Biodiversity & Conservation

The species composition of the biotope may be largely determined by substratum characteristics and hydrodynamics, rather than the interspecific relationships. Environmental conditions allow for a stable, mature and diverse benthic community to develop (Dauvin, 1988).

Suspension feeders are more abundant in coarser sediments, such as this biotope, with the organic content being too low for an abundance of deposit feeders (Nybakken, 1982). Venerid bivalves, particularly Tellina sp. make up the majority of the biomass, along with other robust bivalves such as Dosinia lupinus, Timoclea ovata, Goodallia triangularis, Chamelea gallina andSpisula solida. Other suspension feeders present include the tube building polychaete, Lanice conchilega. Tanaids, Apseudes latreillii, are commonly present in abundance. Bivalves in this biotope may be subject to predation by starfish (Astropecten sp.), boring gastropods (Polinices sp.) and flatfish (Pleuronectes platessa). Larger, thick shelled venerids are more resistant to predation than the thinner shelled Spisula sp. (Thorson, 1971). Polychaetes are very numerous in this biotope. Tube building species, Lanice conchilega and Owenia fusiformis, deposit feed on suspended particles trapped by the fluctuations in hydrodynamic regime around their tubes (Fish & Fish, 1996). Scoloplos armiger and Chaetazone setosa are burrowing deposit feeders and Notomastus latericeus is an infaunal detritivore (Fish & Fish, 1996). Glycera sp. are active carnivorous polychaetes which predate upon smaller annelids and amphipods (e.g. Bathyporeia pelagic and Guernea coalita).

Temporal changes are likely to occur in the community due to seasonal recruitment processes. There is a seasonal variation in planktonic production in surface waters which probably affects the food supply of the benthos in the biotope. Increased production by phytoplankton in spring and summer due to increased temperatures and irradiance is followed by phytoplankton sedimenting events which are correlated with seasonal variations in the organic content of benthic sediments (Thouzeau et al., 1996). These variations directly influence the food supply of the deposit feeders and suspension feeders in the biotope.

The biotope consists of coarse, gravelly, mixed sediment with very little structural complexity. The species composition of the biotope is likely to vary according to the composition of the substratum. Deposit feeders, such as infaunal annelids and echinoderms, Echinocyamus pusillus, are likely to be more abundant where fine sediment exists in the interstitial areas surrounding the gravel and coarse sand.

Tube building polychaetes such as Lanice conchilega, Owenia fusiformis and Spiophanes bombyx stabilize the sediment and provide additional structural complexity.

Some structural complexity is provided by the burrows of infauna although these are generally simple. 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 thus enhances the survival of a considerable variety of small species (Pearson & Rosenberg, 1978).

Primary productivity is derived from red, brown and green algae that may be present, Chondrus crispus, Polyides rotundus, Gracilaria gracilis, Laminaria saccharina and Ulva. However, the contribution to secondary production by suspension feeders is likely to be greater with high numbers of suspension feeders present. This biotope is likely to be predominantly supported by pelagic production and detrital materials emanating from the coastal fringe (Barnes & Hughes, 1992). The amount of planktonic food available to benthic suspension feeders is related to; depth, pelagic production, water movement and proximity of detrital sources (Barnes & Hughes, 1992).

Guillou & Sauriau (1985) investigated reproduction and recruitment in a population of venerid bivalves (Venus striatula). Spawning was reported to occur in spring and summer. Larvae are planktotrophic, with metamorphosis occurring 3 weeks after fertilization (Ansell, 1961; cited in Guillou & Sauriau, 1985). Recruitment occurred in spring and autumn. Life span ranges from 5-10 years. Recruitment patterns for the other venerid bivalves are not reported to differ significantly. Timoclea ovata recruitment occurs in July-August (Dauvin, 1988). However, considerable pluriannual variations in recruitment were found, suggesting that perhaps recruitment is patchy and/or post settlement processes are highly variable. Olafsson et al. (1994) reviewed the potential effects of pre and post recruitment processes. Recruitment may be limited by predation of the larval stage or inhibition of settlement due to intraspecific density dependent competition. High juvenile mortality is likely with post settlement processes affecting survivability including; predation by epibenthic consumers, physical disturbance of the substratum and density dependent starvation of recent recruits. Hence, recruitment of venerid bivalves is sporadic. The recruitment in the heart urchin, Echinocardium cordatum, is also reported to be sporadic, occurring in 3 years out of a 10 year period (Buchanan, 1967). The tube building polychaetes, Lanice conchilega and Owenia fusiformis, disperse via a pelagic larval stage lasting up to 2 months (Fish & Fish, 1996) and therefore recruitment may occur from distant populations. However, dispersal of the infaunal deposit feeders, such as Scoloplos armiger, occurs through burrowing of the benthic larvae and adults. Recruitment must therefore occur from local populations or by longer distance dispersal during periods of bed load transport. Recruitment is therefore likely to be predictable if local populations exist but will most likely be patchy and sporadic otherwise. Spisula solida reaches sexual maturity during its first year, which is a function of age, not of size (Gaspar & Monteiro,1999; Fahy et al., 2003) (see full species review).

No information was found concerning the time taken for the MoeVen community to reach maturity. However, venerid bivalves reach sexual maturity relatively quickly (1-2 years for Venus striatula (Guillou & Sauriau, 1985)), reproduction is sporadic and have an average life span of 5 years or less (Guillou & Sauriau, 1985; Dauvin, 1985). Allowing for patchy recruitment, it is expected that the venerid community would reach maturity within 5 years.