Abra alba and Nucula nitidosa in circalittoral muddy sand or slightly mixed sediment

09-08-2006
Researched byGeorgina Budd Refereed byThis information is not refereed.
EUNIS CodeA5.261 EUNIS NameAbra alba and Nucula nitidosa in circalittoral muddy sand or slightly mixed sediment

Summary

UK and Ireland classification

EUNIS 2008A5.261Abra alba and Nucula nitidosa in circalittoral muddy sand or slightly mixed sediment
EUNIS 2006A5.261Abra alba and Nucula nitidosa in circalittoral muddy sand or slightly mixed sediment
JNCC 2004SS.SSa.CMuSa.AalbNucAbra alba and Nucula nitidosa in circalittoral muddy sand or slightly mixed sediment
1997 BiotopeSS.CMS._.AbrNucCorAbra alba, Nucula nitida and Corbula gibba in circalittoral muddy sand or slightly mixed sediment

Description

Muddy sands or slightly mixed sediments in sheltered or slightly reduced salinity environments may be characterized by the presence of the bivalves Abra alba, Nucula nitidosa and Corbula gibba as well as Nucula nucleus, Lagis koreni and Nephtys species. The echinoderms Echinocardium cordatum, Ophiura albida and Ophiura ophiura may also be present. Sandier habitats contain the CMS.AfilEcor (Amphiura filiformis and Echinocardium cordatum in circalittoral clean or slightly muddy sand) biotope and increasing silt (and depth) gives rise to the CMU.BriAchi (Brissopsis lyrifera and Amphiura chiajei in circalittoral mud) biotope. The relative density of the characterizing species in this biotope is known to vary from year to year (Molander 1962); Nucula nitidosa can, in some cases, be at least if not more prevalent than Abra alba (Salzwedel, Rachor & Gerdes 1985). (Information taken from the Marine Biotope Classification for Britain and Ireland, Version 97.06: Connor et al., 1997a, b).

Recorded distribution in Britain and Ireland

Recorded in the inshore fishing grounds of St. Andrews and Aberdeenshire, off the Cumberland coast, in waters off South West England, the outer Bristol Channel and at various locations in the Irish Sea. The biotope is also present in inner Danish waters and in the German Bight.

Depth range

10-20 m, 20-30 m

Additional information

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Habitat review

Ecology

Ecological and functional relationships

  • A hydrodynamic regime of weak tidal streams and shelter from waves (owing to the depth of the seabed offshore) creates conditions for the formation of circalittoral muddy sands. Sediments in areas of seabed largely undisturbed by water movement are less well sorted, with substantial amounts of silt and organic matter, which favours deposit and suspension feeders of all types.
  • The CMS.AbrNucCor biotope is dominated by bivalve molluscs. Bivalves that inhabit muddy low energy environments are typically deposit feeders, although suspension feeders, e.g. Corbula gibba may also be abundant, or species may be able to switch between feeding methods, e.g. Abra alba. When deposit feeding, bivalves remove, microzooplankton, organic and inorganic particles, and microbes including bacteria, fungi and microalgae from the sediment. They also probably absorb dissolved organic materials in much the same manner as when filter feeding (Dame, 1996). Deposit feeding bivalves adopt two approaches to feeding: bulk feeding and particle sorting. Some may ingest large amounts of sediment in a relatively nonselective manner, or may sort particles before they are ingested and reject the majority as pseudofaeces. Deposit feeding bivalves may process up to 20 times their body weight in sediments per hour with as much as 90 % of the sediment egested as pseudofaeces (Lopez & Levinton, 1987). Suspension feeders also demonstrate some selection of particles ingested, the efficiency of which is related to palp size rather than gill type or turbidity in the bivalve's environment. For instance, Corbula gibba demonstrated a 10% selection efficiency in feeding experiments by Kiørboe & Møhlenberg (1981).
  • Polychaetes are also characteristic of the infauna of the biotope. Members of the families Spionidae (e.g. Prionospio spp., Spiophanes bombyx, Spio filicornis) and Cirratulidae (e.g. Chaetozone setosa, Tharyx spp.) are small slender worms which burrow through the sediment and use their long anterior palps or tentacles to collect organic particles. Nephtys hombergii is carnivorous and captures molluscs, crustaceans and other polychaetes with its eversible, papillated proboscis. Other carnivorous polychaetes include glycerid polychaetes such as Goniada maculata, and Glycera alba. The flabelligerid worm, Diplocirrus glaucus is a commensal of sea urchins, e.g. Echinocardium cordatum and feeds on its faecal material (Hayward & Ryland, 1996). Some polychaetes, however, are less mobile and construct tubes or burrows in the sediment. Lagis koreni constructs a tapered tube of sand grains, open at both ends, but orientated so that the worm's head is down in the sediment, drawing water and food into its burrow below the surface. Echiuran worms, e.g. Echiurus echiurus, also create burrows within the sediment and Thomsen & Altenbach (1993) found that the numbers and biomass of bacteria and foraminifera were up to three times higher around burrows of Echiurus echiurus than in surrounding sediment.
  • The heart urchin, Echinocardium cordatum occurs in both muddy and clean sands, although it grows at a considerably slower rate in the former than the latter (Buchanan, 1966). It is a relatively large infaunal species whose burrowing activity may serve to enhance oxygenation of the sediments and make them less compact.
  • The burrowing and feeding activities of deposit feeding macrofauna, are likely to modify the fabric and increase the mean particle size of the upper layers of the substrata by aggregation of fine particles into faecal pellets. Such actions create a more open sediment fabric with a higher water content which affects the rigidity of the seabed (Rowden et al., 1998). Such alteration of the substratum surface can affect rates of particle resuspension.
  • Bioturbation by the infauna on a variety of scales is also likely to be of particular importance in controlling chemical, physical and biological processes in marine sediments, especially when the influences of physical disturbances such as wave action or strong currents are minimized (Widdicombe & Austen, 1999).
  • In summary, a mix of infaunal burrowers (bivalves, polychaetes and echinoderms) in a sedimentary biotope such as this will generate a complex and continually changing 'mosaic' of habitat patches experiencing different types and levels of disturbance. The differing responses of individual species to such patchiness are likely to be a factor in the maintenance of local species diversity. The depth of penetration into the sediment by infaunal species is also likely to be enhanced by the physical and chemical consequences of infaunal activity (Hughes, 1998).
  • Epifaunal species include brittlestars, Ophiura albida and Ophiura ophiura, these species compete with neighbours for space, as they are surface deposit feeders. Like other echinoderms inhabiting soft sediments, they have pointed rather than suckered tube feet, the latter being of little use for attachment to soft sediment (Wood, 1988). Other epifaunal organisms associated with muddy sands are predominantly mobile species, including the crabs Liocarcinus depurator, Atelecyclus rotundatus and Macropodia spp. Predatory fish are also likely to frequent the biotope to feed upon bivalves, polychaetes and brittlestar arms, and include Dover sole, Solea solea and members of the cod family. The infaunal, tube-building, polychaete Lagis koreni is a significant food-source for commercially important demersal fish, especially dab and plaice, e.g. Macer (1967), Lockwood (1980) and Basimi & Grove (1985).

