on the substratum and cause turbulent re-suspension of surface sediments. When used over fine muddy sediments, trawls are often fitted with shoes designed to prevent the boards from digging too far into the sediment (M.J. Kaiser, pers. obs., cited in Jennings & Kaiser, 1998). Trawling can create suspended sediment plumes up to 10 m above the bottom (Churchill, 1989 cited in Clarke & Wilber, 2000). The effects may persist for variable lengths of time depending on tidal strength and currents and may result in a loss of biological organization and reduce species richness (Hall, 1994; Bergman & Van Santbrink, 2000; Reiss et al., 2009) (see 'change in suspended solids' and 'smothering' pressures). A meta-analysis of over 100 experimental fishing impact studies showed that beam trawling, scallop dredging and otter trawling all had significant short-term impacts in muddy sand habitats, with the most severe effect on suspension feeders (Kaiser et al., 2006). Jennings et al. (2001) found that trawling in the muddy sand region led to significant decreases in infaunal biomass and production in the North Sea, with the abundance of larger individuals depleted more than smaller ones.

Sensitivity assessment. A large proportion of the characterizing species in the biotope is likely to be lost or severely damaged, depending on the scale of the activity (see abrasion pressure). Therefore, a resistance of Low is suggested. Muddy sand habitats have been reported as having the longest recovery times, whilst mud habitats had an ‘intermediate’ recovery time (compared to clean sand communities which had the most rapid recovery rate) (Dernie et al., 2003). Resilience is probably 'High', and therefore the biotope’s sensitivity to this pressure is likely to be 'Low'.

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 suspended sediment and siltation rate (resulting from changes in the hydrographic regime, run-off from the land or coastal construction) are likely to result in changes in the sediment composition of the surface layers and hence the communities present. Increased suspended sediment may lead to decreased light penetration, possible clogging of feeding organs of suspension feeders such as Phaxas pellucidus, and the possibility of smothering whole organisms (see smothering pressure). However, community members of this biotope live beneath the sediment surface or are mobile, and are unlikely to be directly exposed to changes in suspended solids. An increase in turbidity, and reduced light availability will reduce primary production in the biotope. However, the majority of productivity in SS.SMu.CSaMu.LkorPpel is secondary (detritus) and so is not likely to be significantly affected by changes in turbidity. Nevertheless, primary production by pelagic phytoplankton and microphytobenthos do contribute to benthic communities and long-term increases in turbidity may reduce the overall organic input to the detritus, which are the main source of food for characterizing species Lagis koreni. For most benthic deposit feeders, food is suggested to be a limiting factor for populations (Levington, 1979; Hargrave, 1980). Consequently, an increase in suspended particulates and subsequent increased deposition of organic matter in sheltered environments where sediments have high mud content will increase food resources to deposit feeders.

Buchanan & Moore (1986) found that a decline in quantities of organic matter changed the infauna of a deposit-feeding community, which is essentially food limited. This may lead to a shift in community structure with an increased abundance of deposit feeders and a lower proportion of suspension feeders (as feeding is inhibited where suspended particulates are high and the sediment is destabilised by the activities of deposit feeders) (Rhoads & Young, 1970).

An increase in suspended solids in the water can considerably reduce the quantity of dissolved oxygen, as well as increase the production of mucus to protect the gills from clogging, which consequently can impair the metabolism of filter-feeding bivalves, such as Phaxas pellucidus (Moore, 1977). A decrease in siltation may equally affect growth and fecundity if the supply of organic particulate matter declines, given that the particles taken are not discriminated upon nutritional value (Moore, 1977 and references therein). According to Widdows et al. (1979) growth of filter-feeding bivalves may be impaired at suspended particulate matter (SPM) concentrations >250 mg/l. The dominance of Phaxas pellucidus in areas subject to dredge soil dumping and subsequent further deposition (Rees et al., 1992) suggest that this species would not be sensitive to increased turbidity, to either increased seston or subsequent deposition following re-suspension of sediments.

Sensitivity assessment. Resistance and resilience of the biotope are assessed as High, so the biotope is considered Not Sensitive to a change in suspended solids at the pressure 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

No direct evidence concerning the tolerance of the biotope or the characterizing species to overburden was found. However, the biotope is characterized by burrowing species such as Lagis koreni and Phaxas pellucidus that are likely to be able to burrow upwards and therefore unlikely to be adversely affected by smothering by 5 cm sediment. For example, adults of Lagis koreni are capable of upwardly migrating if lightly buried by additional sediment (Schäfer, 1972 cited in Rees & Dare, 1993).

