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

In a review of the ecology of hydroids, Gili & Hughes, (1995) report that temperature is a critical factor stimulating or preventing reproduction and that most species have an optimal temperature for reproduction. However, limited evidence for thermal thresholds and thermal ranges were available for the characterizing species recorded in this biotope.  Berrill (1949) reported that growth in Obelia commissularis (syn. dichotoma) was temperature dependant but ceased at 27°C.  Hydranths did not start to develop unless the temperature was less than 20°C and any hydranths under development would complete their development and rapidly regress at ca 25°C.  Berrill (1948) reported that Obelia species were absent from a buoy in July and August during excessively high summer temperatures in Booth Bay Harbour, Maine, USA.  Berrill (1948) reported that the abundance of Obelia species and other hydroids fluctuated greatly, disappearing and reappearing as temperatures rose and fell markedly above and below 20°C during this period.  The upwelling of cold water (8-10°C colder than surface water) allowed colonies of Obelia sp. to form in large numbers.  Cantero et al. (2002) and refs. thererin describe the presence and year-round fertility of Obelia dichotoma, Kirchenpaureria pinnata, Nemertesia ramosa and Halecium spp.in the Mediterranean, indicating probable tolerance to temperature increases at the benchmark figure.  Ciona intestinalis is considered a cold water or temperate species but has been found as far south as Cape Verde, although these tropical populations are likely transitory (Monniot & Monniot 1994). Temperature tolerance varies among geographical populations or ecotypes. Adult Ciona intestionalis is reported as tolerant of temperatures up to 30°C (Dybern, 1965; Therriault & Herborg, 2008), although Petersen & Riisgard (1992) noted that filtration rates declined above 21°C which suggested thermal stress, and indicated that long-term survival was likely to require temperatures lower than the 30°C.  Other studies also indicated that Ciona intestinalis exhibits a decline in ammonia excretion rate and oxygen consumption rate above 18°C (Zhang and Fang 1999, Zhang et al., 1999).  The effect of higher temperatures on Ascidia mentula is not as well researched.  It is distributed from Norway through to the Mediterranean and Black Sea, and the species appears to tolerate a broad range of temperatures.  Svane (1984) found that in Sweden, whilst lower temperatures decreased recruitment, populations responded positively to the “warm period” of 1972-1976 (Glantz, 2005), with an increase in population density across all sites in the study and a gradual decrease during the ensuing “cold period”, and minor fluctuations throughout.  Unusually high mean temperatures in 1975 did result in higher recruitment, with colder temperatures in January 1976 and spring 1979 coinciding with very little recruitment.  Svane (1984) found that, unlike recruitment, mortality was regulated locally and independent of temperature within the range of the study (mean monthly deviation of ±3°C) (Svane, 1984).  Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Ursin (1960) reported Echinus esculentus occurred at temperatures between 0-18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11-19°C, however, continued exposure to 19°C disrupted gametogenesis. Embryos and larvae developed abnormally after 24 hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998).  Antedon bifida is found from Scotland to Portugal (WoRMS, 2015) so is probably able to tolerate a long-term increase in temperature of 2°C. However, as a subtidal species, Antedon bifida is less likely to be able to tolerate a short-term increase in temperature of 5°C. Antedon petasus has a more northerly range and is therefore thought to be more at risk of temperature increases (Goodwin et al., 2008; Goodwin et al., 2013).  Exposure of laboratory cultures of Antedon petasus to temperatures in excess of 14°C were detrimental or fatal (Gislén, 1924 cited in Khanna, 2005).

Sensitivity assessment.  This biotope occurs in the north west of the UK where sea temperatures vary between 4 and 15°C (Beszczynska-Möller & Dye, 2013) and are typically 9 - 14°C (Huthnance, 2010).  An increase in sea surface temperature of 2°C for a period of 1 year combined with high summer temperatures may approach the upper temperature threshold of Echinus esculentus and would likely affect Antedon petasus, and result in decline in abundance.

Short-term increases in temperatures (i.e. 5°C for a month) may be detrimental for recruitment, or cause mobile echinoderms to move out of the affected area but should not be detrimental to the sessile species.  Any short-term reduction in grazing due to loss of Echinus esculentus is probably also short-term.

Therefore, resistance has been assessed as ‘Medium’, resilience has been assessed as ‘High’ and sensitivity has been assessed as ‘Low’. The effects of increased temperature on the characterizing species are largely well researched, although gaps in the literature for Ascidia mentula result is a confidence rating of Medium.

Temperature decrease (local)

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

Evidence

Orjas et al., 2012 describes studying the feeding ecology of Obelia dichotoma in an Arctic environment (Kongsfjorden, Svalbard) which experiences temperatures of 1-5°C (Beszczynska-Möller & Dye, 2013).  Palerud et al., 2004 also describes the presence in Svalbard of Obelia dichotoma, Halecium Halecinum and Nemertesia sp.  This suggests that these characterizing hydroids are probably tolerant of the lowest temperatures they are likely to encounter in Britain and Ireland of ca 4°C (Beszczynska-Möller & Dye, 2013).  It should be noted that growth rates are reduced at low temperatures. Berrill (1949) reported that for Obelia, stolons grew, under optimal nutritive conditions, at less than 1 mm in 24 hrs at 10-12 °C, 10 mm in 24 hrs at 16-17 °C, and as much as 15-20 mm in 24 hrs at 20 °C.  Tolerance for low temperatures varies among geographical populations. In the Mediterranean, most adult Ciona intestinalis die when temperatures fall below 10°C, and the population is maintained by the survival of younger individuals which are more tolerant of colder temperatures (Marin et al. 1987).  Observation of Scandinavian populations indicated a higher mortality rate of Ciona intestinalis during the coldest period of the year (temperatures down to 1°C) (Dybern, 1965).  In Scandinavian populations, normal egg development requires 8-22°C and larval development occurs between 6-24°C (Dybern, 1965).   Larval temperature tolerances may play a part in successful recruitment in unseasonable temperature fluctuations.  Ciona savigny larvae were found to acclimate to temperature, with embryos collected in the summer dividing normally between 14 - 27°C and embryos collected in the winter dividing normally between 10 - 20°C  (Nomaguchi et al., 1997).  Ascidia mentula is distributed from Norway through to the Mediterranean and Black Sea, and the species appears to tolerate a broad range of temperatures.  Svane (1984) found that in Sweden, whilst lower temperatures (of ±3°C of monthly mean) decreased recruitment, mortality did not significantly increase. Shallow populations (15m) experiencing much greater seasonal variability did exhibit seasonal spawning rather than year-round spawning that occurs in more temperate and deeper populations (Svane, 1984).  Populations responded positively to the “warm period” of 1972-1976 (Glantz, 2005), with an increase in population density across all sites in the study and a gradual decrease during the ensuing “cold period”, with minor fluctuations throughout.  Unusually high mean temperatures in 1975 did result in higher recruitment, with colder temperatures in January 1976 and spring 1979 coinciding with very little recruitment.  Svane (1984) found that, unlike recruitment, mortality was regulated locally and independent of temperature within the range of the study (mean monthly deviation of ±3°C).  Bishop (1985) suggested that Echinus esculentus cannot tolerate high temperatures for prolonged periods due to increased respiration rate and resultant metabolic stress. Ursin (1960) reported Echinus esculentus occurred at temperatures between 0-18°C in Limfjord, Denmark. Bishop (1985) noted that gametogenesis occurred at 11-19°C, however, continued exposure to 19°C disrupted gametogenesis. Embryos and larvae developed abnormally after 24 hr exposure to 15°C but normally at 4, 7 and 11°C (Tyler & Young 1998).  Echinus esculentus has been recorded from the Murmansk Coast, Russia. Due to the high latitude at which Echinus esculentus can occur, it is unlikely to be affected by changes in temperature at the pressure benchmark.  Studies looking at low temperature tolerance of Antedon is lacking with the majority of the literature focusing on high temperature effects.  As these biotopes occur in the mid-range of Antedon bifida’s geographical range and at the high temperature limit of Antedon petasus (Picton, 1993), it is unlikely that a reduction in temperature would negatively impact either Antedon spp. considered in this biotope.