Seasonal and longer term change

  • The relative density of the characterizing species in this biotope is likely to vary from year to year (Molander, 1962). Nucula nitidosa can, in some cases, be at least if not more prevalent than Abra alba (Salzwedel, Rachor & Gerdes, 1985).
  • In Red Wharf Bay on the east coast of Anglesey, many of the species there, for instance the worm Lagis koreni and Abra alba, are short lived species prone to great temporal variations in abundance (Rees et al. 1977; Rees & Walker, 1983). In contrast, longer-lived species such as the bivalve Nucula spp. Are less prone to erratic fluctuations in their abundance.

Habitat structure and complexity

The muddy sand / mixed substratum of the biotope offers little habitat complexity. However, structural diversity may be provided by either localized physiographic features created by the hydrodynamic regime or the biota. Some diversity within the substratum is provided by the burrows and burrowing activity of infauna. Most species living within the sediment are limited to the area above the anoxic layer, the depth of which will vary depending on sediment particle size and organic content. However, the presence of burrows allows a larger surface area of sediment to become oxygenated, and probably enhances the survival of a considerable variety of small species and to a greater depth (Pearson & Rosenberg, 1978).

Productivity

Benthic communities in deeper water, where light is insufficient for primary production, depend almost entirely on an input of energy via sedimentation of organic matter (Wood, 1987). Organic matter may be derived from phytoplankton, zooplankton, bacteria and faecal pellets, the supply of which is one of the main factors affecting production in these communities. If the majority of the phytoplankton and organic material is utilized in the surface waters, productivity of the seabed community would consequently be low. Estimates of productivity are available for individual species in the biotope, but specific community information was not found.
For example:
  • Abra alba: in Kiel Bay, mean annual biomass varied greatly between sites and between years: Biomass (B) =0.1-3 g AFDW (Ash Free Dry Weight) m², with a long-term average (Productivity:Biomass ratio) P:B = c 2.2 (Rainer, 1985); B = 0.1-2 g AFDW m² and P:B = 1.7-2.9 from five years of sampling at al location off the French coast (Dauvin, 1986); B = 0.3 g AFDW m² and P:B = 1.4 in the Bristol Channel, England (Warwick & George, 1980).
  • Lagis koreni: annual production P = 18.3 g AFDW/m²/yr, with an average annual biomass B = 2.5 g AFDW/m² and a productivity/biomass ratio P:B = 7.3 off the North Wales Coast (Nicolaidou, 1983, converted to AFDW after Brey, 1990).
  • Echinocardium cordatum: biomass and productivity of this species is lower than that of the other smaller infaunal species of the biotope. In the central and southern North Sea, Echinocardium cordatum accounted for only 5% of benthic biomass at muddy sites, this contrasted with 50% of the biomass in clean sandy sites (Duineveld & Jenness, 1984). In Carmarthen Bay, North Wales, productivity of the species was P = -0.012 g AFDW/m²/yr, biomass B = 5.138 g AFDW/m², with a productivity:biomass ratio P:B = -0.002 (Warwick et al., 1978).

    Recruitment processes

    Bivalve molluscs
    The bivalves which characterize the biotope typically have an 'r' type life-cycle strategy (from Krebs, 1978, after Pianka, 1970), characteristics of which are high fecundity and rapid development, that allow rapid exploitation of available habitat. For instance, high densities of newly settled Corbula gibba (30, 000-67, 000 m²) and Abra alba (16, 000-22, 000 m²) were found at locations in the Limfjord, Denmark by Jensen (1988) and growth of both species was very rapid. However, recruitment in bivalves is heavily influenced by larval and post-settlement mortality so that large population increases are offset. Larval mortality results from predation during larval pelagic stages, predation from suspension feeding macrofauna (including conspecific adults) prior to settlement, deposit feeders after settlement and from settlement in unsuitable habitats. Mortality of the juveniles of marine benthic invertebrates can exceed 30% in the first day, and several studies report 90% mortality (Gosselin & Qian, 1997). In addition to larval dispersal, dispersal of juveniles and adults occurs via burrowing (Bonsdorff, 1984; Guenther, 1991), floating (Sörlin, 1988), and also possibly by bedload transport. It is expected therefore that recruitment can occur from both local and distant populations.

