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

Ascidiella aspersa is common on all British coasts and is distributed from Norway to the Mediterranean (Hayward & Ryland, 1995b).  Cylista undata is found on all coasts of the British Isles and is distributed from Scandinavia to the Mediterranean (Hayward & Ryland, 1995b).

Both characterizing species are found across the British Isles and are not at their southern distribution limit.  Resistance is, therefore, probably ‘High’ and resilience is assessed as ‘High’ and the biotope is probablly ‘Not sensitive’ at the benchmark level.

Temperature decrease (local)

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

Evidence

Ascidiella aspersa is common on all British coasts and is distributed from Norway to the Mediterranean (Hayward & Ryland, 1995b).  Cylista undata is found on all coasts of the British Isles and is distributed from Scandinavia to the Mediterranean (Hayward & Ryland, 1995b).  Both characterizing species are found across the British Isles and are not at their northern distribution limit.  Resistance is, therefore, probably ‘High’ and resilience is assessed as ‘High’ and the biotope is probablly ‘Not sensitive’ at the benchmark level.

Salinity increase (local)

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

Evidence

The biotope is recorded from full to variable salinity. An increase from full salinity would result in hypersaline conditions.  Naser (2011) described habitats dominated by the burrowing anemone Cerianthus sp. in the areas adjacent to the outlet of the Sitra Power and Water Station, Bahrain.  This desalination outlet is associated with high temperatures, salinities, and a range of chemical and heavy metal pollutants.  Cylista undata has been recorded as occurring in variable to full salinity biotopes (Connor et al., 2004).

No information on Ascidiella aspersa was found. 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 et al., 2014).

Whilst some burrowing anemones have been associated with areas that experience high salinity due to brine effluent from desalination plants, there are is little specie specific information and ‘No evidence’ was found for the characterizing species.

Salinity decrease (local)

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

Evidence

Components of the biotope community are tolerant of variable salinities. Ascidiella aspersa is tolerant of salinities down to 18 psu (ca 18 ‰) and is often common in estuaries, whilst Ciona intestinalis is tolerant of salinities as low as 11 psu (ca 11‰) (Fish & Fish, 1996).  Cylista undata has only been recorded as occurring in variable to full salinity biotopes (Connor et al., 2004).

Sensitivity assessment. A decrease at the benchmark level from variable (18 -35‰) to reduced salinity (18-30‰) is unlikely to result in mortality.  as some of the characterizing species have been recorded in salinities of 18‰ or lower.  Resistance is therefore ‘High’, resilience is ‘High’ and the biotope is probably ‘Not sensitive’ at the benchmark level, although confidence in the assessment is 'Low'.

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

Increased tidal stream velocity may benefit some passive suspension feeders by increasing the supply of food but may also erode the substratum including removal of species attached to the substratum. The long-lived members of the community, the burrowing anemones, are firmly anchored into the sediment and therefore are unlikely to be lost.   SS.SMu.ISaMu.SundAasp occurs from negligible to moderately strong water flow (0 - 1.5 m/sec). sothat a change of 0.1-0.2 m/s is unlikely to be significant. The burrowing anemones are afforded some protection from the direct effects of water flow, however, prolonged increase in water flow could result in restructuring of the substrata.  However, a decrease in water flow is not relevant as the biotope occurs in very weak flow.

Resistance is, therefore, assessed as ‘High’, resilience as ‘High’ and the biotope is ‘Not sensitive’ at the benchmark level.

Emergence regime changes

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

Evidence

This biotope can occur at 0-5 m depth.  The burrowing nature of the anemones would probably confer some resistance in the event of one hour of emergence.  The ascidians, however, are limited to the sublittoral.  Exposure to an emergence regime is likely to cause the population to die.  Therefore, resistance is assessed as ‘Low’, resilience as ‘Medium’ and sensitivity as ‘Medium’. 

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

SS.SMu.ISaMu.SundAasp occurs in wave sheltered to extremely wave sheltered conditions.  Wood (2005) described the characterizing Cylista undata as typically found in sheltered locations.  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.  High energy wave action can be detrimental to ascidian populations, mainly through physical damage to the sea squirts and through the abrasive action of suspended sediment (Jackson, 2008).  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 remove a proportion of the population, especially in the shallower examples of the biotope.

