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information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Solitary ascidians, including Ascidia mentula and Ciona intestinalis, on wave-sheltered circalittoral rock

08-11-2016

Summary

UK and Ireland classification

UK and Ireland classification

Description

This biotope predominantly occurs on the upper faces of wave-sheltered (often sealochs) circalittoral bedrock, boulder and cobble slopes with little tidal flow. Apart from the solitary ascidians Ciona intestinalis and Ascidia mentula, this biotope has a rather barren, pink appearance (due to the encrusting red algae), possibly due to grazing pressure from the sea urchin Echinus esculentus. Other organisms found encrusting the rocky surface include the polychaete Spirobranchus (formerly Pomatoceros) triqueter and the cup coral Caryophyllia smithii. Other species occasionally encountered include Alcyonium digitatumAsterias rubensPagurus berhardusCrossaster papposusAntedon bifida and Metridium senile. Crustaceans such as Munida rugosa and Cancer pagurus may be recorded in crevices. Two variants of this biotope exist: AmenCio.Ant and AmenCio.Bri. AmenCio.Bri occurs where is a dense carpet of brittlestars which sometimes completely cover the rocky substratum. Species present include Ophiothrix fragilisOphiocomina nigra and Ophiura albida (taken from Connor et al., 2004).

Depth range

10-20 m, 20-30 m, 30-50 m, 5-10 m

Additional information

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Sensitivity reviewHow is sensitivity assessed?

Sensitivity characteristics of the habitat and relevant characteristic species

CR.LCR.BrAs.AmenCio and the sub-biotope CR.LCR.BrAs.AmenCio.Ant are typical of wave-sheltered circalittoral bedrock, boulder and cobble slopes with little tidal flow, and often occur in sea lochs. The biotopes have a sparse fauna of solitary ascidians Ciona intestinalis and Ascidia mentula and look barren except for the pink colour of encrusting coralline algae. The barren and sparse fauna is probably due to grazing pressure from sea urchin Echinus esculentus.  Other organisms found encrusting the rocky surface include the polychaete Spirobranchus triqueter and the cup coral Caryophyllia smithiiAlcyonium digitatum and Metridium senile may be present, while Asterias rubensPagurus berhardusCrossaster papposusAntedon bifida are mobile species and crustaceans such as Munida rugosa and Cancer pagurus may be recorded in crevices.  The variant AmenCio.Ant is found in more wave sheltered sites with only weak to very weak tidal flow,  and is characterized by a higher frequency of  Antedon spp., together with more hydroids, the colonial ascidian Clavelina lepadiformis, the brachiopod Novocrania anomala (especially where this biotope occurs shallower than NovPro for example), and the saddle oyster Pododesmus patelliformis.

AmenCio.Bri is characterized by a dense bed of brittlestars, which sometimes completely cover the rocky substratum. Due to the significance of brittlestars, AmenCio.Bri is considered separately. 

Grazing by the sea urchin Echinus esculentus is considered significant in preserving the nature of this biotope and loss of this species is likely to significantly affect the biotope to the extent that reclassification would be necessary.  Loss of the characterizing solitary ascidians, including Ascidia mentula and Ciona intestinalis, may also result in loss or degradation of the biotope.  Antedon spp. may be present in both AmenCio and AmenCio.Ant but is only important characterizing in the sub-biotope (AmenCio.Ant), so that a reduction in its abundance would not result in loss of the AmenCio biotope itself. Other species present in these biotopes are considered transient, mobile or ubiquitous and are therefore not considered significant to assessment of the sensitivity of these biotopes.  However, information on the sensitivity of other characterizing species are included where appropriate. 

Resilience and recovery rates of habitat

Sea squirts (ascidians) are simultaneously hermaphroditic, sessile filter feeding chordates.  Whilst the adults do not have a backbone, their free swimming, short-lived, ascidian larvae possess a notochord which is lost during metamorphosis into its sessile form.  Solitary ascidians are discrete creatures which do not fuse with others (unlike colonial ascidians), but may still form dense beds (e.g. up to 5000 individuals/m² for Ciona intestinalis) (Naylor, 2011). 