    Polychaete worms
    Polychaete worms in the biotope also tend to be 'r' type life-cycle strategists, with a dispersive planktonic larval stage that follows release and fertilization of gametes. Recruitment of Lagis koreni via its pelagic larvae is typically erratic between years, but newly settled juveniles may number several thousand per square metre (e.g. Macer, 1967; Basimi & Grove, 1985). Nichols (1977) recorded an early and late summer recruitment of Lagis koreni in Kiel Bay, but with additional sporadic recruitment occurring throughout the year. However, off the North Wales coast, Nicolaidou (1983) observed only one recruitment event (in June). Colonization of any new or disturbed substrata may also occur by colonization of adults (Eagle, 1975; Rees et al., 1977). Nephtys hombergii matures between two and three years of age. In the Tyne Estuary spawning of Nephtys hombergii occurred in May and September, whilst in Southampton Water the species spawned throughout the year with peaks in July and November (Oyenekan, 1986). The pelagic life cycle of Nephtys hombergii lasts seven to eight weeks at the end of which larvae metamorphose into benthic juveniles.
    Echinoderms:
    Echinocardium cordatum demonstrates a 'K' type life-cycle strategy (Rees & Dare, 1993) and subtidal populations of Echinocardium cordatum are reported to reproduce sporadically, e.g. one population recruited in only three years over a ten year period (Buchanan, 1966). Although, the species is fecund (> 1,000,000 eggs) recruitment is infrequently successful. Recruitment success may depend on temperatures of the preceding winter in some areas (Beukema, 1985).

    Time for community to reach maturity

    Diaz-Castaneda et al. (1989) experimentally investigated recolonization sequences of benthic associations over a period of one year, following defaunation of the sediment. Recovery of the Abra alba community was rapid, recruitment occurring from surrounding populations via the plankton. The abundance, total biomass and diversity of the community all increased until a maximum was reached after 20 to 24 weeks, according to the season. The community within the experimental containers matched that of the surrounding areas qualitatively but quantitatively within 4 to 8 months depending on the seasonal availability of recruits, food supply and faunal interactions. The experimental data suggested that Abra alba would colonize available sediments within the year following environmental perturbation. Summer settled recruits may grow very rapidly and spawn in the autumn, whilst autumn recruits experience delayed growth and may not reach maturity until the following spring/summer. In the worst instance, a breeding population of Abra alba may take up to two years to fully establish. Dittman et al. (1991) observed that Nephtys hombergii was included amongst the macrofauna that colonized experimentally disturbed tidal flats within two weeks of the disturbance that caused defaunation of the sediment. In addition to larval recruitment, recolonization by polychaete worms could also occur via adult migration. In contrast to the dominant characterizing bivalves and polychaetes, Echinocardium cordatum is a long lived species and takes a relatively long time to reach reproductive maturity. Echinocardium cordatum breeds for the first time when two to three years old and recruitment of is often sporadic with reports of recruiting in only three years over a ten year period for a subtidal population (Buchanan, 1966).
    Thus it is likely that the dominant infaunal bivalve and polychaete community of the CMS.AbrNucCor biotope would recover rapidly from a disturbance and mature populations of important characterizing species be present within a year. However, other components of the community that take longer to attain maturity, e.g. Echinocardium cordatum, and in their absence the CMS.AbrNucCor biotope would be recognized but may be considered impoverished.

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Preferences & Distribution

Recorded distribution in Britain and Ireland

Recorded in the inshore fishing grounds of St. Andrews and Aberdeenshire, off the Cumberland coast, in waters off South West England, the outer Bristol Channel and at various locations in the Irish Sea. The biotope is also present in inner Danish waters and in the German Bight.

Habitat preferences

Depth Range 10-20 m, 20-30 m
Water clarity preferences
Limiting Nutrients Field unresearched
Salinity Full (30-40 psu), Variable (18-40 psu)
Physiographic Enclosed coast / Embayment
Biological Zone Circalittoral
Substratum Mixed, Muddy sand
Tidal Weak < 1 knot (<0.5 m/sec.)
Wave Sheltered, Very sheltered
Other preferences Muddy sand

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Species composition

Species found especially in this biotope

Rare or scarce species associated with this biotope

-

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Sensitivity reviewHow is sensitivity assessed?

Explanation

The small bivalves, Abra alba, Nucula nitidosa and Corbula gibba and infaunal polychaetes Lagis koreni and Nephtys hombergii are considered to be important characterizing species as, in their absence, the biotope would not be recognized. These species live within the sediment and their combined burrowing/feeding activities serve to generate a complex and shifting 'mosaic' of habitat patches in an otherwise homogenous sediment, which probably affects species diversity locally. However, no single species activity was identified as the dominant factor determining local biological structure. Although not especially abundant in the biotope, Echinocardium cordatum is a larger species whose burrowing also contributes to the localized habitat mosaic. It has been assessed as an important other species, whose presence may affect the sensitivity of the biotope.