Sensitivity assessment. The ascidians and Cylista undata are likely to be damaged by significant increases in wave exposure, particularly in the shallow examples of this biotope. However, a change at the benchmark level is unlikely to result in mortality and resistance is, therefore, ‘High’, resilience is ‘High’ and the biotope is 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

No information was found on effects of contaminants on the characterizing burrowing anemone Cylista undata. Therefore, examples of anemone response to contaminants is presented.

Mercier et al. (1998) exposed Metridium senile to tri-butyl tin contamination in surrounding water and in contaminated food. The species produced mucus 48 hours after exposure to contaminated seawater. TBT was metabolised but the species accumulated levels of butyl tins leading the authors to suggest that Metridium senile seemed vulnerable to TBT contamination. However, Mercier et al., (1998) did not indicate any mortality and, since Metridium senile is a major component of jetty pile communities immediately adjacent to large vessels coated with TBT antifouling paints, intolerance has been assessed to be low specifically to TBT. No other information has been found on effects of contaminants on other species of the biotopes, so confidence is very low.

Ascidians may be intolerant of synthetic chemicals such as tri-butyl-tin anti-foulants. Rees et al. (2001), working in the Crouch estuary, observed that six ascidian species were recorded at one station in 1997 compared with only two at the same station in 1987, shortly following the banning of TBT in antifouling paints. Also, there was a marked increase in the abundance of ascidians especially Ascidiella aspersa and Ascidia conchilega in the estuary after the ban on TBT was introduced.

However, this pressure is Not assessed but evidence is presented where available.

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.

No information was found on effects of contaminants on the characterizing burrowing anemone Cylista undata. Therefore, examples of anemone response to contaminants is presented. One month after the Torrey Canyon oil spill the dahlia anemone, Urticina felina, was found to be one of the most resistant animals on the shore, being commonly found alive in pools between the tide-marks, which appeared to be devoid of all other animals (Smith, 1968). Ignatiades & Becacos-Kontos (1970) found that Ciona intestinalis can resist oil polluted water and ascidians are frequently found in polluted habitats such as marinas and harbours (Carver et al., 2006; Aneiros et al., 2015).

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. No information was found on effects of contaminants on the characterizing burrowing anemone Cylista undata.

Hoare & Hiscock (1974) reported that the anemone Urticina felina survived near to an acidified halogenated effluent discharge in a 'transition' zone where many other species were unable to survive, suggesting a tolerance to chemical contamination. However, Urticina felina was absent from stations closest to the effluent which were dominated by pollution tolerant species (such as polychaetes). Those specimens closest to the effluent discharge appeared generally unhealthy.

However, this pressure is Not assessed but evidence is presented where available.

Radionuclide contamination

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

Evidence

‘No evidence’ was found to support an assessment.

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.

The ability of solitary ascidians to withstand decreasing oxygen levels has not been well documented. Mazouni et al. (2001) noted that the biofouling community in Thau Lagoon, France (dominated by Ciona intestinali) suffered mortality when exposed to short-term periods of anoxia.  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. 

The burrowing anemones most likely spend significant periods of time in burrows where water movement is likely to be more restricted. Stachowitsch  & Avcin (1988) reported that in two eutrophication events, anemones where relatively resistant to oxygen depletion.   In the Limfjorden, oxygen concentrations fell to below 1 mg/l in the summer of 1975, with the anemones described as the most resistant group to the event (Jeirgensen, 1980).  Stachowitsch (1984) observed mass mortality in the Gulf of Trieste over two weeks apparently due to oxygen deficiency (although no oxygen concentrations were given).  The majority of species perished within a week, with only several types of anemones and certain infaunal forms surviving into the second week, although Cerianthus sp. died after one week (Stachowitsch  & Avcin, 1988).

Sensitivity assessment.

