Neopentadactyla mixta in circalittoral shell gravel or coarse sand

Summary

UK and Ireland classification

Description

Sublittoral plains of clean, shell, maerl, stone gravels or sometimes coarse sands, with frequent Neopentadactyla mixta. Pecten maximus may occur occasionally along with Lanice conchilega. Other epifaunal species may include Ophiura albida, Pagurus spp. and Callionymus spp. These sediments may be thrown into dunes by wave action or tidal streams. Widespread species such as Cerianthus lloydii and Chaetopterus variopedatus are present in many examples of this biotope. Scarcely recorded species such as Molgula oculata, Ophiopsila annulosa and Amphiura securigera may also be found. Ophiopsila annulosa only occurs in records from the south-west of the British Isles. It should be noted that Neopentadactyla may exhibit periodicity in its projection out of, and retraction into, the sediment (Picton 1993). (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 04.05: Connor et al., 2004).

Depth range

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

Additional information

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Habitat review

Ecology

Ecological and functional relationships

  • The gravel sea cucumber, Neopentadactyla mixta, burrows in coarse, typically mobile shell sand, gravel or maerl where water flow is quite strong. The gravel sea cucumber is an infaunal burrower and is only visible when the tentacles are projected above the surface. When extended, the tentacular crown can be up to a quarter of the body length and have a spread of 140 square cm. The body is generally held in a u-shape within the sediment with the tentacles held in the water column and the terminal anus just at the surface. Food particles are trapped using special adhesive areas at the tips of the tentacles. To ingest food, a tentacle is inserted into the mouth, the buccal membrane constricts and the tentacle withdrawn, scraping off any adherent food particles.
  • Neopentadactyla mixta lives gregariously and can reach densities of up to 400 per square metre in loose gravels such as maerl (Smith & Keegan, 1985). Such an abundance of burrowing sea cucumbers may prevent the colonization of other macrofauna and therefore excluding them from this biotope.
  • Other echinoderms are also present and often abundant in this biotope. Brittle stars, Ophiura albida, Ophiospila sp., Amphiura sp., typically inhabit the top layer of sediment. The sea urchin Echinus esculentus is an important grazer.
  • The dominant trophic group is suspension feeders. Neopentadactyla mixta is a passive suspension feeder and requires a reasonable flow of water to provide sufficient food particles. The tentacular crown is held up in the water column in order to feed. Predation is predominantly by fish, Callionymus sp., crabs, Pagarus sp., and starfish, Asterias rubens. If present in high abundance, the arms of Amphiura sp. can be an important food source for demersal fish (Callionymus sp.) providing significant energy transfer to higher trophic levels.
  • Melanella alba, an eulimid gastropod is a temporary ectoparasite on Neopentadactyla mixta, piercing the skin and feeding on the internal organs.
  • Cloak anemones, Adamsia carcinopados, occur attached to the gastropod shells of hermit crabs, Pagurus prideaux. The association appears to be obligatory between the two species and they are not generally found apart in normal circumstances and both degenerate quickly if separated. The base of the sea anemone secretes a chitinous membrane which effectively increases the size of the gastropod shell so as the crab grows it does not need to change shells.

Seasonal and longer term change

Neopentadactyla mixta spend much of the winter buried deep in aerobic mixed sediment. During this winter period, a torpid stage is entered with respiration and activity greatly reduced. Torpor exhibited by this species is marked by a considerable deterioration in body condition, a decline in tissue lipid content, and reduced metabolism. Given sufficient aeration, this species can tolerate long periods (up to 8 months) without feeding due to the use of long-term nutrient reserves stored as lipids and some proteins. The period of feeding cessation and torpor is backed up by previous workers unable to find populations of feeding Neopentadactyla mixta during winter months (Smith & Keegan, 1985). Smith (1981) reported a reduction of tissues in the gonad and substantial loss in gonad weight over the winter period with a concomitant loss of lipid.Neopentadactyla mixta also exhibit daily feeding activity rhythms. Although not necessarily representative of all populations, Neopentadactyla mixta exhibits regular daily and seasonal movements within the substratum. In the Kilkieran Bay population, individuals withdraw further into the sediment between 1-4 hours after sunrise and remain in the substratum for 1-2 hours, re-emerging over a period of up to four hours. The stimuli for the initiation of feeding activity remains unclear but it seems that light and temperature change are major cues affecting daily and seasonal feeding activity rhythms, respectively.

Habitat structure and complexity

The habitat of this biotope is complex. Maerl (dead and live) and gravel are often loose and mobile preventing colonization by many species. However, the majority of species within this biotope live below the gravel surface, notably deep burrowing fauna (Hall-Spencer & Atkinson, 1999). Burrowing fauna and tube building polychaetes (e.g. Lanice conchilega) are important for sediment stabilizing. The tubes modify benthic boundary layer hydrodynamics (Eckman et al., 1981), can provide an attachment surface for filamentous algae (Schories & Reise, 1993) and serve as a refuge from predation (Woodin, 1978; Zühlke et al., 1998). Tubes of Lanice conchilega can penetrate several tens of centimetres into the sediment. Such burrows and tubes allow oxygenated water to penetrate into the sediment indicated by 'halos' of oxidized sediment along burrow and tube walls. Other fauna probably help in stabilizing the substratum. The tube anemone Cerianthus lloydii extends above the sediment surface.