Sensitivity assessment. All species assessed are present in northern/boreal habitats and are unlikely to be affected at the benchmark level. Resistance has been assessed as ‘High’, resilience as ‘High’. Therefore, sensitivity has been assessed as ‘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

Studies on hydroids in general have found that prey capture rates may be affected by salinity and temperature (Gili & Hughes, 1995) although no evidence was found for species that characterize this biotope.  Ciona intestinalis has been classified as euryhaline with a high salinity tolerance range (12-40‰) although it typically occurs in full salinity conditions (>30‰) (Tillin & Tyler-Walters, 2014).  Ciona intestinalis has been found in salinities ranging from 11 to 33 PSU in Sweden, although the same study found that parent acclimation to salinity (high or low) has an overriding and significant effect on larval metamorphic success, independent of parent origins (Renborg, 2014). No information on Ascidia spp. was found.  Echinoderms, including Echinus esculentus and Antedon spp. are generally stenohaline, possess no osmoregulatory organ (Boolootian, 1966) and lack the ability to osmo- and ion-regulate (Stickle & Diehl, 1987). The inability of echinoderms to osmoregulate extracellularly causes body fluid volume to decrease when individuals experience higher external salinity. Over the period of a year, populations are unlikely to survive increased salinity. Echinoderm larvae have a narrow range of salinity tolerance and will develop abnormally and die if exposed to increased salinity. 

Sensitivity assessment

CR.LCR.BrAs.AntAsH is a subtidal full salinity biotope (Connor et al., 2004) and salinity increase to over 40 psu (the benchmark) may adversely impact several members of the community and, in particular, the echinoderms.  Resistance is assessed as ‘None’ and recovery as ‘Medium’ (following restoration of usual salinity). Sensitivity is therefore assessed as ‘Medium’.  In the absence of direct evidence on the effects of hypersaline conditions on the characterizing species, confidence is therefore classed as ‘Low’.

Salinity decrease (local)

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

Evidence

This biotope is recorded in full salinity habitats (Connor et al., 2004).   Little evidence for the characterizing hydroids could be found.   Stebbing, 1981 found that, for the hydroid Campanularia flexuosa, growth was inhabited from 70% seawater(ca 25‰) and that exposure to below 30% seawater (ca 10‰) was lethal after 3 days.   Ciona intestinalis has been classified as euryhaline with a high salinity tolerance range (12-40‰) although it typically occurs in full salinity conditions (>30‰) (Tillin & Tyler-Walters, 2014) but has been found in Scandinavian waters in salinities as low as 11 PSU (Renborg, 2014, Dybern, 1967).  Adult acclimation to salinity was shown to have an overriding and significant effect on larval metamorphic success, independent of parent origins (Renborg, 2014).  ‘Massive die-offs’ of Ciona interstinalis were observed following winter rains in Californian harbours with ‘massive recolonizations usually following in the spring’ (Lambert & Lambert, 1998).  Population collapses of Ciona intestinalis in the Mediterranean have also been reported, and whilst the drivers for these events are not well understood, it has been postulated that low salinity could play a part (Péres, 1943; Caputi et al., 2015).  Oxygen consumption rate has been shown to decline with decreasing salinity and ceased at 19‰ with siphons tightly closed. (Shumway, 1978).  Ascidia mentula is found on the West coast of Norway in salinities greater than 20‰ (Dybern, 1969) and found in a brackish lake in Corsica with a salinity gradient of 6.5 to 18.5 ‰ Cl^- (Verhoeven, 1978).

Echinoderms, including urchins and antedons are generally stenohaline, possess no osmoregulatory organ (Boolootian, 1966) and lack the ability to osmo- and ion-regulate (Stickle & Diehl, 1987). The inability of echinoderms to osmoregulate extracellularly causes body fluid volume to increase when individuals experience lower external salinity. Over the period of a year, populations are unlikely to survive descreased salinity. Echinoderm larvae have a narrow range of salinity tolerance and will develop abnormally and die if exposed to decreased salinity.   At low salinity urchins gain weight, and the epidermis loses its pigment as patches are destroyed; prolonged exposure is fatal (Tillin & Tyler-Walters, 2014).  There is some evidence to suggest Echinus esculentus makes use of intracellular regulation of osmotic pressure due to increased amino acid concentrations. Furthermore Echinus esculentus is found within a number of variable and reduced salinity biotopes, e.g. IR.LIR.KVS.SlatPsaVS (Connor et al., 2004).

Sensitivity assessment.  CR.LCR.BrAs.AntAsH is a subtidal full salinity biotope (Connor et al., 2004) and salinity decrease at the benchmark figure is thought to have some limited impacts on the species of this biotope, most notably the echinoderms.

Resistance is therefore assessed as ‘Medium’ and recovery as ‘Medium’ (following restoration of usual salinity). Therefore, sensitivity is therefore assessed as ‘Medium’.