Hinchey et al. (2006) investigated the responses of estuarine benthic invertebrates to sediment burial and concluded that species-specific responses to burial varied as a function of motility, living position, and inferred physiological tolerance of anoxic conditions while buried. Although the characterizing species were not included in the study, increased overburden stress did not significantly decrease survival and growth of the juvenile bivalve, Macoma balthica, but significantly caused the decline of survival of the studied juvenile polychaete, Streblospio benedicti. The depth of sediment deposited varied between 0-24.6 cm and 0-8.4 cm, respectively. 

Furthermore, a study of the ecological effects of dumping dredged sediments by Essink (1999) reported that the resistance of mobile macrobenthos varied greatly with species. For polychaetes, the author reported tolerances of up to 50 cm of mud for species such as Nepthys and Nereis, and up to 80 cm of sand. For bivalves in the subtidal, Ensis spp. were reported to survive up to 50 cm of both mud and sand, but no further information was available on the rates of survivorship or the time taken to reach the surface.

Furthermore, Lagis koreni was reported as dominant at a dredged-material ground in Liverpool Bay, probably because of the species' opportunistic life cycle (Whomerslwey et al., 2008). Similarly, Rees et al. (1992, from Connor et al., 2004) report that Phaxas pellucidus can become dominant in areas where dredge spoil is dumped. However, it is not clear whether this relates to vertical migration and survivability or an increase in habitat suitability enhancing post-dredging colonization (as seems more likely).

Rees et al. (1992) reported that the biotopes SS.SSa.CMuSa.AalbNuc and SS.SMu.CSaMu.LkorPpel display cyclical behaviour in the Liverpool Bay area, with the community periodically switching from one biotope to the other, and it was suggested this was possibly in relation to the disposal of dredge spoil (Robinson et al., 2001 cited in Rees et al., 1992).

The character of the overburden is an important factor in determining the degree of vertical migration. Individuals are more likely to escape from a covering similar to the sediments in which species are normally found. The biotope occurs in exposed and moderately exposed wave exposure conditions, and a range of tidal streams from very weak to strong (Connor et al., 2004). Dispersion of fine sediments may be rapid, and this could mitigate the magnitude of this pressure by reducing the time exposed, as ‘light’ deposition of sediments is likely to be cleared in a few tidal cycles in areas of higher water flow.

Sensitivity assessment. The evidence suggests that characterizing species Lagis koreni and Phaxas pellucidus are likely to be able to burrow through, although sudden smothering would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on the burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). However, the biotope is likely to resist smothering at the benchmark level. Resistance is therefore assessed as High, and resilience is also High (by default) so that the biotope is considered 'Not Sensitive' to a ‘light’ deposition of up to 5 cm of fine material added to the seabed in a single, discrete event. 

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

No direct evidence concerning the tolerance of the biotope or the characterizing species to overburden was found. However, the biotope is characterized by burrowing species such as Lagis koreni and Phaxas pellucidus that are likely to be able to burrow upwards through deposited material. For example, adults of Lagis koreni are capable of upwardly migrating if lightly buried by additional sediment (Schäfer, 1972 cited in Rees & Dare, 1993).

Hinchey et al. (2006) investigated the responses of estuarine benthic invertebrates to sediment burial and concluded that species-specific responses to burial varied as a function of motility, living position, and inferred physiological tolerance of anoxic conditions while buried. Although the characterizing species were not included in the study, increased overburden stress did not significantly decrease survival and growth of the juvenile bivalve, Macoma balthica, but significantly caused the decline of survival of the studied juvenile polychaete, Streblospio benedicti. The depth of sediment deposited varied between 0-24.6 cm and 0-8.4 cm, respectively. 

Furthermore, a study of the ecological effects of dumping dredged sediments by Essink (1999) reported that the resistance of mobile macrobenthos varied greatly with species. For polychaetes, the author reported tolerances of up to 50 cm of mud for species such as Nepthys and Nereis, and up to 80 cm of sand. For bivalves in the subtidal, Ensis spp. were reported to survive up to 50 cm of both mud and sand, but no further information was available on the rates of survivorship or the time taken to reach the surface.