Both Ascidia mentula and Ciona intestinalis occur across the western, northern and southern coasts of the UK, with more scattered recordings on the eastern coast (NBN, 2015).  Ascidia mentula is found from Norway to the Mediterranean (Picton & Morrow, 2015) and Ciona intestinalis occurs from Norway and Sweden (Svane, 1984) through to Cape Verde, although these latter populations are thought to be transitory (Monniot & Monniot 1994).  Ciona intestinalis is a well-studied species owing to its status as an invasive species in many parts of the world including 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).  It is considered an indigenous species in the UK (Lambert & Lambert, 1998). 

In Ciona intestinalis, spawning has been reported as more or less year round in temperate conditions  (Yamaguchi, 1975, Caputi et al., 2015, MBA, 1957) with seasonal spawning observed in colder climates from May to June on the Canadian coast (Carver et al., 2006) and in shallower habitats in Sweden (Svane & Havenhand, 1993).  Oviparous solitary ascidians generally spawn both oocytes and sperm into the water column, where the resultant fertilized eggs develop into free swimming, non-feeding larvae.

The eggs are negatively buoyant and slightly adhesive and are either released freely or in mucus strings which are especially adhesive.  These strings have a tendency to settle close to or on the parent ascidian.  In vitro studies conclude that fertilization proceeds normally whether in the water column or attached to the mucus string.  The hatched free-swimming larvae settle nearby, are held by the mucus string until settlement or escape as plankton.  Retention in the mucus string may explain the dense aggregations of adults found (Svane & Havenhand, 1993).  In vitro studies 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 (Whittington, 1967; Svane & Havenhand, 1993 and references therein). 

In the Mediterranean, population collapses of Ciona intestinalis were observed, followed by recovery within 1-2 years (Caputi et al., 2015).  The collapses are still poorly understood, although low salinity (Pérès, 1943) and temperature (Sabbadin, 1957) are suggested as possible drivers.

Ascidia mentula is a larger (up to 18 cm long) and longer lived (up to 7 years) ascidian compared to Ciona intestinalis (Rowley, 2008).  Recruitment has been reported to occur year round in Sweden at depths greater than 20 m, with seasonal spawning occurring at 15 m (where sea temperature variability is much greater).  Svane (1984) noted that, whilst predation by sea urchins did not appear to be substantial, mortality caused by disturbance and dislocation were significant.   Sea urchins were observed to leave cleared tracks as they moved across the substratum but their feeding was curtailed in the presence of more densely aggregated ascidians that had restricted urchin movement.

Both active larvae settlement distribution and passive deposition of larvae (i.e. purely hydrodynamic processes) have been proposed (Havenhand & Svane, 1991 see also Meadows & Campbell, 1972; Scheltema, 1974; Butman, 1987).  Long-term data from populations of the ascidian Ascidia mentula on subtidal vertical rock indicated that recruitment of Ascidia mentula larvae was positively correlated with adult population density, and then by subsequent active larval choice at smaller scales. Factors influencing larval settlement have been listed as light, substratum inclination and texture (Havenhand & Svane, 1989). The presence of hydroids may also be important in recruitment of ascidians. Schmidt (1983) described how the hydroid Tubularia larynx attracted a 'bloom' of the ascidians Ciona intestinalis and Ascidiella aspersa on settlement panels.  However, the swimming power of an ascidian tadpole larva is relatively low (Chia, Buckland-Nicks & Young, 1984). Therefore, on a larger scale, hydrodynamics probably determine distribution (Olson, 1985; Young, 1986).

Sebens (1985; 1986) described the recolonization of epifauna on vertical rock walls.  Rapid colonizers such as encrusting corallines, encrusting bryozoans, amphipods and tubeworms recolonized within 1-4 months. Ascidians such as Dendrodoa carnea, Molgula manhattensis and Aplidium spp. achieved significant cover in less than a year, and, together with Halichondria panicea, reached pre-clearance levels of cover after 2 years. A few individuals of Alcyonium digitatum and Metridium senile colonized within 4 years (Sebens, 1986) and would probably take longer to reach pre-clearance levels.