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 key characterizing hydroids are typically found in places of low to moderate water movement although Hayward & Ryland (1995) note that the abundant communities occur in narrow straits and headlands which may experience high levels of water flow. Hydroids can bend passively with water flow to reduce drag forces to prevent detatchment and enhance feeding (Gili & Hughes, 1995). Hydroid growth form also varies to adapt to prevailing conditions, allowing species to occur in a variety of habitats (Gili & Hughes, 1995). Hiscock, (1979) assessed feeding behaviour of the hydroid Tubularia indivisa in response to different flow rates.  At flow rates <0.05 m/s, polyps actively moved tentacles.  Increasing the flow rate to 0.2 m/s increased capture rates but at higher flow rates from 0.5-0.9 m/s the tentacles were extended downcurrent and pushed together and feeding efficiency was reduced. In general, flow rates are an important factor for feeding in hydroids and prey capture appears to be higher in more turbulent conditions that prevent self-shading by the colony (Gili & Hughes, 1995). The capture rate of zooplankton by hydroids is correlated with prey abundance (Gili & Hughes, 1995), thus prey availability can compensate for sub-optimal flow rates. Water movements are also important to hydroids to prevent siltation which can cause death (Round, 1961). Tillin & Tyler-Walters (2014) suggest that the range of flow speeds experienced by biotopes in which hydroids are found indicate that a change (increase or decrease) in the maximum water flow experienced by mid-range populations for the short periods of peak spring tide flow would not have negative effects on this ecological group. 

As sessile filter feeders, ascidians generally require a reasonable water flow rate in order to ensure sufficient food availability.  It was shown that in stagnant water, phytoplankton density became reduced in a 20-30 cm layer immediately above a dense colony of Ciona intestinalis (Riisgård et al., 1996).  However, Ciona intestinalis has been recognised as tolerant of low water flow environments which it uses as a competitive advantage in areas with minimal water exchange and renewal such as harbours, marinas and docks, outcompeting other species (Carver et al., 2006).  Whilst Ciona intestinalis is typically found in areas of low flow, it can reportedly withstand flow rates up to 3 knots (Jackson 2008). If dislodged, juveniles and adults have a limited capability to re-attach, given calm conditions and prolonged contact with the new substrata (Carver et al., 2006; Jackson, 2008; Millar 1971).  Hiscock (1983) found that, for the solitary ascidian Ascidia mentula, siphons closed when current velocity rose above about 0.15 m/s.  Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec. Outside the beds specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec. Echinus esculentus are also displaced by storm action. After disturbance Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979). Therefore, increased water flow may remove the population from the affected area, probably to deeper water; however individuals would probably not be killed in the process and could recolonize the area quickly.  Antedon spp. occur in areas where there is fast current flow suitable for passive suspension feeding. In a series of unpublished experiments by Hannan (cited in Hiscock, 1983) Antedon bifida was able to maintain its grip on a concrete substratum in currents up to 90 cm/s in a flume. The claw-like cirri enable the species to survive in moderately strong currents and dense populations are found in areas where water flow is likely to be between 1 and 3 knots (ca 0.5 - 1.5 m/s) during maximal tidal flow.  La Touche (1978) observed that Antedon bifida was unable to maintain its arms in a vertical feeding position in all but the weakest of currents.

Sensitivity assessment. The CR.LCR.BrAs.AntAsH biotope is typical of low energy environments, and occurs on rocky substrata with little tidal flow, and weak to very weak tidal flow (Connor et al., 2004).  Therefore, a decrease of 0.1-0.2 m/s is unlikely to be significant.  Mobile Echinus esculentus and Antedon spp. occur in biotopes with higher flow (e.g. BrAs.AmenCio) and are unlikely to be killed should they become dislodged and may recolonize quickly.  Ciona intestinalis has been shown to withstand flow rates up to the upper limit of the biotope (Jackson, 2008).  An increase in flow may reduce grazing pressure and allow other suspension feeders to colonize the biotope or increase in abundance.  However, it is unlikely that a change of 0.1-0.2 m/s would be significant.

Therefore, Resistance has been assessed as ‘High’, resilience has been assessed as ‘High’, and sensitivity has been assessed as ‘Not Sensitive’.

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

Changes in emergence are not relevant to this biotope as it is 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

Jackson (2004) reported that Nemertesia ramosa was intolerant of high wave exposure and was only found in sheltered areas.   Faucci et al. (2000) recorded hydroid communities at two sites of different wave exposure and recorded the presence of Obelia dochotoma and Halecium spp. in both the exposed and sheltered sites, but only found Kirchenpaueria sp. in the sheltered site.  High energy wave action can be detrimental to ascidian populations. This is mainly through physical damage to the sea squirts and through the abrasive action of suspended sediment (Jackson, 2008).  Ciona intestinalis is often dominant in highly sheltered areas such as harbours (Carver et al., 2006). Decreases in wave exposure are unlikely to have any effect.  If dislodged, juvenile and adult Ciona intestinalis have a limited capability to re-attach, given calm conditions and prolonged contact with the new substratum (Carver et al., 2006; Jackson 2008; Millar 1971) but increases in wave exposure above moderately exposed are likely to cause a proportion of the population to die, especially in the shallower examples of the biotope if the cobbles and pebbles on which the biotope occurs are mobilized by wave action.  Ascidia mentula has rarely been recorded at depths shallower than 15 m (Svane, 1984), it is possible that damage could occur if subjected to increased wave exposure.  Echinus esculentus occurred in kelp beds on the west coast of Scotland in currents of about 0.5 m/sec. Outside the beds, specimens were occasionally seen being rolled by the current (Comely & Ansell, 1988), which may have been up to 1.4 m/sec. Urchins are removed from the stipe of kelps by wave and current action. Echinus esculentus are also displaced by storm action. After disturbance, Echinus esculentus migrates up the shore, an adaptation to being washed to deeper water by wave action (Lewis & Nichols, 1979). Keith Hiscock (pers. comm.) reported Echinus esculentus occurred in significant numbers as shallow as 15 m below low water at the extremely wave exposed site of Rockall, Scotland. Therefore, localised increases in wave height may remove the population from the affected area; probably to deeper water although individuals would probably not be killed in the process and could recolonize the area quickly.  Antedon bifida and Antedon petasus are not generally found in areas subject to wave exposure stronger than ‘exposed’ or tidal streams stronger than ‘moderately strong’ (1-3 kn) (Connor et al., 2004). Studies have noted that the upper limit of Antedon bifida has been limited in some cases by excessive wave action in shallower habitats (8-9 m) (La Touche, 1978).  Being mobile, Antedon spp. should recover quite quickly following a short-term event.

Sensitivity assessment.