Furthermore, Lagis koreni was reported as dominant at a dredged-material ground in Liverpool Bay, probably because of the species' opportunistic life cycle (Whomerslwey et al., 2008). Similarly, Rees et al. (1992, from Connor et al., 2004) report that Phaxas pellucidus can become dominant in areas where dredge spoil is dumped. However, it is not clear whether this relates to vertical migration and survivability or an increase in habitat suitability enhancing post-dredging colonization (as seems more likely).

Rees et al. (1992) reported that the biotopes SS.SSa.CMuSa.AalbNuc and SS.SMu.CSaMu.LkorPpel display cyclical behaviour in the Liverpool Bay area, with the community periodically switching from one biotope to the other, and it was suggested this was possibly in relation to the disposal of dredge spoil (Robinson et al., 2001 cited in Rees et al., 1992).

The character of the overburden is an important factor in determining the degree of vertical migration. Individuals are more likely to escape from a covering similar to the sediments in which species are normally found. The biotope occurs in exposed and moderately exposed wave exposure conditions, and a range of tidal streams from very weak to strong (Connor et al., 2004). Dispersion of fine sediments following a ‘heavy’ deposition of sediments will likely need a few tidal cycles to clear, enhancing the magnitude of exposure to this pressure.

Sensitivity assessment. The evidence suggests that characterizing species Lagis koreni and Phaxas pellucidus are likely to be able to burrow through, although sudden smothering would temporarily halt feeding and respiration, compromising growth and reproduction owing to energetic expenditure. Beyond re-establishing burrow openings or moving up through the sediment, there is evidence of synergistic effects on the burrowing activity of marine benthos and mortality with changes in time of burial, sediment depth, sediment type and temperature (Maurer et al., 1986). Bivalve and polychaete species have been reported to migrate through depositions of sediment greater that the benchmark (30 cm of fine material added to the seabed in a single discrete event) (Bijkerk, 1988; Powilleit et al., 2009; Maurer et al., 1982). However, it is not clear whether the characterizing species are likely to be able to migrate through a maximum thickness of fine sediment because muds tend to be more cohesive and compacted than sand. Some mortality of the characterizing species is likely to occur. Resistance is therefore assessed as Low (25-75% loss) and resilience as 'High' and the biotope is considered to have 'Low' sensitivity to a ‘heavy’ deposition of up to 30 cm of fine material in a single discrete event.

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 is available on which to assess this pressure.

Underwater noise changes

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

Evidence

Species in the biotope may respond to vibrations from predators or excavation by retracting their palps into their tubes or by burrowing deeper into the sediment. However, the characterizing species are unlikely to be affected by noise pollution and so the biotope is assessed as 'Not sensitive'.

Introduction of light or shading

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

Evidence

Lagis koreni can sense light and react to it, showing to be governed by an endogenous diurnal rhythm (Nicolaidou, 1988). It is likely the species is more active in darkness to decrease the possibility of being preyed upon, since some of its predators, such as plaice, flounder and dab are all predominantly visual feeders (De Groot, 1971). A change in the incidence of lighting that results in continuous lighting may affect the growth and survival of Lagis koreni, by limiting feeding opportunities and increasing the risk of predation by visual predators. However, Peer (1970) estimated that about 80% of mortality of the related species Pectinaria hyperborean was due to predation in Canadian waters, demonstrating that the species is already able to cope with high levels of predation. It is therefore unlikely that an increase would impact the community. Additionally, SS.SMu.CSaMu.LkorPpel is a circalittoral biotope (JNCC, 2022), not characterized by the presence of primary producers and therefore, not directly dependent on sunlight.

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' to biotopes restricted to open waters.

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

Lagis koreni (studied as Pectinaria koreni) can sense light and react to it, showing more activity in darkness to avoid visual predators (Nicolaidou, 1988). However, no evidence was found of the species demonstrating a defence mechanism which could be triggered by visual disturbance. In addition, both the characterizing species of the biotope live infaunally, so are unlikely to be affected by visual disturbance such as shading.

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

The important characterizing species in the biotope are not cultivated or likely to be translocated. This pressure is therefore considered Not Relevant.

