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

Halcampa chrysanthellum has been recorded and described across all coasts in the British Isles (Hayward & Ryland, 1995a; NBN, 2015).  Its distribution beyond France is uncertain (Hayward & Ryland, 1995a) but could extend throughout most of northern Europe (Manuel, 1988).

Edwardia timida has previously been described as only being found in the south and west (Hawyward & Ryland, 1995a), however it has been recorded as occurring on the west coast of Scotland and Outer Hebrides (NBN, 2015).  Picton & Morrow (2015) note that the species is only known from a few localities (including the northern coast of France) and that it may be more widely distributed.  It is described as easily overlooked unless deliberately sought (Picton & Morrow 2015).  Manuel (1988) noted that, while edwardsiids as a group have a worldwide distribution, knowledge of distribution is patchy.

Sensitivity assessment. Neither of the characterizing burrowing anemones are at their southern distribution limit and are unlikely to be affected by an increase in temperature at the benchmark level.  Resistance is ‘High’, resilience is ‘High’ and the biotope is probably ‘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

Halcampa chrysanthellum has been recorded and described across all coasts in the British Isles (Hayward & Ryland, 1995a; NBN, 2015).  Its distribution beyond France is uncertain (Hayward & Ryland, 1995a) but could extend throughout most of northern Europe (Manuel, 1988).

Edwardia timida has previously been described as only being found in the south and west (Hawyward & Ryland, 1995a), however it has been recorded as occurring on the west coast of Scotland and Outer Hebrides (NBN, 2015).  Picton & Morrow (2015) note that the species is only known from a few localities (including the northern coast of France) and that it may be more widely distributed.  It is described as easily overlooked unless deliberately sought (Picton & Morrow 2015).  Manuel (1988) noted that, while edwardsiids as a group have a worldwide distribution, knowledge of distribution is patchy.

Sensitivity assessment

No evidence was found to suggest mortality of the characterizing species due to cold temperatures and the species appear to be widely distributed across the United Kingdom.  Records further afield are sparse, although it has been noted that these species are not well documented. Resistance is assessed as  ‘High’, resilience as ‘High’ and the biotope is assessed as ‘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 only recorded from full salinity subtidal situations. 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.  Edwardsia timida and Halcampa chrysanthellum have only been recorded in full salinity biotopes (Connor et al., 2004).

Whilst some burrowing anemones have been associated with areas that experience hypersalinity due to brine effluent from desalination plants, there are few species specific details 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

No information was found on the likely intolerance of the burrowing anemones, although it should be noted that Edwardsia timida and Halcampa chrysanthellum have only been recorded in full salinity biotopes (Connor et al., 2004). Due to the lack of evidence for the characterizing species within this biotope an assessment of ‘No evidence’ has been given.

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

SS.SCS.ICS.HchrEdw occurs across a wide range of water flow, from strong to weak (0.5 – 3 m/s).  The burrowing anemones are afforded some protection from the direct effects of water flow, however, prolonged increase in water flow could result in a restructuring of the substrata. Tidal flow may be an important structuring factor in this biotope, which has been described as periodically disturbed.  High tidal flow or wave action during spring tides is a possible mechanism by which succession is prevented (Connor et al., 2004).   A significant change in water flow could result in reclassification of the biotope, especially if a decrease resulted in the deposition of fine sediments.  However, this is unlikely at the benchmark level (a change of 0.1-0.2 m/s).  Resistance is, therefore, assessed as ‘High’, resilience as ‘High’ and the biotope is assessed as ‘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 in the 0-5 m range and may, therefore, be exposed to this pressure.  The burrowing nature of the anemones would probably confer some resistance in the event of  one hour of emergence. Whilst burrowing anemones usually occur in the sublittoral, they may be found on sheltered lower shores (Manuel, 1988).

Sensitivity assessment. The burrowing anemones would probably tolerate an increase at the benchmark level, as they could retreat into their burrows. However, a some mortality is possible and resistance is, therefore, assessed as ‘Medium’, resilience as ‘Medium’ and sensitivity is assessed as ‘Medium’ at the benchmark level.

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.SCS.ICS.HchrEdw occurs in moderately wave exposed to extremely wave sheltered locations.  Wood (2005) noted that Edwardsia timida is generally found in current swept locations without wave action and most records for Halcampa chrysanthellum are in sheltered localities (Wood, 2005).  The characterizing species have been recorded as occurring from wave exposed to sheltered habitats (Connor et al., 2004) suggesting that whilst an increase in wave action would be detrimental, the species are present in biotopes that occur at higher and lower wave exposures than are experienced in SS.SCS.ICS.HchrEdw.