Whilst Ciona intestinalis and Echinus esculentus are thought to be quite resistant to wave exposure, some of the hydroids, Asidia mentula and Antedon spp. are considered to be more at risk of damage and mortality when subject to excessive wave exposure. CR.LCR.BrAs.AntAsH occurs in sheltered to extremely sheltered conditions, a 3-5% change in significant wave height is unlikely to be significant.  Therefore resistance has been assessed as ‘High’, resilience has been assessed as ‘High’ and sensitivity has been 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

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

Although no information on the effects of heavy metals on the assessed hydroids was found, evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination. Various heavy metals have been shown to have sublethal effects on growth in the few hydroids studied experimentally (Bryan, 1984). Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1976) reported that 1 µg/l Hg²⁺ was stimulatory, although the effect was transitory, exposure resulting in reduced growth towards the end of his 11 day experiments. Cadmium (Cd) was reported to cause irreversible retraction of 50% of hydranths in Laomedea loveni after 7 days exposure at concentrations between 3 µg/l (at 17.5 °C and 10 ppt salinity) and 80 µg/l (at 7.5 °C and 25 ppt salinity) (Theede et al., 1979). Laomedea loveni was more tolerant of Cd exposure at low temperatures and low salinities. Karbe (1972, summary only) examined the effects of heavy metals on the hydroid Eirene viridula (Campanulidae). He noted that Cd and Hg caused cumulative effects, and morphological changes. Mercury (Hg) caused irreversible damage at concentrations as low as 0.02 ppm. He reported threshold levels of heavy metals for acute effects in Eirene viridula of 1.5-3 ppm Zn, 1-3 ppm Pb, 0.1-0.3 ppm Cd, 0.03-0.06 ppm Cu and 0.001-0.003 ppm Hg. Karbe (1972, summary only) suggested that Eirene viridula was a sensitive test organism when compared to other organisms. Although no information on the effects of heavy metals on assessed hydroid species was found, the above evidence suggests that hydroids may suffer at least sub-lethal effects and possibly morphological changes and reduced growth due to heavy metal contamination. Trace metals (particularly mercury and copper) have been found to affect embryogenesis and larval settlement in Ciona intestinalis (Bellas et al., 2004). Whilst there are extensive studies of larval intolerance to TBT (Mansueto et al., 1993, Pellerito et al., 1996, Bellas, 2005) and zinc pyrithione (Bellas, 2005), data appears non-existent for the adult stage. Chesher (1971) found that Ascidia niagra was surprisingly intolerant of desalination effluent (50% mortality in 5.8% effluent solution after 96 hours), far less tolerant than the other species included in the study (echinoids, crabs and gorgonians). Whilst presence of copper was considered the most deleterious factor across the study, the increased sensitivity of the ascidians was attributed to synergistic copper and temperature effects, although presence of other contaminants (e.g. nickel) could not be ruled out (Chesher, 1971). Little is known about the effects of heavy metals on echinoderms. Bryan (1984) reported that early work had shown that echinoderm larvae were sensitive to heavy metals contamination, for example Migliaccio et al. (2014) reported exposure of Paracentrotus lividis larvae to increased levels of cadmium and manganese caused abnormal larval development and skeletal malformations. Kinne (1984) reported developmental disturbances in Echinus esculentus exposed to waters containing 25 µg / l of copper (Cu).

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.

CR.LCR.BrAs.AntAsH is a sub-tidal biotope (Connor et al., 2004). Oil pollution is mainly a surface phenomenon its impact upon circalittoral turf communities is likely to be limited. However, as in the case of the Prestige oil spill off the coast of France, high swell and winds can cause oil pollutants to mix with the seawater and potentially negatively affect sub-littoral habitats (Castège et al., 2014). Echinus esculentus was reported absent after the oil spill however returned after 2-5 years. Large numbers of dead Echinus esculentus were found between 5.5 and 14.5 m in the vicinity of Sennen cove, presumably due to a combination of wave exposure and heavy spraying of dispersants following the Torrey canyon’s oil spill (Smith 1968). Smith (1968) also demonstrated that 0.5 -1ppm of the detergent BP1002 resulted in developmental abnormalities in its echinopluteus larvae. Echinus esculentus populations in the vicinity of an oil terminal in A Coruna Bay, Spain, showed developmental abnormalities in the skeleton. The tissues contained high levels of aliphatic hydrocarbons, naphthalenes, pesticides and heavy metals (Zn, Hg, Cd, Pb, and Cu) (Gomez & Miguez-Rodriguez 1999). Ignatiades & Becacos-Kontos (1970) found that Ciona intestinalis can resist the toxicity of oil polluted water and ascidia are frequently found in polluted habitats such as marinas and harbours, etc. (Carver et al., 2006) as well as Ascidia mentula (Aneiros et al., 2015).

Although there is no information available on the effect of hydrocarbons on Antedon bifida or Antedon petasus, echinoderms in general appear to be highly intolerant. long-term chronic pollution is thought to be responsible for reduced abundance of Asterias rubens (Bokn et al., 1993) and Echinocardium cordatum (Daan & Mulder, 1996). Crude oil from the Torrey Canyon in 1967, and the subsequent use of detergent caused mass mortalities of echinoderms including Asterias rubens, Echinocardium cordatum, Psammechinus miliaris, Echinus esculentus, Marthasterias glacialis and Acrocnida brachiata (Smith, 1968).

Little information of the effects of hydrocarbons on hydroids was found although hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995).

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.

The species richness of hydroid communities decreases with increasing pollution but hydroid species adapted to a wide variation in environmental factors and with cosmopolitan distributions tend to be more tolerant of polluted waters (Boero, 1984; Gili & Hughes, 1995). Stebbing (1981) reported that Cu, Cd, and tributyl tin fluoride affected growth regulators in Laomedea (as Campanularia) flexuosa resulting in increased growth. Stebbing (1981) cited reports of growth stimulation in Obelia geniculata caused by methyl cholanthrene and dibenzanthrene. Bryan & Gibbs (1991) reported that virtually no hydroids were present on hard bottom communities in TBT contaminated sites and suggested that some hydroids were intolerant of TBT levels between 100 and 500 ng/l. No information concerning the intolerance of the hydroids assessed was found. However, the above evidence suggests that several species of hydroid exhibit sublethal effects due to synthetic chemical contamination and lethal effects due to TBT contamination. Prolonged exposure to low concentrations of polychlorinated biphenyls (PCB's) have been shown to result in growth and regenerative abnormalities in the feather-star Antedon mediterranea but there have been no reports of mortality (Carnevali et al., 2001). Hoare & Hiscock (1974) reported that Antedon bifida appeared to be completely intolerant of conditions within the vicinity of an acidified, halogenated effluent discharge in Anglesey, Wales.

Radionuclide contamination

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

Evidence

No 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

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

In general, respiration in most marine invertebrates does not appear to be significantly affected until extremely low concentrations are reached. For many benthic invertebrates this concentration is about 2 ml/l (Herreid, 1980; Rosenberg et al., 1991; Diaz & Rosenberg, 1995). Cole et al. (1999) suggest possible adverse effects on marine species below 4 mg/l and probable adverse effects below 2 mg/l.  Hydroids mainly inhabit environments in which the oxygen concentration exceeds 5 ml/l (Gili & Hughes, 1995). Although no information was found on oxygen consumption for the characterizing hydroids, Sagasti et al. (2000) reported that epifaunal species, including several hydroids and Obelia bidentata (as bicuspidata) in the York River, Chesapeake Bay, tolerated summer hypoxic episodes of between 0.5 and 2 mg O₂/l (0.36 and 1.4 ml/l) for 5-7 days at a time, with few changes in abundance or species composition.  The ability of solitary ascidians to withstand decreasing oxygen levels has not been well documented. Mazouni et al. (2001) noted that whilst oysters (Magallana gigas) can survive short-term exposure to periods of anoxia (Thau Lagoon, France), the associated biofouling community dominated by Ciona intestinalis suffered heavy mortality.  It should be noted, however, that this species is frequently found in areas with restricted water renewal where oxygen concentrations may drop (Carver et al., 2006). Whist adverse conditions could affect health, feeding, reproductive capability and could eventually lead to mortality, recovery should be rapid.