Introduction or spread of invasive non-indigenous species

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

Evidence

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., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40) although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. 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 in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

High densities of Crepidula fornicata cause ecological impacts on sedimentary habitats. The species can form dense carpets that can smother the seabed in shallow bays, changing and modifying the habitat structure. At high densities, the species physically smothers the sediment, and the resultant build-up of silt, pseudofaeces, and faeces is deposited and trapped within the bed (Tillin et al., 2020, Fitzgerald, 2007, Blanchard, 2009, Stiger-Pouvreau & Thouzeau, 2015). The biodeposition rates of Crepidula are extremely high and once deposited, form an anoxic mud, making the environment suitable for other species, including most infauna (Stiger-Pouvreau & Thouzeau, 2015, Blanchard, 2009). For example, in fine sands, the community is replaced by a reef of slipper limpets, that provide hard substrata for sessile suspension-feeders (e.g., sea squirts, tube worms and fixed shellfish), while mobile carnivorous microfauna occupy species between or within shells, resulting in a homogeneous Crepidula dominated habitat (Blanchard, 2009). Blanchard (2009) suggested the transition occurred and became irreversible at 50% cover of the limpet. De Montaudouin et al. (2018) suggested that homogenization occurred above a threshold of 20-50 Crepidula /m². 

Impacts on the structure of benthic communities will depend on the type of habitat that Crepidula colonizes. De Montaudouin & Sauriau (1999) reported that in muddy sediment dominated by deposit-feeders, species richness, abundance and biomass increased in the presence of high densities of Crepidula (ca 562 to 4772 ind./m²), in the Bay of Marennes-Oléron, presumably because the Crepidula bed provided hard substrata in an otherwise sedimentary habitat. In medium sands, Crepidula density was moderate (330-1300 ind./m²) but there was no significant difference between communities in the presence of Crepidula. Intertidal coarse sediment was less suitable for Crepidula with only moderate or low abundances (11 ind./m²) and its presence did not affect the abundance or diversity of macrofauna. However, there was a higher abundance of suspension–feeders and mobile Crustacea in the absence of Crepidula (De Montaudouin & Sauriau, 1999). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm) but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action which may be less suitable (Tillin et al., 2020). 

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile and unsuitable for most of the invasive non-indigenous species currently recorded in the UK. However, the above evidence suggests that Crepidula fornicata could colonize sandy mud habitats in the subtidal, typical of this biotope, where shell and gravel occur, but in low densities due to the mobility of the substrata. Powell-Jennings & Calloway (2018) noted that Crepidula is killed by sudden burial and this could mitigate high densities of Crepidula. In addition, this habitat is wave exposed to moderately wave exposed, so storms may mobilise the sediment (JNCC, 2022), which may also mitigate or prevent colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). Crepidula fornicata has the potential to colonize this habitat, especially where water movement is meditated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. 

Therefore, resistance is assessed as 'Medium' as colonization may be limited to low densities due to wave action or examples of the biotope subject to spoil deposition, which could result in potential burial. Resilience is assessed as 'Very low' as it would require the removal of Crepidula, probably by artificial means. Hence, sensitivity is assessed as 'Medium' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope so the confidence in the assessment is 'Low' and further evidence is required. 

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

Zoogonoides viviparous is a helminth parasite known to use Lagis koreni as a host (Peoples, 2013). Although polychaetes have been shown to combat parasitism with immune responses, details of the physiological tool of this infection were not provided.

More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may, in some instances, cause death (Dame, 1996). However, no information specifically concerning the effects of microbial pathogens and parasites on the viability of the characterizing species was found.

Sensitivity assessment. No direct evidence of the biotope being affected by the introduction of microbial pathogens was found with which to assess this pressure.

Removal of target species

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

Evidence

No obligate life-history or ecological associations were identified between this ecological group and currently targeted species although removal of predators may be beneficial for Lagis koreni, which is a significant food-source for commercially important demersal fish, especially dab and plaice (e.g Macer, 1967; Lockwood, 1980; Basimi & Grove, 1985). However, it is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. This pressure is therefore considered Not Relevant.

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

Direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures, while this pressure considers the ecological or biological effects of by-catch. Species in these biotopes, including the characterizing species, may be damaged or directly removed by static or mobile gears that are targeting other species (see abrasion and penetration pressures). Loss of these species would alter the character of the biotope resulting in re-classification and would alter the physical structure of the habitat resulting in the loss of the ecosystem functions such as secondary production performed by these species.

Sensitivity assessment. Removal of the characterizing species would result in the biotope being lost or reclassified. Thus, the biotope is considered to have a resistance of 'Low' to this pressure.  Resilience is assessed as 'High' and sensitivity as 'Low' but with 'Low' confidence.