Productivity

Production in the biotope is mostly secondary, dependent upon detritus and organic material. Some primary production comes from benthic macroalgae and water column phytoplankton. The dominant trophic group therefore is suspension feeders. In the relatively shallow waters around the British Isles secondary production in the benthos is generally high, but shows seasonal variation (Wood, 1987). Generally, secondary production is highest during summer months, when temperatures rise and primary productivity is at its peak. Spring phytoplankton blooms are known to trigger, after a short delay, a corresponding increase in productivity in benthic communities (Faubel et al., 1983).

Recruitment processes

The majority of benthic marine invertebrates, particularly echinoderms, suffer high juvenile/post-settlement mortality (Gosselin & Qian, 1997), various environmental factors play an important role in the recruitment processes of echinoderms, such as, predation, disease and migration. Very little is known about settlement in holothuroids and no information has been found in relation to the life history strategies of Neopentadactyla mixta. Breeding is presumed to occur between April and September when the population is at the substratum surface. Holothuroids are predominantly gonochoristic and broadcast spawners, some are brooders or hermaphrodites. The larvae of some species show planktotrophy, others lecithotrophy, some direct development, others indirect. The scallop Pecten maximus appears to have a long breeding period with peaks in spring and autumn (Fish & Fish, 1996). The veliger larvae are planktonic for about three to four weeks and settle on a wide range of substrate including algae, bryozoans and hydroids. Lanice conchilega is a polychaete species with separate sexes. The species has two larval stages, the last stage; an aulophora larva lives for about 4-6 weeks in the plankton (Kessler, 1963). This species has a reported lifespan of 1-2 years (Beukema et al., 1978). Kuhl (1972) reported that the larvae of Lanice conchilega are released between April and October. Experimental data and field studies from the Wadden Sea revealed that the existence of 'hard substrate', preferentially tubes of conspecific adults, was a requirement for initial settlement of Lanice conchilega larvae, although single juveniles were also observed to settle on eroded shells of cockles and soft-shelled clams (Heuers, 1998; Heuers et al., 1998). Tyler (1977) found that populations of Ophiura albida in the Bristol Channel had a well-marked annual reproductive cycle, with spawning taking place in May and early June. Spent adults and planktonic larvae were observed up to early October. In contrast the larger Ophiura ophiura had a more protracted breeding season.

Time for community to reach maturity

No information was found on the life history strategy of Neopentadactyla mixta. Amphiura sp. and Pecten maximus are long lived and take a relatively long time to reach reproductive maturity. It takes approximately 5-6 years for Amphiura sp. to reach maturity. Mortality of settling Amphiura sp. is reported to be extremely high, with less than 5% contributing to the adult population in any given year (Muus, 1981). Pecten maximus reaches sexual maturity within the first 2-3 years and has a lifespan of 10-20 years. The suggested lifespan for Ophiura ophiura in the west of Scotland is 5-6 years (Gage, 1990).

Additional information

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Preferences & Distribution

Habitat preferences

Depth Range 10-20 m, 20-30 m, 30-50 m
Water clarity preferencesNo information
Limiting Nutrients No information
Salinity preferences Full (30-40 psu)
Physiographic preferences Enclosed coast or Embayment, Open coast
Biological zone preferences Circalittoral, Lower infralittoral
Substratum/habitat preferences Coarse clean sand, Gravel / shingle
Tidal strength preferences Moderately strong 1 to 3 knots (0.5-1.5 m/sec.), Very weak (negligible), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferences Exposed, Moderately exposed
Other preferences

Additional Information

Species composition

Species found especially in this biotope

    Rare or scarce species associated with this biotope

    -

    Additional information

    Sensitivity review

    Sensitivity characteristics of the habitat and relevant characteristic species

    The coarse sediment and the abundance of Neopentadactyla mixta define this biotope (SS.SCS.CCS.Nmix). Neopentadactyla mixta is recorded as frequent (ca 1-9 /100m2) in coarse gravel (biotope SS.SCS.CCS.Nmix) and maerl (biotope SS.SMp.Mrl.Pcal.Nmix) and can reach high densities, for example, >400/m2 on the west coast of Ireland (Könnecker & Keegan, 1973; Keegan et al., 1985).  It is the dominant and only important characterizing species within the CCS.Nmix biotope. The other characteristic species are found in a range of coarse sediment biotopes or are otherwise widespread.  A significant reduction in the abundance of, or loss of, Neopentadactyla mixta would result in loss of the biotope as described in the habitat classification. Therefore, the sensitivity of the biotope is dependent on the sensitivity of Neopentadactyla mixta.  The sensitivity of other characteristic species is mentioned where relevant.

    Resilience and recovery rates of habitat

    Little is known about the population dynamics of Neopentadactyla mixta, or their life history.  Their abundance in coarse sediments might suggest either good local recruitment and or sporadic but high-level recruitment. For example, Keeghan et al. (1985) recorded adult densities of ca 420/m2 together with juvenile densities of ca 15,000/m2 (at different locations) on the west coast of Ireland. Breeding is presumed to occur between April and September when the population is at the substratum surface.  Neopentadactyla mixta is dioecious, with large eggs (ca 300 µm in size) (Smith & Keegan, 1985). As a result, Southward & Campbell (2006) been suggested that larval development is lecithotrophic.

    As a group, echinoderms are highly fecund; producing long-lived planktonic larvae with high dispersal potential.  However, recruitment in echinoderms is poorly understood, often sporadic and variable between locations and dependent on environmental conditions such as temperature, water quality, and food availability.  For example, in the heart urchin Echinocardium cordatum recruitment was recorded as sporadic, only occurring in 3 years out of a 10 year period (Buchanan, 1967).  Millport populations of Echinus esculentus showed annual recruitment, whereas few recruits were found in Plymouth populations during Nichols studies between 1980 and 1981 (Nichols,  1984).  Similarly, 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.