Species indicative of sensitivity

Community ImportanceSpecies nameCommon Name
Important characterizingAbra albaA bivalve mollusc
Important characterizingCorbula gibbaBasket shell
Important otherEchinocardium cordatumSea potato
Important characterizingLagis koreniA bristleworm
Important characterizingNephtys hombergiiA catworm
Important characterizingNucula nitidosaA bivalve

Physical Pressures

 IntoleranceRecoverabilitySensitivitySpecies RichnessEvidence/Confidence
High High Moderate Major decline Moderate
Most species in the CMS.AbrNucCor biotope are infaunal or epifaunal. Although many are mobile burrowing species, they are not very fast moving and so are likely to be removed along with the substratum. The intolerance of the biotope has been assessed to be high. Recoverability has been assessed to be high, for instance, Abra alba recovered to former densities following loss of a population from Keil Bay within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986). However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).
Low Immediate Not sensitive No change High
The biotope will probably have a low intolerance to smothering by 5 cm of sediment because most species are capable of burrowing through sediment to feed, e.g. Abra alba and Lagis koreni are capable of upwardly migrating if lightly buried by additional sediment (Schafer, 1972). There may be an energetic cost expended by species to either re-establish burrow openings, to self-clean feeding apparatus or to move up through the sediment, though this is not likely to be significant. Most animals will be able to reburrow or move up through the sediment within hours or days so recovery has been assessed to be immediate. Intolerance to smothering with sediment atypical for the biotope, viscous or impermeable material would be expected to be higher.
Tolerant* Not relevant Not sensitive* No change High
Deposit feeders are the dominant trophic group in this biotope and therefore are not directly reliant on suspended matter as a food resource, although Abra alba and probably some other bivalves are also facultative filter feeders and can switch to suspension feeding should the food supply become more profitable (Lin & Hines, 1994; Salzwedel, 1979). An increase in suspended sediment will increase the rate of siltation at the sediment surface, potentially enhancing the food supply for all deposit feeders in the biotope. The community of the biotope has been assessed to be not sensitive* with the potential for growth and reproduction to be enhanced by the enhanced food supply.
Low Moderate Not relevant
Deposit feeders are the dominant trophic group in this biotope and therefore are not directly reliant on suspended matter to be able to feed. However a decrease in siltation may result in a decreased rate of deposition on the substratum surface and therefore a reduction in food availability for deposit feeders. This would be likely to impair growth and reproduction. The benchmark states that this change would occur for one month and therefore would be unlikely to cause mortality. An intolerance of low is therefore recorded. As soon as suspended sediment levels increase, feeding activity would return to normal and hence recovery is recorded as very high.
Not relevant Not relevant Not relevant No change Not relevant
The biotope occurs in the circalittoral where it is continually immersed and not subjected to desiccation. An assessment of not relevant has been made.
Not relevant Not relevant Not relevant No change Not relevant
The biotope occurs in the circalittoral where it is continually immersed. An assessment of not relevant has been made.
Not sensitive* No change
The biotope occurs in the circalittoral where it is continually immersed. An assessment of not relevant has been made.
High High Moderate Decline Moderate
The intensive working of the uppermost few centimetres of the sediment by the largely deposit feeding community, especially bivalves, produces a fluid faecal-rich surface that is easily re-suspended by even low velocity tidal currents (Rhoads & Young, 1970). The biotope is found in locations of weak (< 0.5 m/sec) tidal streams, so the benchmark increase would expose the biotope to strong currents (1.5 -3 m/sec). Over the period of one year loss of the muddy sand surface substratum is likely, along with much of the organic matter upon which the infaunal deposit feeders consume. Whilst infaunal species buried relatively deeply, such as Echinocardium cordatum are unlikely to be washed out, smaller bivalves buried at shallower depths may be periodically displaced. The intolerance of the biotope has been assessed to be high owing to the fact that benthic food deposits may become limiting and that the biotope may begin to change to another characterized by cleaner sand and more probably dominated by suspension feeding species. On return to prior conditions, increased siltation would again favour deposit feeders and a transition in community composition back to the CMS.AbrNucCor biotope may be observable within five years.
Tolerant Not sensitive* No change Moderate
The CMS.AbrNucCor biotope occurs in areas of weak water flow so the benchmark decrease in water flow rate will expose the community to conditions of almost negligible water flow. Whilst a decreased water flow would favour the deposition of particulate organic matter from suspension, the additional food resource is unlikely to be of any particular significance in this already organically enriched environment. More importantly, a decreased water flow rate may limit the dispersion of planktonic larvae, to the extent that larvae settle back into the parent population where larvae in the earliest stages are likely to be preyed upon by deposit feeders, including their parents. An intolerance assessment of low has been made owing to the reduced viability of the population that may result from poor larval recruitment. Recovery has been assessed to be very high as the adults of many of the important characterizing species will remain and produce again. In addition, larvae in the plankton are likely to be transported into the biotope from other locations and re-colonization of the substrata may also occur through re-distribution of adults.
Low Immediate Not sensitive No change Very low
The bivalves that characterize the biotope, Abra alba, Nucula nitidosa and Corbula gibba, are distributed to the south of the British Isles so are likely to be tolerant of warmer water temperatures than those around the British Isles. Growth of bivalves may be stimulated by elevated temperature. For instance, growth of Fabulina fabula (studied as Tellina fabula) correlated positively with water temperature increases up to 16°C after which temperature increase inhibited growth (Salzwedel, 1979). Growth of Echinocardium cordatum is also more rapid higher in warmer waters (Duineveld & Jenness, 1984), so several species may benefit from a chronic increase of 2°C. An acute temperature increase may cause physiological stress to infaunal organisms, owing to elevated respiration rates, although temperature changes in the water column are likely to be buffered to some extent by the depth of overlying water and the infaunal position of characterizing species. Elevated temperatures may initially enhance growth but later inhibit it as a result of energetic cost associated with sub-optimal metabolic function, therefore intolerance has been assessed to be low. Metabolic activity should return to normal within a few days or weeks at lower temperatures so recoverability has been assessed to be immediate.
Intermediate High Low Decline High
Arntz & Rumohr (1986) noted the intolerance of Abra alba to extreme low temperatures in Kiel Bay with recovery to former densities taking some two years. Significant mortality of coastal populations of Echinocardium cordatum was reported from around the British Isles and in the German Bight during the severe winter of 1962/63 (Crisp, 1964; Ziegelmeier, 1978). Arntz & Rumohr (1986) reported Lagis koreni to be intolerant of extremely low bottom temperatures. Intolerance has been assessed to be intermediate as the populations of important characterizing species may be partially destroyed by the factor. Recoverability has been assessed to be high as evidence suggests that important characterizing species are capable of recovery within 1-2 years, e.g. Abra alba recovered to former densities following loss of a population from Keil Bay owing to deoxygenation within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986).