Wave action may be an important structuring factor in this biotope, which has been described as periodically disturbed, with high tidal flow or wave action during spring tides as a possible mechanism by which succession is halted (Connor et al., 2004).  a significant increase in wave action may remove the gravel while a significant decrease may result in deposition of fine sands and muds.

Sensitivity assessment. Whilst an increase in wave action is likely to result in a decline in the biotope, the burrowing anemones are afforded some protection from the direct effects of wave action, however, prolonged increase in wave exposure could result in a restructuring of the substrata.  A decrease in wave action could result in less disturbance and, therefore allow the colonization of the sediment by other species, resulting in a change in biotope classification.  However, a 3-5% change in significant wave height (the benchmark) is unlikely to cause significant change.  Therefore, resistance is assessed as ‘High’, resilience as ‘High’ and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

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

Evidence

No information was found on effects of contaminants on the characterizing species of the biotopes and high-level anemone response to contaminants is presented. 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. 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 information was found on effects of contaminants on the characterizing species of the biotopes.

Nevertheless, 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 species of the biotopes and high-level 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 and was 'commonly found alive' in pools between the tide-marks which appeared to be devoid of all other animals (Smith, 1968).

Synthetic compound contamination

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

Evidence

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

No information was found on effects of contaminants on the characterizing species of the biotopes. 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.

Radionuclide contamination

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

Evidence

'No evidence' was found.

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 mg/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 biotope occurs in areas where tidal flow is moderate and, therefore, oxygenation is good. Decrease in oxygenation due to stagnation or smothering is likely to have an adverse effect on a community attuned to well-oxygenated conditions. However, as the burrowing anemones most likely spend significant periods of time in burrows where water movement is likely to be more restricted.

Diaz & Rosenberg (1995) noted that anemones include species that were reported to be particularly tolerant of hypoxia (e.g. Cerianthus sp and Epizoanthus erinaceus). 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). A major hypoxic event due a pycnocline in the Gulf of Trieste resulted in a mass mortality of benthos between 12 and 26^th September 1983 (Stachowitsch  & Avcin, 1988; Stachowitsch, 1992b), during which the oxygen levels fell below 4.2 mg/l, became anoxic, and hydrogen sulphide and ammonia were released (Faganeli et al., 1985). Amongst the epifauna, the even hypoxia resistant polychaetes and bivalves died after 4-5 days and the only organism to survive after one week were the anemones Cerianthus sp and Epizoanthus erinaceus, the gastropods Aporrhais pespelecani and Trunculariopsis trunculus and the sphinuculid Sipunculus nudis (Stachowitsch, 1992b). Reidel et al. (2001) also noted that anemones were amongst the most resistant of the species they encountered in ‘in situ’ deoxygenation experiments.

Sensitivity assessment. Whilst no evidence for the characterizing species was found, anemones have been reported as being relatively resistant to oxygen depletion.  However, mortality at the benchmark level cannot be ruled out and resistance is, therefore, assessed as ‘Medium’, resilience is ‘Medium’ and sensitivity is ‘Medium’ at the benchmark level.

Nutrient enrichment

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

Evidence

No information was available on the effect of nutrient enrichment on the characterizing 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

Borja et al. (2000) assessed Halcampa sp. as Group I 'very sensitive to organic enrichment and present under unpolluted conditions' and Edwardsia sp. as Group II 'indifferent to enrichment, always present in low densities with non-significant variations with time'.  The basis for their assessment and relation to the pressure benchmark is not clear (Tillin & Tyler-Walters, 2014).  It should be noted that both Borja et al. (2000) and Gittenberger & van Loon (2011) assessed the burrowing anemone group Cerianthus spp. as Group I 'sensitive to organic enrichment'.

Sensitivity assessment. The two characterizing anemones have been recorded as either ‘very sensitive’ or ‘indifferent’ to organic enrichment.  A cautious resistance assessment of ‘Low’ is, therefore applied, with ‘Medium’ resistance and ‘Medium’ sensitivity but 'Low' confidence.

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. A 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.SCS.ICS.HchrEdw is characterized as occurring on ‘sublittoral clean stone gravel’ (Connor et al., 2004).  The characterizing species of SS.SCS.ICS.HchrEdw 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 represent a fundamental change in the character of the biotope, with re-classification of the biotope necessary. Resistance is, therefore, assessed as None based on a change from gravel to mixed sediment. As this is a permanent change, resilience is ‘Very low’ and sensitivity is, therefore, assessed as ‘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. 