Both the ascidians and anemones have been reported as being relatively resistant to oxygen depletion (Carver et al., 2006; Stachowitsch  & Avcin, 1988).  However, mortality at the benchmark level cannot be ruled out and resistance is, therefore, ‘Medium’, resilience as ‘Medium’ and sensitivity as ‘Medium’ at the benchmark level.

Nutrient enrichment

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

Evidence

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).  No information was available on the effect of nutrient enrichment on the burrowing anemones. 

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

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). Gittenberger & Van Loon (2011) assessed Cylista undata (studied as Sagartiogeton undatus) as Group II (indifferent to enrichment, always present in low densities with non-significant variations with time).  But the basis for their assessment and relation to the pressure benchmark is not clear (Tillin & Tyler-Walters, 2014).

Sensitivity assessment. Both ascidians and Cylista undata may be tolerant of organic enrichment and resistance is, therefore, assessed as ‘High’, but with 'Low' confidence.  Resilience is assessed as ‘High’ and the biotope is assessed as ‘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 sediment were replaced with rock or artificial substrata, this would represent a fundamental change to the biotope with reclassification necessary. Change from a mixed sediment substrata to rock would also result in loss of the infaunal component. 

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

SS.SMu.ISaMu.SundAasp is characterized as occurring on sandy mud (Connor et al., 2004).  The characterizing species of may tolerate a change in one Folk class (based on the Long, 2006 simplification), as similar species have been noted to inhabit muddy sand and fine shell breccia (see Shäfer, 1972). However, this shift in substrata would necessitate a re-classification of the biotope. Resistance is, therefore, assessed as None based on a change from sandy mud or mud to muddy sand or gravelly mud. As this is a permanent change, resilience is ‘Very low’ and sensitivity is therefore ‘High’.

Habitat structure changes - removal of substratum (extraction)

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

Evidence

Sedimentary communities are likely to be highly intolerant of substratum removal, which will lead to partial or complete defaunation, expose underlying sediment which may be anoxic and/or of a different character and lead to changes in the topography of the area (Dernie et al., 2003). Any remaining species, given their new position at the sediment / water interface, may be exposed to unsuitable conditions.

Shäfer (1972) conducted a review of locomotion of the Ceriantharia. The burrowing anemones require open tubes as they breathe with their entire body surface (Shäfer, 1972).  Cylista undata can be found 10-15 cm into the sediment (Manuel, 1988). Extraction of 30cm would probably result in total loss of the burrowing anemones as they would be unlikely to escape rapidly. 

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

Sensitivity assessment. Extraction of 30 cm of sediment will remove the characterizing biological component of the biotope.  Assuming that the revealed substratum is not altered (in terms of the Folk scale), resistance is assessed as ‘None’ and biotope resilience is assessed as ‘Low’.   Sensitivity is therefore assessed as ‘High’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

No specific evidence for the characterizing burrowing anemone Cylista undata was found, however it was noted that Cerianthus lloydii is rarely caught by fishing boats since it retreats into the burrow as the trawl net approaches (Grzimek, 1972). While Langton & Robinson (1990) reported a 25-27 % decline in abundance of cerianthids following a marked increase in scallop dredging in the Gulf of Maine, this decrease could be down to penetrative disturbance (see below).

Ascidiella aspersa and Ciona intestinalis 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 some ascidians become more abundant following disturbance events (Bradshaw et al., 2000), presumably due to high resilience and a reduction in spatial competition.

Sensitivity assessment 

Given the ability to retract into the sediment, the burrowing anemones would be quite resistant to surface abrasion events.  However, given the sessile, emerged nature of Ascidiella aspersa, damage and mortality following a physical disturbance effect are likely to be significant.  Resistance has been assessed as ‘Low’, resilience as ‘Medium’ and sensitivity as ‘Medium’.

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

No specific evidence for the characterizing Cylista undata was found, however it was noted that the similar Cerianthus lloydii is rarely caught by fishing boats since it retreats into the burrow as the trawl net approaches (Grzimek, 1972).  Langton & Robinson (1990) reported a 25-27 % decline in abundance of cerianthids following a marked increase in scallop dredging in the Gulf of Maine.  Cylista undata grows to a length of ca 12 cm (Manuel, 1988) and burrows 10-15 cm into the sediment (Manuel, 1988).  The ascidians are epifaunal and disturbance effects are likely to be analogous to the surface abrasion pressure (see above). However, Cylista undata may be more affected.