Low Very high Very Low No change Moderate
The light attenuating effects on primary productivity resulting from an increase in turbidity are unlikely to directly affect a community dependent upon detrital organic matter for its productivity. However, the benthic fauna rely on nutrient input from pelagic and coastal fringe production (Barnes & Hughes, 1992). Increased turbidity in these areas may reduce primary production and in the long term reduce the food supply to the benthos. The fauna in the biotope may therefore suffer reduced viability as competition for food increases. However, the nutrient input to the biotope originates from a very wide area and the decrease in food supply is not likely to cause mortality over a year so the biotope intolerance has been assessed to be low. Primary production would be stimulated as turbidity decreased so recoverability has been assessed to be very high.
Tolerant* Not sensitive No change Moderate
Primary producers are not present in the biotope. However, a decrease in turbidity will mean more light is available for photosynthesis by phytoplankton in the water column above the seabed. This would increase primary production and eventually lead to greater food availability for suspension and deposit feeding species in benthic habitats as the phytoplankton die and fall to the seabed. An assessment of not sensitive* has been made as the community would benefit from elevated levels of organic matter upon which to feed.
High High Moderate Major decline Moderate
The biotope typically occurs at depths of between 10-30 m and in areas 'sheltered' to 'very sheltered' (Connor et al., 1997a) from wave action. Thus it is likely that the community would be intolerant of a period of increased wave action. Areas of the biotope in shallower regions are likely to be more susceptible to wave disturbance generated by storms than the community in deeper areas (>25 m), as wave orbital velocities decay exponentially with depth, although that is not to say that the effects of waves can be totally discounted with depth (Hall, 1994), e.g. Drake & Cacchione (1985) showed that even at 100 m depth, winter storms could transport approximately 1000 kg/m²/day of resuspended sediment across the continental shelf.
Eagle (1975) observed dramatic variations in the abundance of three community dominants (Abra alba, Lagis koreni and Lanice conchilega) in sedimentary habitats in coastal areas of Liverpool Bay, N. England, following severe storms. Eagle (1975) hypothesized that during such storm periods the infauna had been washed out relatively easily after the surface sediment fabric had been loosened by the feeding activities of the deposit feeders, e.g. bivalves and polychaetes. Following storms on the North Wales coast, Rees et al. (1977) also reported Abra alba, Lagis koreni and Echinocardium cordatum to be vulnerable to wave-induced bottom disturbance as these species were found washed up on the shore. Intolerance to an increase in wave exposure has been assessed to be high as important characterizing species are likely to be displaced from the substratum and transported from the biotope. Recoverability has been assessed to be high, for instance, Abra alba recovered to former densities following loss of a population from Keil Bay within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986). However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).
Tolerant Not sensitive* No change Moderate
The biotope occurs in areas 'sheltered' to 'very sheltered' (Connor et al., 1997a) from wave exposure so a reduction in wave exposure is not likely to have a significant impact on the biotope. At the benchmark level the biotope has been assessed not to be sensitive. If a decrease in wave exposure were also combined with a decrease in water flow rate, deoxygenation may occur in bottom waters to which some species in the biotope would be intolerant.
Tolerant Not relevant Not relevant No change Low
Little information concerning the specific effects of noise on infaunal species in the biotope was found. Bivalves maybe able to detect shear-wave vibrations that propagate along the sediment, e.g. the response of Macoma balthica to noise induced vibrations in the frequency range 50-200 Hz, consisted of frequent and intense digging attempts (Franzen, 1995). In response to localized noise species buried at a shallow depth may make attempts to bury deeper into the substratum which requires an energetic expenditure, but such action is unlikely to have a detectable effect on species viability considering that the species which characterize this biotope are active burrowers. At the benchmark level an assessment of not sensitive has been made.
Tolerant Not relevant Not relevant Not relevant Moderate
The majority of the species particularly characteristic of this biotope are infaunal and probably have little or no visual acuity. Predators such as fish have visual acuity and may be temporarily scared away or cease hunting in response to the visual presence of objects not normally found in the marine environment (see benchmark). However, the biotope is unlikely to be affected as the infauna will remain in situ. Therefore an assessment of not sensitive has been made.
Intermediate High Low No change Moderate
The infaunal element of the biotope is probably not generally subject to much human induced physical disturbance of the substratum because it does not support quantities of commercial species that require penetration into the sediment to harvest, e.g. scallop dredging. However, fishing for demersal species will disturb the surface layer of sediment and any protruding or shallow burrowing species. Shells of Abra alba, Corbula gibba and Nucula nitidosa are probably vulnerable to physical damage (e.g. by otter boards; Rumohr & Krost, 1991) but their small size relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals that pass through. Bivalves such as Ensis spp., Corbula gibba, and Chamelea gallina together with starfish were relatively resistant (Bergman & van Santbrink, 2000) to direct mortality due to bottom trawling in sandy sediments. Schafer (1972) noted that adults of Lagis koreni were incapable of re-constructing their delicate sand-tubes once removed from them, and that mortality following physical disturbance to the substratum, e.g. from trawl/tickler chain damage, is likely to be significant (de Groot & Apeldoorn, 1971). For other infaunal species that burrow deeper into the sediment, e.g. Echinocardium cordatum, immediate effects are dependant on the depth of penetration of an object, e.g. an anchor or fishing gear relative to the distribution of animals in the sediment. Smaller individuals tend to live nearer the surface and are likely to be more vulnerable. Houghton et al. (1971), Graham (1955), de Groot & Apeldoorn (1971) and Rauck (1988) refer to significant trawl-induced mortality of Echinocardium cordatum. Echinocardium cordatum has a fragile test that is likely to be damaged by an abrasive force, such as movement of trawling gear over the seabed. A substantial reduction in the numbers of the species due to physical damage from scallop dredging has been observed (Eleftheriou & Robertson, 1992). Echinocardium cordatum was reported to suffer between 10 and 40% mortality due to fishing gear, depending on the type of gear and sediment after a single trawl event (Bergman & van Santbrink, 2000). They suggested that mortality may increase to 90% in summer when individuals migrate to the surface of the sediment during their short reproductive season. Bergman & van Santbrink (2000) suggested that Echinocardium cordatum was one of the most vulnerable species to trawling.