Both Halcampa chrysanthellum and Edwardsia timida have columns up to 7 cm long (Wood, 2005; Picton & Morrow, 2015).  Whilst no information for the characterizing species was found, Schäfer (1972) conducted a review of locomotion of the Cerianthids, reporting that the burrowing anemones require open tubes as they breathe with their entire body surface.

Extraction of 30 cm 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,  resistance is assessed as ‘None’ and  resilience is assessed as ‘Medium’.  Sensitivity is, therefore, assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

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

Evidence

No specific evidence for the characterizing burrowing anemones was found, however, it was noted that the similar burrowing anemone Cerianthus lloydii was 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). In addition, Cerianthus lloydii is larger and burrows to greater depths (ca 40 cm) than the characterizing burrowing anemones.

Sensitivity assessment. The biotope was reported to experience seasonal disturbance due to storms or winter wave action and tidal flows (Connor et al., 2004). Therefore, the resident fauna are probably adapted to some disturbance and surface abrasion and resistance is assessed as High. Hence, resilience is High and the biotope is assessed as Not sensitive. 

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 burrowing anemones 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).  Both Halcampa chrysanthellum and Edwardsia timida have columns up to 7 cm long (Wood, 2005; Picton & Morrow, 2015) while Cerianthus lloydii is larger and burrows may be up to 40 cm deep.  Langton & Robinson (1990) reported a 25-27 % decline in abundance of cerianthids following a marked increase in scallop dredging in the Gulf of Maine.

Sensitivity assessment. Therefore, the resistance to penetrative activities may be Low, based on the effects monitoring of cerianthid populations exposed to scallop dredging by Langton & Robinson (1990), and the fact that both Halcampa chrysanthellum and Edwardsia timida are smaller and burrow to shallower depths than Cerianthus lloydii. Hence, resilience is probably ‘Medium’ and sensitivity is assessed as ‘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

SS.SCS.ICS.HchrEdw occurs on clean gravel and across a range of water flow from weak to strong, where turbidity is likely to be low. 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.

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

Smothering and siltation rate changes (light)

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

Evidence

Whilst no information for the characterizing species 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 Cerianthus spp. compensates by the 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). 

Both Halcampa chrysanthellum and Edwardsia timida have columns up to 7 cm long (Wood, 2005; Picton & Morrow, 2015).  It is probable that the majority of the anemones would tolerate 5 cm of deposition by burrow extension and those unable would likely escape burial by abandoning their burrow.  The permanent addition of sediment would result in a change in substrata, and therefore reclassification of the biotope. However, SS.SCS.ICS.HchrEdw occurs in strong to weak water flow and the sediment would be probably be removed within a few tidal cycles in areas of strong to moderately strong flow or as a result of periodic seasonal increases in flow or wave action.

Sensitivity assessment. It is likely that the burrowing anemones would be able to extend burrows to cope with deposition of 5 cm of sediment. In the event of burial, the anemones are capable of some vertical movement and would probably escape.  Mortality is unlikely and resistance is, therefore, assessed as ‘High’, with ‘High’ resilience and the biotope is assessed as ‘Not sensitive’ at the benchmark level.

Smothering and siltation rate changes (heavy)

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

Evidence

Whilst no information for the characterizing species 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 Cerianthus spp. compensates by the 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). 

Both Halcampa chrysanthellum and Edwardsia timida have columns up to 7 cm long (Wood, 2005; Picton & Morrow, 2015).  It is probable that the majority of the anemones would tolerate 5 cm of deposition by burrow extension and those unable would likely escape burial by abandoning their burrow.  The permanent addition of sediment would result in a change in substrata, and therefore reclassification of the biotope. However, SS.SCS.ICS.HchrEdw occurs in strong to weak water flow and the sediment would be probably be removed within a few tidal cycles in areas of strong to moderately strong flow or as a result of periodic seasonal increases in flow or wave action.

Sensitivity assessment. 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, but it is assumed that the majority of anemones would probably escape by abandoning their burrows.  Periodic seasonal increases in flow or wave action will probably remove the deposited sediment in a few tidal cycles but some mortality may occur in the meantime.  Therefore, a precautionary resistance of ‘Medium’ is suggested.  Hence, resilience is probably ‘Medium’ and sensitivity is assessed as ‘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 to assess this pressure.

Underwater noise changes

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

Evidence

'No evidence' was found to assess this pressure.

Introduction of light or shading

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

Evidence

'No evidence' was found to assess this pressure.