Antedon bifida is an aerobic organism and oxygen uptake is by the tube feet and across the body wall. It is typically found in areas of fast tidal flow where water will be oxygenated. Although there is no evidence regarding the effect of low oxygen conditions.  Mass mortality of species including Echinus esculentus was observed due to a stratified hypoxic event below 8 m caused by a phytotplankton bloom ( Griffiths et al., 1979).  Hiscock & Hoare (1975) reported an oxycline forming in the summer months (Jun-Sep) in a quarry lake (Abereiddy, Pembrokeshire) from close to full oxygen saturation at the surface to <5% saturation below ca 10 m.  During these summer events, no echinoderms or Ascidia mentula were recorded at depths below 10 - 11 m.

Sensitivity assessment:

CR.LCR.BrAs.AmenCio and CR.LCR.BrAs.AmenCio.Ant are typically low energy biotopes; a hypoxic event is likely to remain for some time, depending on local conditions.   The evidence suggests that Ciona intestinalis, Ascidia mentula and Echinus esculentus would be lost in hypoxic conditions.  Resistance is therefore recorded as ‘Low’, with a resilience of ‘Medium’ and sensitivity is classed as 'Medium'. Due to the lack of specific data for these species, confidence is recorded as ‘Low’. 

Nutrient enrichment

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

Evidence

Witt et al., 2004 found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids.  Ascidia mentula has been reported in Iberian bays subject to both nutrient-rich upwelling events and anthropogenic pollution (Aneiros et al., 2015).  There is some suggestion that there are possible benefits to ascidians from increased organic content of water; ascidian ‘richness’ in Algeciras Bay was found to increase in higher concentrations of suspended organic matter (Naranjo et al. 1996).  It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition. Nutrient enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability. Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age.  However, this biotope is considered to be 'Not 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

Witt et al., 2004 found that the hydroid Obelia spp. was more abundant in a sewage disposal area in the Weser estuary (Germany) which experienced sedimentation of 1 cm for more than 25 days.  It should be noted that another hydroid (Sertularia cupressina) was reduced in abundance when compared with unimpacted reference areas.   As suspension feeders, an increase in organic content at the benchmark is likely to be of benefit to the characterizing hydroids.  There is some suggestion that there are possible benefits to the ascidians from increased organic content of water; Ascidian ‘richness’ in Algeciras Bay was found to increase in higher concentrations of suspended organic matter (Naranjo et al. 1996).  Kocak & Kucuksezgin (2000) noted that Ciona intestinalis was one of the rapid breeding opportunistic species that tended to be dominant in Turkish harbours enriched by organic pollutants and was frequently found in polluted environments (Carver et al., 2006).  Ascidia mentula has been reported in Iberian bays subject to both nutrient-rich upwelling events and anthropogenic organic pollution (Aneiros et al., 2015).  It was suggested by Comely & Ansell (1988) that Echinus esculentus could absorb dissolved organic material for the purposes of nutrition. Organic enrichment may encourage the growth of ephemeral and epiphytic algae and therefore increase sea-urchin food availability. Lawrence (1975) reported that sea urchins had persisted over 13 years on barren grounds near sewage outfalls, presumably feeding on dissolved organic material, detritus, plankton and microalgae, although individuals died at an early age.  Antedon bifida has been reported in Iberian bays subject to both nutrient-rich upwelling events and anthropogenic organic pollution (Aneiros et al., 2015).

Sensitivity assessment: The above evidence suggests that resistance to this pressure is s 'High'.  Therefore, resilience is assessed as 'High' and the biotope 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

If rock were replaced with sediment, this would represent a fundamental change to the physical character of the biotope and the species would be unlikely to recover. The biotope would be lost.

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very low’. Sensitivity has been assessed as ‘High’.

Physical change (to another sediment type)

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

Evidence

‘Not relevant’ to biotopes occurring on bedrock.

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

The species characterizing this biotope are epifauna or epiflora occurring on rock and would be sensitive to the removal of the habitat. However, extraction of rock substratum is considered unlikely and this pressure is considered to be ‘Not relevant’ to hard substratum habitats.

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 available evidence indicates that hydroids can be entangled and removed by abrasion.  Drop down video surveys of Scottish reefs exposed to trawling showed that visual evidence of damage to bryozoans and hydroids on rock surfaces was generally limited and restricted to scrape scars on boulders (Boulcott & Howell, 2011).  The study showed that damage is incremental with damage increasing with frequency of trawls rather than a blanket effect occurring on the pass of the first trawls.  Re-sampling of grounds that were historically studied (from the 1930s) indicates that some species have increased in areas subject to scallop fishing (Bradshaw et al., 2002).  This study also found (unquantified) increase in abundance of tough stemmed hydroids including Nemertesia spp., its morphology may have prevented excessive damage.  Bradshaw et al. (2002) suggested that as well as having high resistance to abrasion   pressures, Nemertesia spp. have benthic larvae that could rapidly colonize disturbed areas with newly exposed substrata close to the adult. Hydroids may also recover rapidly as the surface covering of hydrorhizae may remain largely intact, from which new uprights are likely to grow. In addition, the resultant fragments of colonies may be able to develop into new colonies.  Hydroid colonies were still present in the heavily fished area, albeit at lower densities than in the closed area. This may largely be because the Isle of Man scallop fishery is closed from 1^st June to 31^st October (Andrews et al., 2011), so at the time the samples were taken for the study in question, the seabed had been undredged for at least 3.5 months. The summer period is also the peak growing/breeding season for many marine species (Bradshaw, 2003).  Both Ciona intestinalis and Ascidia mentula are large, emergent, sessile ascidians, and physical disturbance is likely to cause damage with mortality likely.  Emergent epifauna are generally very intolerant of disturbance from fishing gear (Jennings & Kaiser, 1998).  However, studies have shown Ascidia spp. to become more abundant following disturbance events (Bradshaw et al., 2000).  Species with fragile tests, such as Echinus esculentus were reported to suffer badly as a result of scallop or queen scallop dredging (Bradshaw et al., 2000; Hall-Spencer & Moore, 2000a). Kaiser et al. (2000) reported that Echinus esculentus were less abundant in areas subject to high trawling disturbance in the Irish Sea. Jenkins et al. (2001) conducted experimental scallop trawling in the North Irish sea and recorded the damage caused to several conspicuous megafauna species.  The authors used simultaneous assessment of both bycatch and organisms left on the seabed to estimate capture efficiency for both target and non-target organisms. This found 16.4% of Echinus esculentus were crushed/dead, 29.3% would have >50% spine loss/minor cracks, 1.1% would have <50% spine loss and the remaining 53.3% would be in good condition. Sea urchins can rapidly regenerate spines, e.g. Psammechinus miliaris were found to re-grow all spines within a period of 2 months (Hobson, 1930).  The trawling examples mentioned above were conducted on sedimentary habitats and thus the evidence is not directly relevant to rock based, however it does indicate the likely effects of abrasion on Echinus esculentus.  Antedon spp. are likely to be intolerant of abrasion as individuals would probably be killed or damaged by a force equivalent to a scallop dredge dragged across them (Hill, 2008). The species can regenerate body parts even when most arms and part of the disc have been lost so most damaged individuals are likely to recover. However, Cook et al. (2013) noted a significant decline in abundance of Antedon bifida one year after a trawling event on a protected reef.  Boulcott & Howell (2011) conducted experimental Newhaven scallop dredging over a circalittoral rock habitat in the sound of Jura, Scotland and recorded the damage to the resident community. The authors highlight physical damage to faunal turfs (erect bryozoans and hydroids) was difficult to quantify in the study. However, the faunal turf communities did not show large signs of damage and were only damaged by the scallop dredge teeth, which was often limited in extent (approximately. 2cm wide tracts). The authors indicated that faunal turf communities were not as vulnerable to damage through trawling as sedimentary fauna and whilst damage to circalittoral rock fauna did occur, it was of an incremental nature, with loss of faunal turf communities increasing with repeated trawls.