Sensitivity assessment. Based on monitoring of cerianthid populations exposed to scallop dredging by Langton & Robinson (1990), resistance is likely to be ‘Low’, resilience ‘Medium’ and sensitivity ‘Medium’.

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

A significant increase in suspended sediment may have a deleterious effect on the suspension feeding community.  It may clog feeding apparatus, which would result in a reduced ingestion over the benchmark period and, subsequently, a decrease in growth rate (Jackson, 2004).  A decrease in suspended sediment is likely to benefit the community associated with this biotope. The suspension feeders may be able to feed more efficiently due to a reduction in time and energy spent cleaning feeding apparatus.

For examples, 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 (1985b) 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.  Specific data on the characterizing Ascidiella aspersa was not found. 

No evidence on the effect of a change in suspended solids was found for the burrowing anemones Cylista undata  or Cerianthus lloydii. 

Sensitivity assessment. Resistance to this pressure is assessed as 'High', but with 'Low' confidence.  Resilience is assessed as 'High' by default and the biotope is probably 'Not Sensitive' at the benchmark level.

Smothering and siltation rate changes (light)

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

Evidence

The solitary ascidians considered in this report are permanently attached to the substratum and are active suspension feeders. Ciona intestinalis and Ascidiella aspersa grow up to 15 cm and 10 cm in length respectively (Hayward & Ryland, 1995b; Fish & Fish, 1996).  Smothering with 5 cm of sediment is likely to only affect a small proportion of the population.

Whilst no information for the characterizing Cylista undata was found, cerianthids require open tubes as they breathe with their entire body surface (Schäfer, 1972).  In the event of gradual sedimentation, the burrowing anemone compensates by upward construction of its tube.  In the event of rapid sedimentation resulting in burial, cerianthids abandon their burrow, pushing vertically to the surface of the sediment (Schäfer, 1972).  Cylista undata is up to 12 cm long (Wood, 2005).  It is probable that the majority of the anemones would tolerate the deposition by burrow extension and those unable would likely escape burial by abandoning their burrow. 

Resistance is, therefore, assessed as ‘High’, with ‘High’ resilience, and the biotope is probably ‘Not sensitive’ at the benchamrk 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

The solitary ascidians considered in this report are permanently attached to the substratum and are active suspension feeders. Ciona intestinalis and Ascidiella aspersa grow up to 15 cm and 10 cm in length respectively (Hayward & Ryland, 1995b; Fish & Fish, 1996).  When considering that his biotope occurs on sandy mud, smothering with 30 cm of sediment would likely result in significant mortality of the population, unless the sediment is rapidly removed.

Whilst no information for the characterizing anemones was found, cerianthids require open tubes as they breathe with their entire body surface (Schäfer, 1972).  In the event of gradual sedimentation, the burrowing anemone compensates by upward construction of its tube.  In the event of rapid sedimentation resulting in burial, cerianthids abandon their burrow, pushing vertically to the surface of the sediment (Schäfer, 1972). 

Cylista undata is up to 12 cm long (Wood, 2005).  Whilst it is unlikely that the characterizing species would be able to extend their burrows in the event of burial by 30 cm in a single event, the majority of anemones would probably escape by abandoning their burrows.  

It should be noted that permanent addition of sediment could result in a change in substrata, and therefore reclassification of the biotope would be necessary,  as SS.SMu.ISaMu.SundAasp occurs in negligible and weak water flow and the sediment might not be removed.

Sensitivity assessment. While a propotion of the burrowing anemones would likely escape a burial event, addition of 30 cm of fine sediment would smother the ascidians.  As SS.SMu.ISaMu.SundAasp can occur in areas that experience negligible water flow, the deposited sediment could be a permanent addition and necessitate a reclassification, given that the biotope is characterized as occurring on ‘sandy mud’.  Assuming that the sediment was removed, resistance is, therefore, assessed as ‘Low’, resilience is ‘Medium’ and sensitivity is ‘Medium’.