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

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

Coastal and estuarine areas are among the most biologically invaded systems in the world, especially by molluscs such as the slipper limpet Crepidula fornicata and the pacific oyster Magallana gigas (OSPAR, 2009b). The two species have not only attained considerable biomasses from Scandinavian to Mediterranean countries but have also generated ecological consequences such as alterations of benthic habitats and communities, or food chain changes. In the Wadden Sea, the main issue of concern is the Pacific oyster (Magallana gigas), which has also spread in the Thames estuary and along French intertidal flats. Padilla (2010) predicted that Magallana gigas could either displace or overgrow mussels on rocky and sedimentary habitats of low or high energy.  However, Padilla (2010) also noted that there were no examples of Magallana gigas invading sedimentary habitats where there are no native ecosystem engineers (bivalves or Sabellaria).  

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; 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; Helmer et al., 2019; Hinz et al., 2011; Minchin et al., 1995; 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). Changes in habitat structure and reduced abundance of suspension-feeding organisms upon which the flatfish feed were also linked to slipper limpet extent (Decottignies et al., 2007; Blanchard et al. 2008; and Kostecki et al., 2011 cited in Sewell & Sweet, 2011). The presence of Crepidula as an ecosystem engineer has created a range of new niche habitats, reducing biodiversity as it modifies habitats (Fitzgerald, 2007). De Montaudouin et al. (1999) concluded that Crepidula did not influence macroinvertebrate diversity or density significantly under experimental conditions, on fine sands in Arcachon Bay, France. De Montaudouin et al. (2018) noted that the limpet reef increased the species diversity in the bed, but homogenised diversity compared to areas where the limpets were absent. In the Milford Haven Waterway (MHW), the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell or gravel or both (grain sizes 16-256 mm) but, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). Bohn et al. (2015) suggested that high densities of Crepidula in high-energy environments were possible in the subtidal but not the intertidal, suggesting the availability of this substratum type is beneficial for its establishment. Hinz et al. (2011) reported a substantial increase in the occurrence of Crepidula off the Isle of Wight, between 1958 and 2006, at a depth of ca 60 m, on hard substrata (gravel, cobbles, and boulders), swept by strong tidal streams. Presumably, Crepidula is more tolerant of tidal flow than the oscillatory flow caused by wave action which may be less suitable (Tillin et al., 2020). 

Sensitivity assessment. The sediments characterizing this biotope are likely to be too mobile or otherwise unsuitable for most of the invasive non-indigenous species currently recorded in the UK. Magallana gigas is predicted to invade sedimentary habitats, although no direct examples exist to date and Magallana gigas recruitment is lower in the subtidal (Diederich 2005, 2006; Padilla, 2010). The above evidence suggests that Crepidula fornicata could colonize coarse sediment habitats in the subtidal, typical of this biotope. Bohn et al. (2015) demonstrated that Crepidula had a preference for gravelly habitats, while De Montaudouin & Sauriau (1999) and Bohn et al. (2015) noted that Crepidula densities were low in intertidal coarse sediments. Therefore, Crepidula has the potential to colonize, and modify the habitat and its associated community due to the introduction of Crepidula shell biomass, silt, pseudofaeces and faeces (Blanchard, 2009; Tillin et al., 2020), as occurs in maerl gravels (Grall & Hall-Spencer, 2003) resulting in the loss of the biotope. Both species require hard substrata in the form of stones, debris or, preferably, the shells conspecifics to colonize the habitat. 

This is a moderately exposed to extremely sheltered energy habitat, in which wave action and storms may mobilise the sediment (JNCC, 2022), which may mitigate or prevent colonization by Crepidula at high densities, although it has been recorded from areas of strong tidal streams (Hinz et al., 2011). Therefore, the habitat may be more suitable for Crepidula in wave sheltered areas of the biotope and where water movement is mediated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. However, Crepidula reduced the density of suspension feeders and mobile Crustacea in coarse sediment even at low densities (De Montaudouin & Sauriau, 1999). 

Therefore, resistance is assessed as 'Low', especially in wave sheltered areas dominated by tidal flow. Resilience is assessed as 'Very low' as it would require the removal of Crepidula or Magallana gigas, probably by artificial means. Hence, sensitivity is assessed as 'High' based on the worst-case scenario. Crepidula has not yet been reported to occur in this biotope and there is a lack of direct evidence so the confidence in the assessment is 'Low' and further evidence is required. 

Introduction of microbial pathogens

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

Evidence

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

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 easily 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.

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