    Overall, there is no direct evidence of larval development, recruitment and/or population dynamics in Neopentadactyla mixta.  As many echinoderms show sporadic and variable recruitment, any population could take anywhere from one year to perhaps ten years to recruit and recolonize a habitat from which they were reduced in abundance and or removed.  Therefore, resilience is given a precautionary rank of Medium (2-10 years).  However, the assessment of resilience is made by inference from the life history of members of the same phylum, so confidence is Low based on expert judgement. 

    Hydrological Pressures

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    ResistanceResilienceSensitivity
    Temperature increase (local) [Show more]

    Temperature increase (local)

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

    Evidence

    Little information on temperature tolerances was found. The assessment is based on reported global distribution. The majority of records of Neopentadactyla mixta occur in the British Isles although its range extends from the Faeroe Islands, the west coast of Norway (Molde),the Barents Sea to the Bay of Biscay (Southward & Campbell, 2006; OBIS, 2016).  Based on this evidence, it is likely to tolerate a chronic change in temperature.

    Neopentadactyla mixta is not reported from shallow water, and it is only likely to be exposed to acute temperature changes due to thermal effluents. It is likely to withdraw into the sediment, away from the thermal plume, and be protected by the temperature of the interstitial waters.  Only long-term acute change (greater than the benchmark) is likely to adversely affect the population. In winter months, it is probably too deep to be affected by significant decreases in temperature as it burrows to a depth of 30-60 cm into the substratum (Smith & Keegan, 1985). Smith & Keegan (1985) suggested that light or winter temperature might be one cue for seasonal torpor but noted that winter turbulence and increased turbidity, due to water movement, may also induce Neopentadactyla mixta to overwinter at depth.

    Sensitivity assessment. Therefore, if exposed to a short-term acute change i.e. 5°C for a month, it will probably withdraw into the sediment and be unable to feed, resulting in a temporary loss of condition. Overall, a resistance of High is suggested, with a resilience of High so that the biotope is assessed as Not sensitive at the benchmark level.

    High
    Low
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    High
    High
    High
    High
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    Not sensitive
    Low
    Low
    Low
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    Temperature decrease (local) [Show more]

    Temperature decrease (local)

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

    Evidence

    Little information on temperature tolerances was found. The assessment is based on reported global distribution. The majority of records of Neopentadactyla mixta occur in the British Isles although its range extends from the Faeroe Islands, the west coast of Norway (Molde),the Barents Sea to the Bay of Biscay (Southward & Campbell, 2006; OBIS, 2016).  Based on this evidence, it is likely to tolerate a chronic change in temperature.

    Neopentadactyla mixta is not reported from shallow water, and it is only likely to be exposed to acute temperature changes due to thermal effluents. It is likely to withdraw into the sediment, away from the thermal plume, and be protected by the temperature of the interstitial waters.  Only long-term acute change (greater than the benchmark) is likely to adversely affect the population. In winter months, it is probably too deep to be affected by significant decreases in temperature as it burrows to a depth of 30-60 cm into the substratum (Smith & Keegan, 1985). Smith & Keegan (1985) suggested that light or winter temperature might be one cue for seasonal torpor but noted that winter turbulence and increased turbidity, due to water movement, may also induce Neopentadactyla mixta to overwinter at depth.

    Sensitivity assessment. Therefore, if exposed to a short-term acute change i.e. 5°C for a month, it will probably withdraw into the sediment and be unable to feed, resulting in a temporary loss of condition. Overall, a resistance of High is suggested, with a resilience of High so that the biotope is assessed as Not sensitive at the benchmark level.

    High
    Low
    NR
    NR
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    High
    High
    High
    High
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    Not sensitive
    Low
    Low
    Low
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    Salinity increase (local) [Show more]

    Salinity increase (local)

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

    Evidence

    Echinoderms are restricted to the marine environment and one of the only stenohaline phyla in the animal kingdom (Russell, 2013).  Although some species can acclimatise to hypo/hypersaline conditions, Russell (2013) did not mention Neopentadactyla mixta amongst them.  Smith (1983) noted that hypo or hypersaline water caused the animal to withdraw its tentacles. 

    Neopentadactyla mixta is not reported from shallow water, and it is only likely to be exposed due to hypo/hypersaline effluents. Roberts et al. (2010b) reported that hypersaline effluents from desalination plants disperse with tens of metres of the discharge point but reported widespread alteration in seagrass and soft sediment communities in poorly flushed environments. Echinoderms and ascidians were amongst the most sensitive to hypersaline brine in the studies examined (Roberts et al., 2010b).  While hypersaline effluents are likely to sink to the seabed, and potentially penetrate into the sediment, the water movement characteristic of this biotope is likely to disperse the effluent and limit the effect to the immediate vicinity of any discharge point.

    Sensitivity assessment.  An increase in salinity above 40 psu is likely to be detrimental to Neopentadactyla mixta and interrupt feeding but if prolonged for a year (see benchmark) may result in the death of individuals in the vicinity of the discharge.  Therefore, a precautionary resistance assessment of Medium is suggested but with Low confidence. Resilience is probably Medium so that sensitivity is assessed as Medium.

    Medium
    Low
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    Medium
    Low
    NR
    NR
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    Medium
    Low
    Low
    Low
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    Salinity decrease (local) [Show more]

    Salinity decrease (local)

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

    Evidence

    Echinoderms are restricted to the marine environment and one of the only stenohaline phyla in the animal kingdom (Russell, 2013).  Although some species can acclimatise to hypo/hypersaline conditions, Russell (2013) did not mention Neopentadactyla mixta amongst them. Smith (1983) noted that hypo or hypersaline water caused the animal to withdraw its tentacles. 