Sensitivity assessment.

Given the sessile, emerged nature of the hydroids, Ciona intestinalis and Asidia mentula, damage and mortality following a physical disturbance effect are likely to be significant, however some studies have brought into question the extent of damage to the faunal turf.  Echinus esculentus and Antedon spp. have been found to be negatively impacted following disturbance events. 

Resistance has been assessed as ‘Low’, resilience has been assessed as ‘Medium’. Sensitivity has been assessed as ‘Medium’.

Please note Boulcott & Howell (2011) did not mention the abrasion caused by fully loaded collection bags on the Newhaven dredges. A fully loaded Newhaven dredge may cause higher damage to community as indicated in their study.

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

The species characterizing this biotope group are epifauna or epiflora occurring on rock which is resistant to subsurface penetration.  The assessment for abrasion at the surface only is therefore considered to equally represent sensitivity to this pressure. This pressure is thought ‘Not Relevant’ to hard rock biotopes

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

An increase in suspended sediment may have a deleterious effect on the suspension feeding community. It is likely to clog their feeding apparatus to some degree, resulting in a reduced ingestion over the benchmark period and, subsequently, a decrease in growth rate (Jackson, 2004). As the hydroids capture small prey in suspension (Gili & Hughes, 1995), a reduction in feeding efficiency could potentially lead to a reduction in overall biomass.   Nemertesia ramosa is a passive suspension feeder, extracting seston from the water column. Increased siltation may clog up the feeding apparatus, requiring energetic expenditure to clear. Recovery is likely to take only a few days (Jackson, 2004).  A decrease in suspended sediment is likely to benefit the community associated with. The suspension feeders may be able to feed more efficiently due to a reduction in time and energy spent cleaning feeding apparatus. Over the course of the benchmark the hydroids may increase in abundance.  Ciona intestinalis frequently occurs in habitats close to harbours and marinas with high levels of silt and suspended matter (Carver et al., 2006; Kocak & Kucuksezgin, 2000). Naranjo et al. (1996) described Ciona intestinalis as having a large body and siphons that have wide apertures that helps prevent blocking. Increased suspended sediment may potentially have some detrimental effects in clogging up feeding filtration mechanisms, however, there are possible benefits from increased suspended sediment as ascidian ‘richness’ in Algeciras Bay was found to increase in higher concentrations of suspended organic matter (Naranjo et al. 1996). In high (up to 300 mg/l of inorganic and 2.5 x10⁷ cells/l) suspended particulate concentrations, the active rejection mechanism (squirting) is increased in Ciona intestinalis with no discrimination between organic and inorganic particulates observed in any of the ascidians observed (Robbins, 1984a).  Despite these observations, the turbidity tolerance level for this species is not well established. Robbins (1985a) found that continual exposure to elevated levels of inorganic particulates (>25 mg/l) arrested the growth rate of Ciona intestinalis and exposure to 600 mg/l resulted in 50% mortality after 12-15 days and 100% mortality after 3 weeks.  It was suggested that because this species can only “squirt” to clear the branchial sac, it may be vulnerable to clogging under heavy sediment loads.  Ascidia mentula has been shown to decrease absolute (instantaneous) rate of pumping in high suspended particulate concentrations, whilst filtration efficiency remained unchanged (Robbins, 1984a).  However, specific data on the sensitivity to suspended sediment is lacking.  Moore (1977a) suggested that Echinus esculentus was unaffected by turbid conditions. Echinus esculentus is an important grazer in CR.LCR.BrAs.AmenCio and CR.LCR.BrAs.AmenCio.Ant. Whilst increased turbidity and resultant reduced light penetration may negatively affect algal growth, Echinus esculentus can also feed on alternative prey, detritus or dissolved organic material (Lawrence, 1975, Comely & Ansell, 1988).  Studies on the impact of high suspended sediment conditions on Antedon spp. are lacking, however a study of Antedon bifida showed 17% of gut content was inorganic particles and that this was consistent throughout the year and for all locations studied (La Touche & West, 1980).  Antedon spp. are not considered directly sensitive to the associated change in light attenuation as this does not impact upon suspension feeding. An increase in turbidity, reducing light availability, may reduce primary production by phytoplankton in the water column and thus influence food availability. However, particulate food supplies are also likely to be derived from distant sources so the long-term impact is not likely to be significant.  Antedon bifida has also been shown to ingest a large fraction of detritus (ca 65% of stomach contents), which is considered an important source of nutrition (La Touche & West, 1980).

Sensitivity assessment

CR.LCR.BrAs.AntAsH is found in silty, circalittoral rock, wave-sheltered conditions.  A change at the benchmark level is unlikely to have significant effects on the species considered in this study.

 Resistance at the benchmark has been assessed as ‘High’, Resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

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

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al., 1961 reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement.  Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered but although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible. Smothering with 5 cm of sediment may cover over some individuals, others may just have the lower section of the main stem covered (Hayward & Ryland, 1994).