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’ was found.

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 settled much faster in the presence of noise (2 h- 20 h compared with 6 h-26 h for control), underwent metamorphosis more rapidly (between 10 and 20 h 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).  No evidence for burrowing anemones was found.

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

Introduction of light or shading

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

Evidence

The biotope is recorded in the infra and circalittoral and no algae were included in the recorded species list (Connor et al., 2004).  Whilst an increase in light could stimulate algal colonization, growth ceases for a number of red algae (including Chrondrus crispus)  below ca 1.0 μmol m^-2l^-1 (ca 50 Lux), and 2 μmol m^-2l^-1 (ca 100 Lux) for green algae (Leukart & Lüning, 1994) .  A change at the benchmark level (0.1 Lux) is therefore unlikely to be significant.  Resistance to this pressure is assessed as 'High' and resilience as 'High'. This biotope is, therefore, considered to be 'Not sensitive' at the benchmark level.

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' as 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

‘No evidence’ was found 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

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887-1890. It has spread through expansion and introductions along the full extent of the English Channel and into the European mainland (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Helmer et al., 2019; Hinz et al., 2011; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015).

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (18 to 40) although it prefers ca 30 (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy gravelly, shell-rich, substrata that include gravel, or shells of other Crepidula, or other species e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. But it also recorded in a wide variety of habitats including clean sands, artificial substrata, Sabellaria alveolata reefs and areas subject to moderately strong tidal streams (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020). 

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

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

The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). However, Bohn et al. (2015) noted that Crepidula occurred at low density or was absent in areas of homogenous fine sediment and areas dominated by boulders. Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. Blanchard (2009) noted that sandy areas in the Bay of Saint-Mont Michel were not colonized by Crepidula because of surface sand mobility. Thieltges et al. (2003) also noted that storm events removed some clumps of mussels and presumably Crepidula onto tidal flats where they disappeared, which caused their abundance to fluctuate. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at the Swansea east site (Powell-Jennings & Calloway, 2018).

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile and unsuitable for most of the invasive non-indigenous species currently recorded in the UK. The above evidence suggests that this biotope is unsuitable for the colonization of Crepidula fornicata due to a lack of gravel, shells, or any other hard substrata used for larvae settlement (Tillin et al., 2020), despite the sheltered to very sheltered conditions of the habitat which would be suitable for Crepidula. This biotope may be an impoverished or disturbed version of SS.SMu.IFiMu.PhiVir and if disturbed it may be unsuited for Crepidula. Hence, resistance is assessed as 'High' and resilience as 'High' so that the biotope is assessed as 'Not sensitive', although 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

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).  No evidence was found on the effect of microbial pathogens on the characterizing burrowing anemones. 

Sensitivity assessment

Whilst there is evidence of disease in Ciona intestinalis, with mortality possible in severe cases, ‘No evidence’ for the characterizing species was found. 

Removal of target species

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

Evidence

 None of the characterizing species within this biotope are currently directly targeted in the UK and hence this pressure is considered to be ‘Not relevant’.

Removal of non-target species

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

Evidence

Direct, physical impacts from harvesting are assessed through the abrasion and penetration of the seabed pressures.  The characterizing species within this biotope could be incidentally removed from this biotope as by-catch when other species are being targeted.  The loss of these species and other associated species would decrease species richness and negatively impact on the ecosystem function. Langton & Robinson (1990) reported a 25-27 % decline in abundance of cerianthids following a marked increase in scallop dredging in the Gulf of Maine.  Emergent epifauna are generally very intolerant of disturbance from fishing gear (Jennings & Kaiser, 1998).  However, studies have shown some ascidians become more abundant following disturbance events (Bradshaw et al., 2000), presumably due to high resilience and a reduction in spatial competition.

Sensitivity assessment

Removal of a large percentage of the characterizing species would alter the character of the biotope.  The resistance to removal is ‘Low’ based on the effects of scallop dredging on burrowing anemones. Resilience is assessed as ‘Medium’ and overall sensitivity as ‘Medium’.