Venerid bivalves in circalittoral coarse sand or gravel
Image Anon. - Neopentadactyla mixta and venerid bivalves in circalittoral shell gravel or coarse sand. Image width ca XXm.
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Ecological and functional relationships
The species composition of the biotope is probably determined largely by the substratum characteristics and therefore the hydrodynamic regime, rather than the interspecific relationships. Sediment is the most extensive sub-habitat within the biotope and hence infauna dominate.
The predictable environmental conditions in which the biotope occurs allow a stable and mature benthic community with high diversity and evenness to develop (Dauvin, 1988).
The biotope is dominated by suspension feeders. Venerid bivalves, e.g. Clausinella fasciata and Timoclea ovata, make up the majority of the biomass, along with other slow growing, robust bivalve species, such as Glycymeris glycymeris and Astarte sulcata, and faster growing species, such as Spisula elliptica. Other suspension feeders include the burrowing cephalochordate, Branchiostoma lanceolatum, and the epifaunal tube building polychaete, Hydroides norvegica.
Although bivalves dominate the biomass, polychaetes are very numerous (Dauvin, 1988). The 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).
The bivalves are predated by starfish, especially Astropecten sp. (Christensen ,1970; cited in Guillou & Sauriau, 1985), with the larger, thick shelled venerids being more resistant to predation than the thinner shelled Spisula sp. (Thorson, 1971).
Bivalves are predated by boring gastropods, e.g. Polinices sp., and flatfish (Thorson, 1971). Spisula sp. in particular are predated by plaice, Pleuronectes platessa.
The infaunal deposit feeding echinoid, Spatangus purpureus, is opportunistically predated by fish, including the whiting, Merlangius merlangus (Kaiser & Spencer, 1994b), and the common whelk, Buccinum undatum (Evans et al., 1996), especially following disturbance and damage due to fishing activity.
The Glycera sp. are active carnivorous polychaetes which predate smaller annelids and amphipods.
Seasonal and longer term change
- Seasonal changes probably occur in the abundance of the fauna due to seasonal recruitment processes. For example, in Morlaix Bay, Timoclea ovata recruitment occurs in late summer and the population shows pluriannual variations in recruitment, growth and production (Dauvin, 1985).
- There is a seasonal variation in planktonic production in surface waters which affects the food supply of the benthos in the biotope. In the Clausinella fasciata community of the western English Channel, the water temperature varies from 9 °C in February to 15.4 °C in August. There are seasonal variations in sediment organic matter content which are correlated with phytoplankton sedimentation events (Thouzeau et al., 1996). These variations directly influence the food supply of the suspension feeders and deposit feeders in the biotope.
Habitat structure and complexity
- The biotope consists of 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 the echinoid, Spatangus purpureus, 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 of species such as Spatangus purpureus 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).
- The spines of Spatangus purpureus provide a site for the attachment of the small bivalve Montacuta substriata (Fish & Fish, 1996).
CGS.Ven occurs below 30 m depth and therefore there is little primary production in the biotope. The benthos is supported predominantly by pelagic production and by detrital materials emanating from the coastal fringe (Barnes & Hughes, 1992). The amount of planktonic food available to benthic suspension feeders is related to;
- depth of water through which the material must travel
- magnitude of pelagic production
- proximity of additional sources of detritus
- extent of water movement near the sea bed, bringing about the renewal of suspended supplies (Barnes & Hughes, 1992).
Food becomes available to deposit feeders by sedimentation on the substratum surface and by translocation from the water column to the substratum through production of pseudofaeces by suspension feeders. Hence, secondary production in the biotope may be high despite the lack of local primary production.
Recruitment processes for the biotope are assessed through the major component species.
- Guillou & Sauriau (1985) investigated reproduction and recruitment in a Venus striatula population in the Bay of Douarnenez, Brittany. There were 2 periods of spawning activity, one in the spring and then again in late summer. The larvae undergo planktotrophic development, metamorphosis occurring 3 weeks after fertilization (Ansell, 1961; cited in Guillou & Sauriau, 1985). There were 2 periods of recruitment, one at the end of the spring and the second in autumn. The mean life span was 5 years and the maximum 10 years. No evidence was found to suggest that recruitment patterns for the other venerid bivalves differed significantly.
- Dauvin (1985) reported that Timoclea ovata (studied as Venus ovata) recruitment occurred in July-August in the Bay of Morlaix. However, the population showed considerable pluriannual variations in recruitment, which suggests that 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. Post settlement processes affecting survivability include predation by epibenthic consumers, physical disturbance of the substratum and density dependent starvation of recent recruits. Hence, annual predictable recruitment of venerid bivalves probably does not occur in the biotope.
- The purple heart urchin, Spatangus purpureus, is gonochoristic, a broadcast spawner and disperses via a pelagic larva (Fish & Fish, 1996). It is likely that recruitment patterns are similar to another heart urchin, Echinocardium cordatum, in which recruitment has been recorded as sporadic, only occurring in 3 years out of a 10 year period (Buchanan, 1967).
- Recruitment of shallow burrowing infaunal species can also depend on adult movement by bedload sediment transport and not just spat settlement. Emerson & Grant (1991) investigated recruitment in the bivalve Mya arenaria and found that bedload transport was positively correlated with clam transport. They concluded that clam transport at a high energy site accounted for large changes in clam density. Furthermore, clam transport was not restricted to storm events and the significance is not restricted to Mya arenaria recruitment. Many infauna, e.g. polychaetes, gastropods, nematodes and other bivalves, will be susceptible to movement of their substratum.
- 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 bedload transport. Recruitment is therefore likely to be predictable if local populations exist but will most likely be patchy and sporadic otherwise.
Time for community to reach maturity
No information was found concerning the time taken for the CGS.Ven community to reach maturity. However, the venerid clams reach sexual maturity relatively quickly (1-2 years for Venus striatula (Guillou & Sauriau, 1985)), reproduce at least once a year 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.
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This review can be cited as follows:
Venerid bivalves in circalittoral coarse sand or gravel.
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=63&code=1997>