Echinus esculentus is a sea urchin found within the north-east Atlantic, recorded from Murmansk Coast, Russia to Portugal (Hansson, 1998). Echinus esculentus is an important algal grazer and is thought, combined with low light levels, to control red algal growth (Connor et al., 2004). Echinus esculentus is estimated to have a lifespan of 8-16 years (Nichols, 1979; Gage, 1992) and reaches sexual maturity within 1-3 years (Tyler-Walters, 2008). Maximum spawning occurs in spring although individuals may spawn over a protracted period throughout the year.  Gonad weight is at its maximum in February/March in English Channel (Comely & Ansell, 1988) but decreases during spawning in spring and then increases again through summer and winter until the next spawning season. Spawning occurs just before the seasonal rise in temperature in temperate zones but is probably not triggered by rising temperature (Bishop, 1985). Echinus esculentus is a broadcast spawner, with a complex larval life history which includes a blastula, gastrula and a characteristic four armed echinopluteus stage, which forms an important component of the zooplankton. MacBride (1914) observed planktonic larval development could take 45-60 days in captivity.

Recruitment is sporadic or variable depending on locality, e.g. Millport populations showed annual recruitment, whereas few recruits were found in Plymouth populations during Nichols’ studies between 1980-1981 (Nichols, 1984). Bishop & Earll (1984) suggested that the population of Echinus esculentus at St Abbs had a high density and recruited regularly whereas the Skomer population was sparse, ageing and had probably not successfully recruited larvae in the previous 6 years (Bishop & Earll, 1984). Comely & Ansell (1988) noted that the largest number of Echinus esculentus occurred below the kelp forest.

Echinus esculentus is a mobile species and could therefore migrate and re-populate an area quickly if removed. For example, Lewis & Nichols (1979) found that adults were able to colonize an artificial reef in small numbers within 3 months and the population steadily grew over the following year.  If completely removed from a site and local populations are naturally sparse then recruitment may be dependent on larval supply which can be highly variable. As suggested by Bishop & Earll (1984) the Skomer, Wales Echinus esculentus population had most likely not successfully recruited for 6 years which would suggest the mature population would be highly sensitive to removal and may not return for several years. The Prestige oil tanker spilled 63 000t of fuel 130 nautical miles off Galicia, Spain in November 2002. High wave exposure and strong weather systems increased mixing of the oil to “some” depth within the water column, causing sensitive faunal communities to be effected. The biological community of Guéthary, France was monitored preceding and for nine years following the oil spill. Following the oil spill, taxonomic richness decreased significantly from 57 recorded species to 41, which included the loss of Echinus esculentus from the site. Two to three years after the oil spill taxonomic richness had increased to pre-spill levels and Echinus esculentus had returned (Castège et al., 2014).

Antedon is a genus of free-swimming, stem-less crinoids.  Two such species are found in CR.LCR.BrAs.AmenCio.Ant; Antedon bifida and Antedon petasus, both of which are ten armed feather-stars which use claw-like cirri on their underside to move across the substratum.  Mature individuals can be recognised by swollen genital papillae at the base of the arms. Eggs escape through splits which appear in the pinnule walls, and adhere in groups to the external wall of the pinnule where fertilization takes place. The embryos are held on the pinnules in a mucous net during which time the female holds its arms close together. Embryos remain attached in groups of up to 20 for four or five days (Chadwick, 1907 cited in Nichols, 1991). The embryos hatch as free-swimming larvae which, after a short pelagic phase, attach to the substratum and develop a short stalk. At this stage they are known as pentacrinoid larvae. The pentacrinoids eventually detach with prehensile cirri having developed on the underside of the disc.

Antedon bifida spawns between May to July. However, Nichols (1991) observed that mature oocytes and active sperm were present in every month of the year although a ‘spawned out’ condition has been intermittently recorded.  

Despite shed embryos only having been observed between May and July, it is possible that, like the Antedon mediterranea, Antedon bifida reproduces all year (Nichols 1991).  In later work Nichols (1994) suggested that the maintenance of gonads at a high level of maturity throughout the year is a life-history trait developed to tolerate the predation of expendable and easily-regenerated gonads.