Whilst monosipphonic Obelia dichotoma stems grow to 5 cm, polysiphonic structures can reach up to 35 cm in height,

Halecium halecinumcan grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Some of the community is therefore likely to survive smothering by 5 cm.  The solitary ascidians considered in this report are permanently attached to the substratum and are active suspension feeders. Because the adults reach up to 15 cm and 18 cm in length for Ciona intestinalis and Ascidia mentula respectively (Rowley, 2008; Jackson, 2008) and frequently inhabit vertical surfaces (Jackson, 2008), smothering with 5 cm of sediment is likely to only affect a small proportion of the population. Recovery should be rapid, facilitated by the remaining adults.  Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt. This suggests that Echinus esculentus is unlikely to be killed by smothering, however, smaller specimens and juveniles may be less resistant. A layer of sediment may interfere with larval settlement.  If retained within the host biotope for extended periods a layer of 5cm of the sediment may negatively affect successive recruitment events.  Antedon bifida has been found to be limited to vertical surfaces in the presence of a heavy layer of sediment (Eleftheriou et al., 1997). Antedons are also unlikely to be able to move above the sediment as they require a hard substratum for attachment and the feeding and respiratory structures are likely to become clogged (Hill, 2008).  As CR.LCR.BrAs.AmenCio.Ant is often found on vertical rocks, it is possible that most of the population would survive the event in order to repopulate.

Sensitivity assessment. Smothering by 5 cm of sediment is likely to impact hydroids, ascidian and antedon species, however it is likely that enough of the population would survive to recover quite rapidly should the thin layer of sediment be removed. Resistance has been assessed as ‘Medium’, resilience as ‘High’ and sensitivity has been assessed as ‘Low’ at the benchmark level.

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

In general, it appears that hydroids are sensitive to silting (Boero, 1984; Gili & Hughes, 1995) and decline in beds in the Wadden Sea have been linked to environmental changes including siltation.  Round et al., 1961 reported that the hydroid Sertularia (now Amphisbetia) operculata died when covered with a layer of silt after being transplanted to sheltered conditions.  Boero (1984) suggested that deep water hydroid species develop upright, thin colonies that accumulate little sediment, while species in turbulent water movement were adequately cleaned of silt by water movement.

Nemertesia ramosa is an upright hydroid with a height of up to 15 cm. The colony structure is fairly tough and flexible (Hayward & Ryland, 1994). Monosipphonic  Obelia dichotoma stems grow to 5 cm, polysiphonic structures can reach up to 35 cm in height, Halecium halecinumcan grow up to 25 cm and Kirchenpaueria pinnata can grow to ca 10 cm (Hayward & Ryland, 1994).  Smothering by 30 cm of material is likely to cover almost all the hydroids.  Hughes (1977) found that maturing hydroids that had been smothered with detritus and silt lost most of the hydrocladia and hydranths. After one month, the hydroids were seen to have recovered but, although neither the growth rate nor the reproductive potential appeared to have been affected, the viability of the planulae may have been affected.  Therefore, if the deposition is removed fairly rapidly, impact may be limited, however given that CR.LCR.BrAs.AntAsH experiences weak water movement, the effects are likely to persist for some time.

The solitary ascidians considered in this report are permanently attached to the substratum and are active suspension feeder. Because the adults reach up to 15 cm and 18 cm in length for Ciona intestinalis and Ascidia mentula respectively (Rowley, 2008; Jackson, 2008) and frequently inhabit vertical surfaces(Jackson, 2008), smothering with 30 cm of sediment is likely to affect the vast majority of the population.  Comely & Ansell (1988) recorded large Echinus esculentus from kelp beds on the west coast of Scotland in which the substratum was seasonally covered with "high levels" of silt. This suggests that Echinus esculentus is unlikely to be completely removed by smothering, however, smaller specimens and juveniles may be less resistant. A layer of sediment may interfere with larval settlement. 

Smothering by 30 cm of sediment is likely to result in emigration of feather-stars as they require a hard substratum for attachment and the feeding and respiratory structures are likely to become clogged (Hill, 2008).  Antedon bifida has been found to be limited to vertical surfaces in the presence of a heavy layer of sediment (Eleftheriou et al., 1997).

Sensitivity assessment. Smothering by 30 cm of sediment is likely to cause mortality amongst the majority of characterizing and important species of these biotopes and impact recovery unless the sediment is removed, especially where the epifauna occur on cobbles and pebbles which would be completely covered by sediment. However, vertical surfaces may protect a proportion of the population, so that the effects will depend on the topography of the substratum. Resistance at the benchmark has been assessed as ‘Low’.  Resilience is assessed as ‘Low’ as recovery is dependent on the removal of the sediment by tidal flow and wave action, both of which are limited in this low energy biotope. Sensitivity has been assessed 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

McDonald (2014) studied the effect of generator noise on fouling of four vessels by Ciona intestinalis and found that fouling was highest at locations closest to the generators and lowest furthest away from the generators.  Subsequent in vitro experiments demonstrated that larvae in the presence of noise settled much faster (2h- 20h compared with 6h-26h for control), underwent metamorphosis more rapidly (between 10 and 20h compared with ca 22h) and had a markedly increased survival rate to maturity (90-100% compared with 66%).  Other studies also reported that noise emissions from vessels promoted fouling by organisms including ascidians (Stanley et al., 2016).  Echinus esculentus has no hearing perception but vibrations may cause an impact, however no studies exist to support an assessment.  No evidence could be found for the effects of noise or vibrations on the characterizing hydroids or Antedon spp.

Sensitivity assessment: Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is therefore considered to be 'Not sensitive'.  Confidence has to be assessed as ‘Low’ given the lack of literature for echinoderms and hydroids.

Introduction of light or shading

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

Evidence

Gili & Hughes (1995) reviewed the effect of light on a number of hydroids and found that there is a general tendency for most hydroids to be less abundant in well-lit situations.  Whilst hydroid larvae can be positively or negatively photoactic, the planulae of Nemertesia antennina show no response to light (Hughes, 1977).  In vitro studies of solitary ascidians indicate that both spawning and settlement are controlled by light, however Ciona intestinalis in vivo has been observed to spawn and settle at any time of the day.  (Svane & Havenhand, 1993 and references therein).  Whilst there is some evidence that the basiepithelial nerve plexus below the entire outer skins of echinoderms is sensitive to light (Hill, 2008), the species considered in this study are not thought to be sensitive at the benchmark level.

Sensitivity assessment: Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is therefore considered to be 'Not sensitive'.  Confidence has to be assessed as ‘Low’ given the lack of literature for echinoderms.

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: barriers and changes in tidal excursion are 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

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

Echinus esculentus was identified by Kelly & Pantazis (2001) as a species suitable for culture for the urchin Roe industry. However, at present no evidence could be found to suggest that significant Echinus esculentus mariculture was present in the UK. If industrially cultivated it is feasible that Echinus esculentus individuals could be translocated.  Ciona intestinalis is considered a fouling species and adheres readily to the hulls of vessels, and is considered an invasive species in the USA, Chile, Western Australia, New Zealand, Canada and South Africa (Millar 1966; McDonald 2004; Blum et al. 2007; Ramsay et al. 2008, 2009; Dumont et al.,2011 )   

Whilst there have been novel proposals to farm Ciona intestinalis as potential feedstock for aquaculture in Sweden (Laupsa, 2015), there is no evidence to suggest such farming exists.  Therefore, there is currently ‘No evidence’ on which to assess this pressure.