    Sensitivity assessment.  The biotope and Neopentadactyla mixta are only recorded from ‘full’ marine conditions. A reduction is salinity to reduced (18- <30 psu) for a year is likely to reduce feeding or drive Neopentadactyla mixta into the sediment where it cannot feed. Its seasonal torpor lasts from September to March each year, during which it loses condition significantly, it is unlikely to survive for a year without feeding.  Therefore, a resistance of None is suggested but with Low confidence.  Resilience is probably Medium so that sensitivity is assessed as Medium.

    None
    Low
    NR
    NR
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    Medium
    Low
    NR
    NR
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    Medium
    Low
    Low
    Low
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    Water flow (tidal current) changes (local) [Show more]

    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

    Neopentadactyla mixta occurs in maerl beds and coarse gravel sediments, both of which are associated with water flow either due to tidal streams (moderately strong to weak, Connor et al. 2004) or wave mediated water movement (exposed to moderately wave exposed).  For example, the beds of Neopentadactyla mixta in coarse sediments examined by Konnecker & Keegan (1973) were found in tidal currents of up to 2.5 knots (ca 1.28m/s).  Nevertheless, artificially increased current beyond the calm weather, spring tide, maximum of ca 1.5 m/s caused Neopentadactyla mixta to stop feeding a withdraw into its burrow, as did bombardment with dislodged sediment (Smith & Keegan 1985). Similarly, a heavy gale in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days (Smith & Keegan, 1985).  The species regularly undertakes a ca six month long torpor period, during which it loses condition and lipid energy stores.  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food availability coupled with increased turbulence experienced in winter at their study site.  An increase in water flow may also modify the sediment, causing a loss of the sediment from the surface and mobilisation of the bed, although these sediments routinely bear mega-ripples caused by current flow and storms.  However, a decrease in flow will probably result in deposition of fine sediments and detritus, resulting in a change in sediment type, and a complete change in the biological community,

    Sensitivity assessment.  Water flow (due to tidal flow or wave action) is an important structuring factor in habitats dominated by Neopentadactyla mixta (e.g. SS.SCS.CCS.Nmix and SS.SMp.Mrl.Pcal.Nmix), maintaining an open matrix of maerl or coarse sediment, removing fine sediments, allowing oxygenation deep within the sediment and providing adequate food supply to suspension feeders such as Neopentadactyla mixta. In areas of weak flow the biotope probably experience higher wave action, while in areas of moderate wave exposure, tidal flow is probably more important.  However, a change in water flow of 0.1-0.2 m/s is probably of limited effect in the biotopes normal range of <0.5 to 1.5 m/s, especially if low flow occurs in wave exposed areas. Therefore, a resistance of High is suggested, with a resilience of High so that the biotope is probably Not sensitive at the benchmark level.  

    High
    Low
    NR
    NR
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    High
    High
    High
    High
    Help
    Not sensitive
    Low
    Low
    Low
    Help
    Emergence regime changes [Show more]

    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

    The pressure benchmark is relevant only to littoral and shallow sublittoral fringe biotopes.

    Not relevant (NR)
    NR
    NR
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    Not relevant (NR)
    NR
    NR
    NR
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    Not relevant (NR)
    NR
    NR
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    Wave exposure changes (local) [Show more]

    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

    Neopentadactyla mixta occurs in maerl beds and coarse gravel sediments, both of which are associated with water flow either due to tidal streams (moderately strong to weak, Connor et al. 2004) or wave mediated water movement (Exposed to Moderately exposed). Smith & Keegan (1985) noted that a  heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days.  The species regularly undertakes a ca six month long torpor period, during which it loses condition and lipid energy stores.  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food availability coupled with increased turbulence experienced in winter at their study site. 

    Sensitivity assessment.  Water flow (due to tidal flow or wave action) is an important structuring factor in habitats dominated by Neopentadactyla mixta (e.g. SS.SCS.CCS.Nmix and SS.SMp.Mrl.Pcal.Nmix), maintaining an open matrix of maerl or coarse sediment, removing fine sediments, allowing oxygenation deep within the sediment and providing adequate food supply to suspension feeders such as Neopentadactyla mixta.  An increase in wave action may also modify the sediment, causing a loss of the sediment from the surface and mobilisation of the bed, although these sediments routinely bear mega-ripples caused by current flow and storms.  However, a decrease in wave action (in areas of low water flow) will probably result in deposition of fine sediments and detritus, resulting in a change in sediment type, and a complete change in the biological community. However, a change in nearshore wave height of >3% but <5%) is unlikely to be significant. Therefore, a resistance of High is suggested, with a resilience of High so that the biotope is probably Not sensitive at the benchmark level.  

    High
    Low
    NR
    NR
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    High
    High
    High
    High
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    Not sensitive
    Low
    Low
    Low
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    Chemical Pressures

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    ResistanceResilienceSensitivity
    Transition elements & organo-metal contamination [Show more]

    Transition elements & organo-metal contamination

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

    Evidence

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

    Not Assessed (NA)
    NR
    NR
    NR
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    Not assessed (NA)
    NR
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    NR
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    Not assessed (NA)
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    NR
    NR
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    Hydrocarbon & PAH contamination [Show more]

    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.

    Not Assessed (NA)
    NR
    NR
    NR
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    Not assessed (NA)
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    NR
    NR
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    Not assessed (NA)
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    Synthetic compound contamination [Show more]

    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.