Little information is available on recovery of the Antedon. However, it should be noted that Antedon are mobile and judging from life-history traits, Antedon bifida should be able to recover within five years. The species reaches sexual maturity within the first or second year and is iteroparous, spawning for 2-3 months every year (Nichols, 1991). Eggs are brooded on the arms of the feather-star and pelagic larvae are then released into the water column. However, the pelagic phase is fairly short so dispersal distances may not be great and recruitment may rely on relatively local populations. Therefore, if populations are completely removed by a factor recovery may take longer than five years.

Resilience assessment.

Spawning has been reported as more or less year round in temperate conditions for both Ciona intestinalis (Yamaguchi, 1975, Caputi et al., 2015; MBA, 1957) and Ascidia mentula (Fish & Fish 1996).  Ciona intestinalis reaches sexual maturity at a body height of ca 25-30mm, with one to two generations per year and longevity of ca 1.5 years.  (Fish & Fish 1996).  Sebens (1985, 1986) found that ascidians such as Dendrodoa carnea, Molgula manhattensis and Aplidium spp. achieved significant cover in less than a year, and, together with Halichondria panicea, reached pre-clearance levels of cover after 2 years. A few individuals of Alcyonium digitatum and Metridium senile colonized within 4 years (Sebens, 1986) and would probably take longer to reach pre-clearance levels.  Echinus esculentus can reportedly reach sexual maturity within 1-2 years (Tyler-Walters, 2008), however as highlighted by Bishop & Earll (1984) and Castège et al., (2014) recovery may take 2-6 years (possibly more if local recruitment is poor).  Antedon spp. are mobile, reach sexual maturity within the first or second year and are iteroparous, spawning for 2-3 months every year (Nichols, 1991). Eggs are brooded on the arms of the feather-star and pelagic larvae are then released into the water column. However, the pelagic phase is fairly short so dispersal distances may not be great and recruitment may rely on relatively local populations. Therefore, if populations are completely removed, recovery will take longer.

If the community is completely removed from the habitat (resistance of ‘None’ or ‘Low’) resilience is assessed as ‘Medium’ (recovery within 2-10 years).  However if resistance is assessed as ‘Medium’ or ‘High’ then resilience will be assessed as ‘High’ (recovery within 2 years).

 

NB: The resilience and the ability to recover from human induced pressures is a combination of the environmental conditions of the site, the frequency (repeated disturbances versus a one off event) and the intensity of the disturbance.  Recovery of impacted populations will always be mediated by stochastic events and processes acting over different scales including, but not limited to, local habitat conditions, further impacts and processes such as larval-supply and recruitment between populations. Full recovery is defined as the return to the state of the habitat that existed prior to impact.  This does not necessarily mean that every component species has returned to its prior condition, abundance or extent but that the relevant functional components are present and the habitat is structurally and functionally recognizable as the initial habitat of interest. It should be noted that the recovery rates are only indicative of the recovery potential.

Hydrological Pressures

 ResistanceResilienceSensitivity
Medium High Low
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

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.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

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 likely that a reduction in temperature would not negatively impact either Antedon sp.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’.

None Medium Medium
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Medium
Q: Low
A: Low
C: Low

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.AmenCio 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.

A reduction in the abundance of Antedon spp. in CR.LCR.BrAs.AmenCio.Ant would result in its replacement by CR.LCR.BrAs.AmenCio, however a reduction in Echinus esculentus abundance and hence grazing pressure is likely to result in loss of the AmenCio biotope. 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 classed as ‘Low’.

Medium Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Low
C: Medium
Q: Low
A: Low
C: Low

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.LsacPsaVS (Connor et al., 2004).

Sensitivity assessment.  CR.LCR.BrAs.AmenCio is a subtidal full salinity biotope (Connor et al., 2004) and salinity decrease at the benchmark level may have some impact 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’.  In the absence of direct evidence on the effects of hyposaline conditions on the characterizing species, confidence is therefore classed as ‘Low’.