Brittlestars such as Ophiura albida may be more tolerant of abrasion. Bergman & Hup (1992) for example, found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Brittlestars can tolerate considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration. Ramsay et al. (1998) suggest that Amphiura spp. may be less susceptible to beam trawl damage than other species like echinoids or tube dwelling amphipods and polychaetes. In an analysis of long-term effects of scallop dredging on benthic communities in the Irish Sea, Bradshaw et al. (2002) noted a decline in the sedentary, filter feeding brittlestars Ophiothrix fragilis and Ophiopholis aculeata but an increase in surface detritivores or scavenging brittlestars such as Amphiura filiformis, Ophiocomina nigra and Ophiura albida.

At the benchmark level, intolerance has been assessed to be intermediate as mortality of important characterizing species may occur. However, the community is unlikely to change significantly and recovery is likely to be rapid owing to larval recruitment, e.g. Abra alba recovered to former densities following loss of a population from Keil Bay within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986). Such evidence suggests that recoverability of important characterizing species of the biotope would be high. However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).

Intermediate Very high Low Minor decline Low
The bivalve species of this biotope are likely to be tolerant of displacement owing to their ability to reburrow into the sediment, as would Echinocardium cordatum. Errant polychaetes, such as Nephtys hombergii will return to the sediment after displacement. However, the polychaete Lagis koreni is incapable of reconstructing its delicate sand-tube once removed from it (Schafer, 1972), and hence mortality following displacement would be expected to be high for this species. The intolerance of this biotope has been assessed to be intermediate owing to mortality of an important characterizing species and the fact that normally infaunal species displaced to the surface will be prone to predation by fish and epibenthic predators such as crabs. On balance recoverability has been assessed to be very high as the majority of species would seek protection and any that were lost to predators would be replaced by new recruits within the year.