    Not Assessed (NA)
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    NR
    NR
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    Not assessed (NA)
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    Not assessed (NA)
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    Radionuclide contamination [Show more]

    Radionuclide contamination

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

    Evidence

    No evidence was found

    No evidence (NEv)
    NR
    NR
    NR
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    Not relevant (NR)
    NR
    NR
    NR
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    No evidence (NEv)
    NR
    NR
    NR
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    Introduction of other substances [Show more]

    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.

    Not Assessed (NA)
    NR
    NR
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    Not assessed (NA)
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    Not assessed (NA)
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    De-oxygenation [Show more]

    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

    Neopentadactyla mixta probably needs coarse sediments to survive, as the open matrix provided by coarse sediments or maerls at depth, together with water flow, ensures that the water is oxygenated at depth in the sediment. Neopentadactyla mixta reduces its metabolism and oxygen consumption from 0.11 ml O2/ gm dry wt. to 0.03 ml O2/ gm dry wt. during its overwinter torpor (Smith & Keegan, 1985). Therefore, it might be able to survive lower oxygen levels overwinter than in spring, summer and autumn.

    Lawrence (1996) reported mass mortality of echinoderms in the Gulf of Trieste due to hypoxia caused by a strong thermocline combined with high pelagic productivity and eutrophication. The brittlestar Ophiura quinquemaculata was killed with a few days, holothurians including Ocnus planci (as Cucumaria planci), starfish Asteropecten sp. and the remaining brittlestars were killed within a week. Echinoderms were shown to be intolerant of the effects of algal blooms, resulting in mortalities of the sea urchins Echinus esculentus and Paracentrotus lividus, and the holothurian Labidoplax digitata amongst other echinoderms, probably due to hypoxia caused by death of the algal bloom algae (Boalch, 1979; Forster, 1979; Griffiths et al., 1979; Lawrence, 1996). Diaz & Rosenberg (1995, Figure 5) suggested that shrimp and crustaceans were lost as oxygen levels dropped below ca 0.75 ml/l and that the macroinfauna was reduced below ca 0.4ml/l.

    Vaquer-Sunyer & Duarte (2008) suggested a median sublethal oxygen concentration of 1.22 mg O2/l (± 0.25) for a number of echinoderms reviewed in their study.  Echinoderms were neither the most or the least sensitive of the taxonomic groups examined. 

    Sensitivity assessmentNeopentadactyla mixta may be more resistant of decrease oxygen levels while in it winter torpor. No information on juveniles was found. However, the species has a preference of coarse, mobile, deposits in areas of moderate to strong water flow (Konnecker & Keegan, 1973; Keegan et al., 1985). This suggests that it prefers well oxygenated habitats.  The evidence from the Gulf of Trieste also suggests that echinoderms are sensitive to hypoxia. Therefore, a resistance of Low is suggested based on expert judgement. Resilience is probably Medium so that the biotope is assessed as Medium sensitivity at the benchmark level.

    Low
    Low
    NR
    NR
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    Medium
    Low
    NR
    NR
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    Medium
    Low
    Low
    Low
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    Nutrient enrichment [Show more]

    Nutrient enrichment

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

    Evidence

    Nutrient enrichment can lead to increase in algal growth and algal blooms, whose subsequent death results in hypoxia or even anoxia. Nutrient enrichment can also result in increased bacterial growth in sediments, that also result in hypoxia. However, this biotope occurs in well flushed habitats so that only continuous of extreme enrichment is likely to be detrimental. But no direct evidence was found. However, the biotope is assessed as Not sensitive at the pressure benchmark of compliance with good status as defined by the WFD.

    Not relevant (NR)
    NR
    NR
    NR
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    Not relevant (NR)
    NR
    NR
    NR
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    Not sensitive
    NR
    NR
    NR
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    Organic enrichment [Show more]

    Organic enrichment

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

    Evidence

    Organic enrichment due to sewage and other effluents has been implicated in the loss of maerl beds and a complete shift in their resident communities.  For example in Brittany, numerous maerl beds were affected by sewage outfalls and urban effluents, resulting in increases in contaminants, suspended solids, microbes and organic matter with resultant deoxygenation (Grall & Hall-Spencer, 2003).  This resulted in increased siltation, higher abundance, and biomass of opportunistic species, loss of sensitive species and reduction in biodiversity.  Grall & Hall-Spencer (2003) note that two maerl beds directly under sewage outfalls were converted from dense deposits of live maerl in 1959 to heterogeneous mud with maerl fragments buried, under several centimetres of fine sediment, with communities dominated by only a few species by 1997.  Similarly, changes in sediment community structure from diverse communities to communities dominated by opportunistic deposit feeders is well documented (Pearson & Rosenberg 1978; Diaz & Rosenberg 1995).

    Sensitivity assessment. Although the evidence available could not be compared directly with the benchmark, the evidence suggests that organic enrichment could lead to a complete change in the community and loss of Neopentadactyla mixta populations.  In addition, while this biotope is not characterized by maerl, the coarse sediment provides a similar open matrix, and would probably respond to organic enrichment in a similar manner. However, it is not possible to compare the reported evidence to the benchmark level of impact.  Therefore, a resistance of Low is suggested.  A resilience of Low is suggested as the habitat would need to recover before the species could return.  Sensitivity is, therefore, assessed as High. 

    Low
    Medium
    Medium
    Medium
    Help
    Medium
    Low
    NR
    NR
    Help
    Medium
    Low
    Low
    Low
    Help

    Physical Pressures

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    ResistanceResilienceSensitivity
    Physical loss (to land or freshwater habitat) [Show more]

    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.