Introduction or spread of invasive non-indigenous species

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

Evidence

Crepidula fornicata 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 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; Tillin et al., 2020). Close examination of the literature (2023) shows that evidence of its colonization and density on bedrock in the infralittoral or circalittoral was lacking. Tillin et al. (2020) suggested that Crepidula could colonize circalittoral rock due to its presence on tide-swept rough grounds in the English Channel (Hinz et al., 2011). However, Hinz et al. (2011) reported that Crepidula fornicata only dominated one assemblage (with an average of 181 individuals per trawl) on gravel substratum with boulders. Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas dominated by boulders, and Bohn et al. (2013a, 2013b, 2015) and Preston et al. (2020) showed that while Crepidula could settle on slate panels or ‘stone’ it preferred shell, especially that of conspecifics. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats. It was only recorded at low density (0.1-0.9/m²) in one faunal turf biotope (CR.MCR.CFaVS.CuSpH.As) (JNCC, 2015). Faunal turfs are dominated by suspension feeders so larval predation is probably high, which may prevent colonization by Crepidula. Also, faunal turf species actively compete for space and many are fast growing and opportunistic, so may out-compete Crepidula for space even if it gained a foothold in the community. 

Styela clava was first recorded in the UK at Plymouth in 1952 (Eno et al., 1997). Where Styela clava and Ciona intestinalis co-occur they may compete for space and food (Jackson, 2008).

Didemnum vexillum is an invasive colonial sea squirt native to Asia which was first recorded in the UK in Darthaven Marina, Dartmouth in 2005. Didemnum vexillum can form extensive mats over the substrata it colonizes; binding boulders and cobbles and altering the host habitat (Griffith et al., 2009). Didemnum vexillum can also grow over and smother the resident biological community. Recent surveys within Holyhead Marina, North Wales have found Didemnum vexillum growing on and smothering native tunicate communities, including Ciona intestinalis (Griffith et al., 2009). Due to the rapid-re-colonization of Didemnum vexillum eradication attempts have to date failed.  Presently Didemnum vexillum is isolated to several sheltered locations in the UK (NBN, 2015), however, Didemnum vexillum has successfully colonized the offshore location of the Georges Bank, USA (Lengyel et al., 2009) which is more exposed than the locations which Didemnum vexillum have colonized in the UK. It is therefore possible that Didemnum vexillum could colonize more exposed locations within the UK and could therefore pose a threat to these biotopes.

Sensitivity assessment. The circalittoral rock characterizing this biotope could be suitable for colonization by Crepidula fornicata due to the sheltered to extremely sheltered wave conditions, where water is mediated by tidal flow rather than wave actions. Crepidula has been recorded from the lower intertidal to ca 160 m in depth but is most common in the shallow subtidal above 50 m (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2023; Tillin et al., 2020), therefore, colonization of Crepidula would be limited to low densities in deeper examples of the biotope. In addition, no evidence was found of the effect of Crepidula populations on faunal turf-dominated habitats or infralittoral or circalittoral rock habitats. At present, there is 'Insufficient evidence' to suggest that the circalittoral rock biotopes are sensitive to colonization by Crepidula fornicata or other invasive species; 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

Hydroids exhibit astonishing regeneration and rapid recovery from injury (Sparks, 1972) and the only inflammatory response is active phagocytosis (Tokin & Yaricheva, 1959;1961, as cited in Sparks, 1972).  No record of diseases in the characterizing hydroids could be found.  There appears to be little research into ascidian diseases particularly in the Atlantic.  The parasite Lankesteria ascidiae targets the digestive tubes and can cause ‘long faeces syndrome’ in Ciona intestinalis (although it has also been recorded in other species).Mortality occurs in severely affected individuals within about a week following first symptoms. (Mita et al., 2012).  Echinus esculentus is susceptible to 'Bald-sea-urchin disease', which causes lesions, loss of spines, tube feet, pedicellariae, destruction of the upper layer of skeletal tissue and death. It is thought to be caused by the bacteria Vibrio anguillarum and
Aeromonas salmonicida. Bald sea-urchin disease was recorded from Echinus esculentus on the Brittany Coast. Although associated with mass mortalities of Strongylocentrotus franciscanus in California and Paracentrotus lividus in the French Mediterranean it is not known if the disease induces mass mortality (Bower, 1996).  Like most echinoderms, Antedon spp. are host to symbiotic organisms. The annelid myzostome Myzostoma cirriferum is an obligate associate of echinoderms and lives mainly with crinoids including Antedon bifida (Eeckhaut & Jangoux, 1997). Infestation levels seem to vary between sites (Leonard & Jeal, 1984). Although no diseases associated with these organisms have been reported echinoderms in other parts of the world have been severely affected by epidemic type diseases so there is the potential for this to occur.

Sensitivity assessment: In the absence of evidence of mortalities due to disease both resistance and resilience are assessed as ‘High’; the biotope is therefore ‘Not Sensitive’ to this pressure. However, the assessment has a low confidence score as more research is needed into the effects of microbial pathogen on faunal turfs and associated communities.

Removal of target species

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

Evidence

Despite historic harvesting of the hydroid Sertularia cupressinain in the Wadden Sea (Wagler et al., 2009), no evidence for harvesting of the characterizing hydroids could be found and targeted extraction is highly unlikely.

Despite novel proposals to farm Ciona intestinalis as potential feedstock for aquaculture in Sweden (Laupsa, 2015), it is very unlikely that solitary ascidians, would be targeted for extraction.  Despite historic extraction as a curio (Jangoux, 1980; Nichols, 1984), Echinus esculentus is not thought to be currently targeted. Extraction of Antedon spp. is unlikely because it has no commercial value.  If Echinus esculentus was removed from the biotope, the loss of grazing pressure would result in increasing competition from algae and could lead to a change in biotope classification.  Resistance is therefore recorded as ‘Low’, resilience is recorded as ‘Medium’ and Sensitivity is ‘Medium’.

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

This biotope may be removed or damaged by static or mobile gears that are targeting other species. These direct, physical impacts are assessed through the abrasion and penetration of the seabed pressures. The sensitivity assessment for this pressure considers any biological/ecological effects resulting from the removal of non-target species on this biotope.

While recovery of the characterizing species should be possible within 2-10 years following non-targeted removal (e.g. from static or mobile gears), loss of Echinus esculentus from the biotope and subsequent loss of grazing pressure would result in increasing competition from algae and could lead to a change in biotope classification.  Resistance is recorded as ‘Low’, resilience is recorded as ‘Medium’ and Sensitivity is ‘Medium’.