Chemical Pressures

 IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
High High Moderate Decline Moderate
Deposit feeding may be a particularly important route for exposure to synthetic chemicals within this biotope. Beaumont et al. (1989) concluded that bivalves were particularly sensitive to tri-butyl tin (TBT), the toxic component of many antifouling paints. Abra alba failed to burrow into sediment contaminated with pesticides (6000 ppm parathion, 200 ppm methyl parathion and 200 ppm malathion) (Møhlenberg & Kiørboe, 1983), such behaviour would make it prone to predation. Echinocardium cordatum was found to be highly intolerant of detergents used to disperse oil from the Torrey Canyon oil spill which caused mass mortalities of the species (Smith, 1968). Other species in the biotope, in particular polychaete worms, are generally less intolerant of a range of marine pollutants so a change in the faunal composition may be expected if chemical pollution increases. Polluted areas would be characterized by lower species diversity and a higher abundance and density of pollution tolerant species such as polychaetes Given the likely intolerance of the bivalves, biotope intolerance has been assessed to be high, species richness is expected to decline and the biotope may begin to change to another. On return to prior conditions, recoverability has been assessed to be high. For instance, Abra alba recovered to former densities following loss of a population from Keil Bay within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986). However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).
Heavy metal contamination
Intermediate High Low Decline Low
In Norwegian fjords, Rygg (1985) found a relationship between species diversity in benthic fauna communities and sediment concentrations of heavy metals Cu, Pb and Zn. Cu in particular showed a strong negative correlation and the author suggested a cause-effect relationship. Those species not present at sites where Cu concentrations were greater than ten times higher than the background level, were assessed as non-tolerant species e.g. Amphiura filiformis and several bivalves including Nucula sulcata. The most tolerant species were all polychaete worms. Therefore, increased heavy metal contamination in sediments may cause change in the faunal composition of the community and species diversity may decrease. Some dominant species characteristic of the biotope may disappear and the community may become increasingly dominated by polychaetes. The biotope is considered to have an intermediate intolerance rank owing to the fact that heavy metal contamination of the sediments may change the faunal composition of the community and decrease overall species diversity. In the absence of heavy metal contaminants re-population of the biotope is likely to occur and has been assessed to be high.
Hydrocarbon contamination
Intermediate Moderate Moderate Decline Very low
No information was found on the effect of hydrocarbon pollution specifically on the biotope. However, owing to its position in the circalittoral, the biotope may not come into direct contact with spilt oil until it has become emulsified and dispersed into the water as small droplets. Droplets may become attached to sediment in suspension and drop to the sea bed, where they may be buried and bacterial degradation slower. Deposit feeders are dominant in the biotope and through this route the infauna may be exposed to hydrocarbons. The best documented oil spill for protected habitats with soft mud/sand substrata is the West Falmouth, Florida tanker spill of 1969. Immediately after the spill, virtually the entire benthic fauna was eradicated and populations of the opportunistic polychaete Capitella capitata increased to abundances of over 200,000/m² (Sanders, 1978). Suchanek (1993) reviewed the effects of oil on bivalves. Sublethal concentrations may produce substantially reduced feeding rates and/or food detection ability, probably due to ciliary inhibition. Respiration rates have increased at low concentrations and decreased at high concentrations. Generally, contact with oil causes an increase in energy expenditure and a decrease in feeding rate, resulting in less energy available for growth and reproduction. Sublethal concentrations of hydrocarbons also reduce infaunal burrowing rates. Echinoderms also seem to be especially intolerant of the toxic effects of oil, probably because of the large amount of exposed epidermis (Suchanek, 1993). The high intolerant of Echinocardium cordatum to hydrocarbons was seen by the mass mortality of animals, down to about 20m, shortly after the Amoco Cadiz oil spill (Cabioch et al., 1978). The intolerance of the CMU.AbrNucCor biotope to hydrocarbon contamination has been assessed to be intermediate as the populations of dominant species in the community may be reduced/degraded and the diversity and function of the community become reduced. Oil polluted sediments may remain so for many years so recovery may be protracted. For example, persistent toxicity of Amoco Cadiz oil in sediment prevented the start of the recovery period (Clark, 1997), recovery has been assessed to be moderate.
Radionuclide contamination
No information Not relevant No information Not relevant Not relevant
Insufficient
information.
Changes in nutrient levels
Tolerant* Not relevant Not sensitive* No change Low
Abra alba increased its reproductive output to three spawnings in a year, as opposed to its normally occurring two, as an adaptive response to eutrophic conditions that followed the Amoco Cadiz oil spill (Dauvin & Gentil, 1989). Abra alba occurs in high abundances in moderately nutrient enriched environments (e.g. Caspers, 1987). Lagis koreni may favour moderate organic enrichment, but it is displaced in anoxic sediments (Pearson & Rosenberg, 1978). However, growth levels of Echinocardium cordatum have been observed to be lower in sediments with high organic content although it is suggested that this may be due to higher levels of intraspecific competition (Duineveld and Jenness, 1984). Pearson & Rosenberg (1976) describe the changes in fauna along a gradient of increasing organic enrichment by pulp fibre where Echinocardium cordatum is absent from all but distant sediments with low organic input. Typically an increasing gradient of organic enrichment results in a decline in the suspension feeding fauna and an increase in the number of deposit feeders, in particular polychaete worms, so deposit feeding species in the biotope are likely to benefit from increased nutrient availability. Only in situations of very high nutrient enrichment, where productivity results in accumulation of organic material and deoxygenation is a negative effect likely to be seen. At the benchmark level, nutrient enrichment is likely to be of benefit to the community and an assessment of not sensitive* has been made.
Not relevant Not relevant Not relevant Not relevant Not relevant
The CMU.AbrNucCor biotope is found within locations of either 'fully saline' or 'variable salinity' water (Connor et al. 1997a). It is highly unlikely that the biotope would experience conditions of hypersalinity, e.g. owing to evaporation and precipitation of salts that may occur in rockpools of shallow pools on sand, and in this instance the factor is considered not relevant. However, individually species of the biotope may be intolerant of an increase in salinity.
Not sensitive* Not relevant
The biotope occurs in the circalittoral within locations of either 'fully saline' or 'variable salinity' water (Connor et al. 1997a). It is highly unlikely that the biotope would experience conditions of reduced salinity as described in the benchmark. An assessment of not relevant has been made. However, individually species of the biotope may be intolerant of a decrease in salinity.
Intermediate High Low Decline Moderate
Jorgensen (1980) observed the response of macrofauna to reduced dissolved oxygen levels of 0.2 to 1 mg/l for a period of 3 to 4 weeks off Sweden. Polychaetes were observed to come to the surface, small specimens first. Lagis koreni was observed limp and motionless on the surface, but could be revived in 30 minutes by placing in oxygenated water. Nichols (1977) reported high mortality of Lagis koreni in association with periodic oxygen deficiency of the bottom waters of Kiel Bay, Germany, however it was capable of reaching former densities within a year following larval recruitment. Nephtys hombergii is apparently more tolerant of deoxygenation. After sulphide exposure under anaerobic conditions of >24 h, Arndt & Schiedek (1997) observed Nephtys hombergii to recover completely. During periods of hypoxia, burrowing bivalves were first observed to extend their siphons further into the water column, but as oxygen depletion continued, they emerged and laid on the sediment surface. Abra alba became inefficient in its use of available organic matter over a period of prolonged hypoxia (93 days) in an experiment to examine the interaction between eutrophication and oxygen deficiency (Hylland et al., 1996). Abra alba was also reported to be intolerant of lowered oxygen concentrations arising from eutrophication off the Swedish west coast (Rosenberg & Loo, 1988), whilst lethal effects were noted by Weigelt & Rumohr (1986) and Arntz & Rumohr (1986) in the western Baltic Sea. Echinocardium cordatum is highly intolerant of a fall in oxygenation. In the south-eastern North Sea a period of reduced oxygen resulted in the death of many individuals of Echinocardium cordatum (Niermann, 1997) and in laboratory experiments many individuals were dead at a concentration of 2.4mg/l (Nilsson & Rosenberg, 1994). The intolerance of the community to a reduced oxygen concentration (2 mg/O2/L) for a period of one week has been assessed to be intermediate, as some important characterizing species may die, others may expose themselves to an increased risk of predation, e.g. bivalves leaving the sediment. On return to prior conditions, recovery has been assessed to be high. For instance, Abra alba recovered to former densities following loss of a population from Keil Bay owing to deoxygenation within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986). However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).