    None
    High
    High
    High
    Help
    Very Low
    High
    High
    High
    Help
    High
    High
    High
    High
    Help
    Physical change (to another seabed type) [Show more]

    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 sedimentary substrata were replaced with rock substrata the biotope would be lost, as it would not longer be a sedimentary habitat and would no longer support Neopentadactyla mixta or other infauna or epifauna.

    Sensitivity assessment. Resistance to the pressure is considered ’None‘, and resilience ’Very low‘ or ‘None’ (as the pressure represents a permanent change) and the sensitivity of this biotope is assessed as ’High’.

    None
    High
    High
    High
    Help
    Very Low
    High
    High
    High
    Help
    High
    High
    High
    High
    Help
    Physical change (to another sediment type) [Show more]

    Physical change (to another sediment type)

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

    Evidence

    Neopentadactyla mixta is recorded from coarse sand and gravel sediments. Könnecker & Keegan (1973) reported that it had a preference for gravel type substrata on the west coast of Ireland and that the highest densities of individuals occurred in loose, mobile deposits. Keegan et al. (1985) reported that it occurred in coarse sediments in moderate to strong tidal stream but that it was less common in other deposits. Connor et al. (1997a) reported that Neopentadactyla mixta occurred in biotopes from gravel, algal gravel (maerl) and coarse clean sand, while this biotope (CCS.Nmix is only recorded from sandy gravel habitats (Connor et al., 2004). Therefore, a change in sediment type to fine sediments e.g. to fine sands, sands with gravel or muddy sands would result in a loss of the biotope as described by the habitat classification. Neopentadactyla mixta probably needs coarse sediments to survive, as the open matrix provided by coarse sediments or maerls at depth, together with water flow, ensures that the water is oxygenated at depth in the sediment. This is probably especially important as Neopentadactyla mixta overwinters for ca six months at depth (30-60 cm).  A change is sediment type to 100% gravel wouls also result in loss of the biotope as described by the classification. Neopentadactyla mixta may also be lost, presumably, because the higher water flow associated with gravel habitats would preclude feeding.

    Sensitivity assessment.  Therefore, a resistance of None is recorded. As the change is permanent, resilience is Very low and sensitivity is assessed as High.

    None
    High
    High
    Medium
    Help
    Very Low
    High
    High
    High
    Help
    High
    High
    High
    Medium
    Help
    Habitat structure changes - removal of substratum (extraction) [Show more]

    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

    Neopentadactyla mixta lives in the sediment is a ‘U-shaped’ posture with its oral tentacles raised above the surface and its anus just below the surface of the sediment (Smith & Keegan, 1985).  It is usually found in this position in its burrow 15-25 cm deep in the sediment (Könnecker & Keegan, 1973).  However, in the winter months (ca September to March) its burrows into the sediment to a depth of 30-60 cm.  It maintains this depth, even if the surface of the sediment is eroded or accreted (Smith & Keegan, 1985).

    Sensitivity assessment. In spring to autumn, extraction of the sediment to 30 cm is likely to remove the majority of the resident population but in winter, the majority of the population would survive as long as suitable substratum remained after extraction.  Therefore, a resistance of None is recorded to represent to worst case scenario.  Resilience is probably Medium so that sensitivity is assessed as Medium.

    None
    Medium
    Medium
    Medium
    Help
    Medium
    Low
    NR
    NR
    Help
    Medium
    Low
    Low
    Low
    Help
    Abrasion / disturbance of the surface of the substratum or seabed [Show more]

    Abrasion / disturbance of the surface of the substratum or seabed

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

    Evidence

    The burrow of Neopentadactyla mixta in spring/autumn is 15-25 cm deep, and 30-60 cm deep during its winter torpor (Smith & Keegan, 1985).  Therefore, it is unlikely to be directly impacted by surface abrasion.  For example, in long-term studies of scallop dredging and subsequent recovery (Hall-Spencer & Moore 2000a, 2000b) deep burrowing species including Neopentadactyla mixta were not impacted and their abundance changed little over the four year period.  It should be noted however that no information on juveniles was available.  Therefore, a resistance of High is suggested. Resilience is probably also High (as there is no impact to recover from) so that biotope is assessed as Not Sensitive

    High
    High
    High
    High
    Help
    High
    High
    High
    High
    Help
    Not sensitive
    High
    High
    High
    Help
    Penetration or disturbance of the substratum subsurface [Show more]

    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

    In long-term studies of scallop dredging and subsequent recovery (Hall-Spencer & Moore 2000a, 2000b) deep burrowing species including Neopentadactyla mixta were not impacted and their abundance changed little over the four year period.  However, experimental hydraulic blade dredging removed and damaged deep-burrowing species, including small numbers of Neopentadactyla mixta (Hauton et al. 2003), and affected the maerl bed to a depth of 9 cm.  Hydraulic dredging in coarse sand and gravel may have similar effects.

    Overall, penetrative gear may adversely affect Neopentadactyla mixta populations and a resistance of Medium is suggested. Resilience is likely to be Medium so that sensitivity is assessed as Medium.

    Medium
    Medium
    Medium
    Medium
    Help
    Medium
    Low
    NR
    NR
    Help
    Medium
    Low
    Low
    Low
    Help
    Changes in suspended solids (water clarity) [Show more]

    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

    This biotope occurs in well flushed areas subject to moderately strong to weak flow and/or wave exposed or moderately wave exposed conditions.  Neopentadactyla mixta is a passive suspension feeder. It holds its tentacles into the water column and particles of food and detritus stick to the sticky mucus on the tentacles while filamentous algae lodge amongst the tentacles. The tentacles are them placed into the mouth and the food consumed (Smith, 1983). It feeds on unicellular and filamentous algae, diatoms, dinoflagellates, the exoskeletons of planktonic crustaceans and other organic material (Smith, 1983). Therefore, an increase in suspended sediment may increase food availability while an increase in turbidity may reduce phytoplankton abundance.  