Medium High Low
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low
Q: Low
A: Low
C: Low

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 AmenCio biotope is typical of low energy environments, and occurs on rocky substrata with little tidal flow, and (moderate to very weak tidal flow) (Connor et al., 2004).  Therefore, a decrease of 0.1-0.2 m/s is unlikely to be significant, even in examples of the biotope exposed to moderate tidal flow.  Mobile Echinus esculentus and the Antedon spp. may become dislodged by an increase in water flow but are unlikely to be killed 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 unclear if a change of 0.1-0.2 m/s would be significant.   Therefore, resistance has been assessed as ‘Medium’, resilience has been assessed as ‘High’, and sensitivity has been assessed as ‘Low’.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

High High Not sensitive
Q: Medium
A: Low
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Low
C: Medium

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, Asidia mentula and Antedon spp. are considered to be more at risk of damage and mortality when subject to excessive wave exposure. The AmenCio biotopes occur in moderate wave exposure to very sheltered conditions, although probably at greater depth in moderate conditions. However, a 0.3-0.5% 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’.

Chemical Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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).  However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

CR.LCR.BrAs.AmenCio 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).  However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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. However, this biotope is considered to be 'Not sensitive' at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

'No evidence'.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No benchmark was proposed.  Therefore, sensitivity has been assessed as 'Not sensitive' at the pressure benchmark that assumes compliance with all relevant environmental protection standards.

Low Medium Medium
Q: Low
A: Low
C: Low
Q: Medium
A: Low
C: Medium
Q: Low
A: Low
C: Low

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.

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). While 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’.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

High High Not sensitive
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High
Q: High
A: Medium
C: High

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 Pressures

 ResistanceResilienceSensitivity
None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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.

None Very Low High
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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’.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant’ to biotopes occurring on bedrock.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

Low Medium Medium
Q: Medium
A: Medium
C: Low
Q: Medium
A: Medium
C: Medium
Q: Low
A: Medium
C: Low

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 both 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

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

High High Not sensitive
Q: High
A: High
C: High
Q: High
A: High
C: High
Q: High
A: High
C: High

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 assessmentCR.LCR.BrAs.AmenCio and CR.LCR.BrAs.AmenCio.Ant are found in silty, circalittoral rock, wave-sheltered conditions, with sea lochs.  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’.

Medium High Low
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium
Q: Medium
A: Medium
C: Medium

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 5 cm 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). Antedon spp. 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 5cm of sediment is likely to impact 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.

Low Low High
Q: Medium
A: Low
C: Medium
Q: Medium
A: Low
C: Medium
Q: Medium
A: Low
C: Medium

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 probably ‘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’ at the benchmark level.

Not Assessed (NA) Not assessed (NA) Not assessed (NA)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not assessed

No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No evidence

High High Not sensitive
Q: Low
A: Low
C: Low
Q: Low
A: Low
C: Low
Q: Low
A: Low
C: Low

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 settled much faster in the presence of noise (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 Antedon spp. but whilst they may respond to vibrations by curling their arms in defence, they are unlikely to be considered sensitive.

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. 

High High Not sensitive
Q: Low
A: Low
C: Low
Q: Medium
A: Low
C: Medium
Q: Low
A: Low
C: Low

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.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant: barriers and changes in tidal excursion are not relevant to biotopes restricted to open waters.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Not relevant.

Biological Pressures

 ResistanceResilienceSensitivity
No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

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.

No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

There is ‘No evidence’ regarding known invasive species posing a threat to CR.LCR.BrAs.AmenCio or CR.LCR.BrAs.AmenCio.Ant.

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 matts over the substrata it colonizes; binding boulders, 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.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: Medium
A: Low
C: Medium
Q: Low
A: Low
C: Low

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.

Low Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Low
C: Medium
Q: Low
A: Low
C: Low

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’.

Low Medium Medium
Q: Low
A: NR
C: NR
Q: Medium
A: Low
C: Medium
Q: Low
A: Low
C: Low

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 characterising 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’.

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Citation

This review can be cited as:

Readman, J.A.J., 2016. Solitary ascidians, including [Ascidia mentula] and [Ciona intestinalis], on wave-sheltered circalittoral rock. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/48

Last Updated: 19/01/2016