Biological Pressures

 IntoleranceRecoverabilitySensitivityRichnessEvidence/Confidence
No information Not relevant No information Not relevant Not relevant
More than 20 viruses have been described for marine bivalves (Sinderman, 1990). Bacterial diseases are more significant in the larval stages and protozoans are the most common cause of epizootic outbreaks that may result in mass mortalities of bivalve populations. Parasitic worms, trematodes, cestodes and nematodes can reduce growth and fecundity within bivalves and may in some instances cause death (Dame, 1996). However, information concerning impacts on the community was not found, so an intolerance assessment has not been made.
Not relevant Not relevant Not relevant Not relevant Not relevant
No particular non-native species was identified as posing a current threat to this biotope.
Not relevant Not relevant Not relevant Not relevant Not relevant
It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. The biotope does not support quantities of commercial species that require penetration into the sediment to harvest, e.g. scallop dredging. However, fishing for demersal species may adversely affect the biotope. Shells of Abra alba, Corbula gibba and Nucula nitidosa are probably vulnerable to physical damage (e.g. by otter boards; Rumohr & Krost, 1991) but their small size relative to meshes of commercial trawls may ensure survival of at least a moderate proportion of disturbed individuals that pass through. Bivalves such as Ensis spp., Corbula gibba, and Chamelea gallina together with starfish were relatively resistant (Bergman & van Santbrink, 2000) to direct mortality due to bottom trawling in sandy sediments. Schafer (1972) noted that adults of Lagis koreni were incapable of re-constructing their delicate sand-tubes once removed from them, and that mortality following physical disturbance to the substratum, e.g. from trawl/tickler chain damage, is likely to be significant (de Groot & Apeldoorn, 1971). For other infaunal species that burrow deeper into the sediment, e.g. Echinocardium cordatum, immediate effects are dependant on the depth of penetration of an object, e.g. an anchor or fishing gear relative to the distribution of animals in the sediment. Smaller individuals tend to live nearer the surface and are likely to be more vulnerable. Houghton et al. (1971), Graham (1955), de Groot & Apeldoorn (1971) and Rauck (1988) refer to significant trawl-induced mortality of Echinocardium cordatum. Echinocardium cordatum has a fragile test that is likely to be damaged by an abrasive force, such as movement of trawling gear over the seabed. A substantial reduction in the numbers of the species due to physical damage from scallop dredging has been observed (Eleftheriou & Robertson, 1992). Echinocardium cordatum was reported to suffer between 10 and 40% mortality due to fishing gear, depending on the type of gear and sediment after a single trawl event (Bergman & van Santbrink, 2000). They suggested that mortality may increase to 90% in summer when individuals migrate to the surface of the sediment during their short reproductive season. Bergman & van Santbrink (2000) suggested that Echinocardium cordatum was one of the most vulnerable species to trawling.

Brittlestars such as Ophiura albida may be more tolerant of abrasion. Bergman & Hup (1992) for example, found that beam trawling in the North Sea had no significant direct effect on small brittlestars. Brittlestars can tolerate considerable damage to arms and even the disk without suffering mortality and are capable of arm and even some disk regeneration. Ramsay et al. (1998) suggest that Amphiura spp. may be less susceptible to beam trawl damage than other species like echinoids or tube dwelling amphipods and polychaetes. In an analysis of long-term effects of scallop dredging on benthic communities in the Irish Sea, Bradshaw et al. (2002) noted a decline in the sedentary, filter feeding brittlestars Ophiothrix fragilis and Ophiopholis aculeata but an increase in surface detritivores or scavenging brittlestars such as Amphiura filiformis, Ophiocomina nigra and Ophiura albida.

At the benchmark level, intolerance has been assessed to be intermediate as mortality of important characterizing species may occur. However, the community is unlikely to change significantly and recovery is likely to be rapid owing to larval recruitment, e.g. Abra alba recovered to former densities following loss of a population from Keil Bay within 1.5 years, whilst Lagis koreni took only one year (Arntz & Rumohr, 1986). Such evidence suggests that recoverability of important characterizing species of the biotope would be high. However, the recovery of Echinocardium cordatum may take longer owing to recruitment that is frequently unsuccessful (Rees & Dare, 1993).

Not relevant Not relevant Not relevant Not relevant Not relevant

Additional information

No text entered.

Importance review

Policy/Legislation

Habitats of Principal ImportanceSubtidal sands and gravels
Habitats of Conservation ImportanceSubtidal sands and gravels
UK Biodiversity Action Plan PrioritySubtidal sands and gravels

Exploitation

Survey data suggests that the biotope does not contain significant numbers of any commercially important species such as Nephrops norvegicus and so is not likely to be subject to exploitation.

Additional information

-

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Citation

This review can be cited as:

Budd, G.C. 2006. Abra alba and Nucula nitidosa in circalittoral muddy sand or slightly mixed sediment. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/habitat/detail/62

Last Updated: 09/08/2006