    Smith & Keegan (1985) noted that a  heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days (Smith & Keegan, 1985).  Smith & Keegan (1985) suggested that the overwinter torpor may be a response to poor food quality of the seston in winter months coupled with increased turbulence experienced in winter at their study site.  Perhaps poor food quality was due to lack of phytoplankton in the winter months. Smith & Keegan (1983) also noted that in strong flow the tentacles became heavily turbated but were still held into the water column.

    Sensitivity assessment.  No direct measure of turbidity normally experienced by this biotope was found. Suspension feeders require good or constant water flow and a supply of seston. So an increase in suspended sediment could provide extra food. However, in turbid conditions, the suspension feeding apparatus may become clogged or overwhelmed by particulates and the animal stop feeding.  The evidence of winter torpor in Neopentadactyla mixta may suggest that it avoids a natural increase in turbidity in the more stormy winter months, and/or avoids organic particulates in winter in preference for more energy rich phytoplankton in spring to autumn.  Therefore, a resistance of Medium is suggested to represent the potential loss of feeding and food quality if the turbidity was to increase (e.g. from clear to intermediate; see benchmark) but with Low confidence. Resilience is probably Medium so that sensitivity is assessed as Medium.

    Medium
    Low
    NR
    NR
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    Medium
    Low
    NR
    NR
    Help
    Medium
    Low
    Low
    Low
    Help
    Smothering and siltation rate changes (light) [Show more]

    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

    Neopentadactyla mixta lives in the sediment is a ‘U-shaped’ posture with its oral tentacles raised above the surface and its anus just below the surface of the sediment (Smith & Keegan, 1985).  It is usually found in this position in its burrow 15-25 cm deep in the sediment (Könnecker & Keegan, 1973).  However, in the winter months (ca September to March) its burrows into the sediment to a depth of 30-60 cm.  It maintains this depth, even if the surface of the sediment is eroded or accreted (Smith & Keegan, 1985).

    Sensitivity assessment. The tentacular crown can expand to 140 cm2 (Smith & Keegan, 1985) and probably extends to ca 4-5 cm above the substratum (expert opinion). However, the deposit of 5 cm of fine sediment would probably discourage Neopentadactyla mixta from feeding and it would probably withdraw into its burrow. Fine sediment will also penetrate the surface of the sediment in the affected area, significantly reducing water flow, and increasing the possibility of anoxia within the sediment.  If the smothering sediment remained, it would result in a complete shift of the community and loss of the Neopentadactyla mixta population.  However, in the areas of water movement in which these habitats occur it is unlikely that the smothering sediment would persist, depending on the local hydrography.  As Neopentadactyla mixta can survive ca 6 months without feeding (Konnecker & Keegan, 1973; Smith & Keegan, 1985) it is likely that resistance is High and resilience is also High. Therefore, the biotope is assessed as Not Sensitive at the benchmark level.

    High
    Low
    NR
    NR
    Help
    High
    High
    High
    High
    Help
    Not sensitive
    Low
    Low
    Low
    Help
    Smothering and siltation rate changes (heavy) [Show more]

    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

    Neopentadactyla mixta lives in the sediment is a ‘U-shaped’ posture with its oral tentacles raised above the surface and its anus just below the surface of the sediment (Smith & Keegan, 1985).  It is usually found in this position in its burrow 15-25 cm deep in the sediment (Könnecker & Keegan, 1973).  However, in the winter months (ca September to March) its burrows into the sediment to a depth of 30-60 cm.  It maintains this depth, even if the surface of the sediment is eroded or accreted (Smith & Keegan, 1985).

    Sensitivity assessment. The tentacular crown can expand to 140 cm2 (Smith & Keegan, 1985) and probably extends to ca 4-5 cm above the substratum (expert opinion). However, the deposit of 30 cm of fine sediment would probably discourage Neopentadactyla mixta from feeding and it would probably withdraw into its burrow. Fine sediment will also penetrate the surface of the sediment in the affected area, significantly reducing water flow, and increasing the possibility of anoxia within the sediment.  If the smothering sediment remained, it would result in a complete shift of the community and loss of the Neopentadactyla mixta population.  Smith & Keegan (1985) noted that a heavy gale at their study site in August caused Neopentadactyla mixta to withdraw deep into the sediment for six to ten days.  However, in the areas of water movement in which these habitats occur it is unlikely that the smothering sediment would persist, depending on the local hydrography.  As Neopentadactyla mixta can survive ca 6 months without feeding (Konnecker & Keegan, 1973; Smith & Keegan, 1985) it is likely that resistance is High and resilience is also High. Therefore, the biotope is assessed as Not Sensitive at the benchmark level.

    High
    Low
    NR
    NR
    Help
    High
    High
    High
    High
    Help
    Not sensitive
    Low
    Low
    Low
    Help
    Litter [Show more]

    Litter

    Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

    Evidence

    Not assessed. Neopentadactyla mixta is a passive suspension feeder in which small particulates stick to its mucus-covered tentacles.  It seems logical that microplastics could also stick to its tentacles and be ingested, where they occur in these habitats. However, no evidence was found.

