Abra alba, Nucula nitida and Corbula gibba in circalittoral muddy sand or slightly mixed sediment
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).
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.
- 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).
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 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.
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.
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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This review can be cited as follows:
Abra alba, Nucula nitida and Corbula gibba in circalittoral muddy sand or slightly mixed sediment.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=62&code=1997>