    Not Assessed (NA)
    NR
    NR
    NR
    Help
    Not assessed (NA)
    NR
    NR
    NR
    Help
    Not assessed (NA)
    NR
    NR
    NR
    Help
    Electromagnetic changes [Show more]

    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

    No evidence (NEv)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    No evidence (NEv)
    NR
    NR
    NR
    Help
    Underwater noise changes [Show more]

    Underwater noise changes

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

    Evidence

    Neopentadactyla mixta may respond to sound vibrations and can withdraw into the sediment. Feeding will resume once the disturbing factor has passed. However, most of the species are infaunal and unlikely respond to noise disturbance at the benchmark level. Therefore, this pressure is probably Not relevant in this biotope.

    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Introduction of light or shading [Show more]

    Introduction of light or shading

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

    Evidence

    Neopentadactyla mixta exhibits a diurnal feeding pattern. On the west coast of Ireland, individuals began to withdraw into the sediment about an hour after sunrise, and had all withdrawn within 2-3 hours and remained in the sediment for 1-2 hours before emerging again over a 4 hour period (Könnecker & Keegan, 1973). Yet Könnecker & Keegan (1973) also reported that they did not show an immediate response to strong white light.  Smith & Keegan (1985) suggested that light may not be the cause of the diurnal behaviour.

    Therefore, a change in incident light or shading from artificial structures may not affect feeding behaviour in Neopentadactyla mixta.  Therefore, resistance is assessed as High so that resilience is also High and the biotope is assessed as Not sensitive to this pressure.  

    High
    Low
    NR
    NR
    Help
    High
    High
    High
    High
    Help
    Not sensitive
    Low
    Low
    Low
    Help
    Barrier to species movement [Show more]

    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 - this pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of seed or propagules.  But seed or propagule dispersal is not considered under the pressure definition and benchmark.

    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Death or injury by collision [Show more]

    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. 

    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Visual disturbance [Show more]

    Visual disturbance

    Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

    Evidence

    Most species within the biotope are burrowing and have no or poor visual perception and are unlikely to be affected by visual disturbance such as shading. Epifauna such as crabs have well developed visual acuity and are likely to respond to movement in order to avoid predators. However, it is unlikely that the species will be affected by visual disturbance at the benchmark level.

    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help

    Biological Pressures

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    ResistanceResilienceSensitivity
    Genetic modification & translocation of indigenous species [Show more]

    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 of genetic modification, breeding, or translocation was found.

    No evidence (NEv)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    No evidence (NEv)
    NR
    NR
    NR
    Help
    Introduction or spread of invasive non-indigenous species [Show more]

    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; 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 from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; 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 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 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.  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 (Tillin et al., 2020). 

    Sensitivity assessment. 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, if Crepidula colonized this biotope it would probably 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. Neopentadactyla requires well-oxygenated open substrata and is likely to be excluded from silted, smothered sediments  This is a high to moderate energy habitat, in which storms may mobilise the sediment (Smith & Keegan, 1985), which may mitigate or prevent colonization by Crepidula at high densities, although Crepidula has been recorded from areas of strong tidal streams (Hinz et al., 2011). Crepidula fornicata has the potential to colonize this habitat, especially where water movement is meditated by tidal flow rather than wave action, e.g., the deeper examples of the biotope. 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 'Medium' in examples where wave action is high and subject to storms but 'Low' in areas dominated by tidal flow. Resilience is assessed as 'Very low' as it would require the removal of Crepidula, 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 so the confidence in the assessment is 'Low' and further evidence is required. 

    Low
    Low
    NR
    NR
    Help
    Very Low
    High
    High
    High
    Help
    High
    Low
    NR
    NR
    Help
    Introduction of microbial pathogens [Show more]

    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 available on the effect of microbial pathogens.

    No evidence (NEv)
    NR
    NR
    NR
    Help
    Not relevant (NR)
    NR
    NR
    NR
    Help
    No evidence (NEv)
    NR
    NR
    NR
    Help
    Removal of target species [Show more]

    Removal of target species

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

    Evidence

    Scallops may be targeted in coarse sand and gravel habitats. Their removal may result in the physical effects discussed under ‘abrasion’ and ‘penetration’ pressures above. However, there are no clear relationships between the dominant important characterizing species Neopentadactyla mixta  and other characterizing species.  Therefore, a resistance of High is suggested so that resilience is also High and the biotope is assessed as Not sensitive.

    High
    Low
    NR
    NR
    Help
    High
    High
    High
    High
    Help
    Not sensitive
    Low
    Low
    Low
    Help
    Removal of non-target species [Show more]

    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

    Scallops may be targeted in coarse sand and gravel habitats. Their removal may result in the physical effects discussed under ‘abrasion’ and ‘penetration’ pressures above. If the dominant important characterizing species Neopentadactyla mixta was removed as bycatch and its abundance reduced significantly, then the biotope would be lost. However, experimental hydraulic blade dredging removed and damaged deep-burrowing species, including small numbers of Neopentadactyla mixta (Hauton et al. 2003b), and affected a maerl bed to a depth of 9 cm.  Hydraulic dredging in coarse sand and gravel may have similar effects.  Due to the depth of it burrow, if only a few individual or juvenile Neopentadactyla mixta are vulnerable as bycatch, then a resistance of Medium is suggested. Resilience is likely to be Medium so that sensitivity is assessed as Medium.

    Medium
    Medium
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    Citation

    This review can be cited as:

    Tyler-Walters, H.,, Durkin, O.C. & Watson, A., 2023. Neopentadactyla mixta in circalittoral shell gravel or coarse sand. 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. [cited 02-03-2024]. Available from: https://www.marlin.ac.uk/habitat/detail/389

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    Last Updated: 25/08/2023