Venerupis senegalensis and Mya truncata in lower shore or infralittoral muddy gravel
Ecological and functional relationships
The species composition of the biotope is probably determined largely by the substratum characteristics and therefore the hydrodynamic regime and sediment supply, rather than the interspecific relationships. Sediment is the most extensive sub-habitat within the biotope and hence infauna dominate.
The suspension feeding infaunal bivalves, e.g. Venerupis senegalensis, Abra alba, Mysella bidentata and Mya truncata, compete for nutrients among themselves and with epifauna, e.g. Mytilus edulis.
Spatial competition probably occurs between infaunal suspension feeders and deposit feeders. Reworking of sediment by deposit feeders, e.g. Arenicola marina, makes the substratum less stable, increases the suspended sediment and makes the environment less suitable for suspension feeders (Rhoads & Young, 1970). Tube building, e.g. by Lanice conchilega, and byssal attachment, e.g. by Venerupis senegalensis, stabilize the sediment and arrest the shift towards a community dominated by deposit feeders.
Amphipods, e.g. Corophium sp., and the infaunal annelid species in this biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows, while high densities of amphipods can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986). Arenicola marina has been shown to have a strong negative effect on Corophium volutator due to reworking of sediment causing the amphipod to emigrate (Flach, 1992).
Carcinus maenas is a significant predator in the biotope. It has been shown to reduce the density of Mya arenaria, Cerastoderma edule, Abra alba, Tubificoides benedii, Aphelochaeta marioni and Corophium volutator (Reise, 1985). A population of Carcinus maenas from a Scottish sea-loch preyed predominantly on annelids (85% frequency of occurrence in captured crabs) and less so on molluscs (18%) and crustaceans (18%) (Feder & Pearson, 1988).
Carnivorous annelids such as Nephtys hombergii and Pholoe inornata operate at the trophic level below Carcinus maenas (Reise, 1985). They predate the smaller annelids and crustaceans.
Cerastoderma edule and Mya arenaria are common prey for several bird species. Ensis sp. and Venerupis sp. are also heavily predated (Meire, 1993). The main bird predator in the biotope is probably the oystercatcher, Haematopus ostralegus. Drianan (1957, cited in Meire, 1993) estimated that oystercatchers remove 22% of the cockle population annually in Morecambe Bay. It should be noted that only the upper portion of the biotope will be vulnerable to predation by shore birds at low tide.
Macroalgae, e.g. Fucus serratus, colonize the hard substrata where present. The low energy environment allows colonization of gravel and pebbles which in higher energy environments would be too unstable.
Littorina littorea and Gibbula cineraria graze microalgae and ephemeral green algae, preventing domination by the faster growing species. Calcareous species, e.g. the Corallinaceae, are resistant to grazing.
Seasonal and longer term change
Seasonal changes occur in the abundance of the fauna due to seasonal recruitment processes. Venerupis senegalensis
exhibits pronounced year class variability in abundance (Johannessen, 1973; Perez Camacho, 1980) probably due to patchy recruitment and/or variable post recruitment processes. Variation in abundance is very pronounced in the polychaete Aphelochaeta marioni
. In the Wadden Sea, peak abundance occurred in January (71,200 individuals per m²) and minimum abundance occurred in July (22,500 individuals per m²) following maximum spawning activity between May and July (Farke, 1979). However, the spawning period varies according to environmental conditions and so peak abundances will not necessarily occur at the same time each year. Adult densities of the bivalve, Abra alba
, may exceed 1000 per m² in favourable conditions but typically fluctuate widely from year to year due to variation in recruitment success or adult mortality (see review by Rees & Dare, 1993).
Macroalgal cover typically varies through the year due to temperature and light availability. Fucus serratus
plants, for example, lose fronds in the winter, followed by regrowth from existing plants in late spring and summer, so that summer cover can be about 250% of the winter level (Hartnoll & Hawkins, 1980). Production by microphytobenthos and microalgae is also likely to be higher in spring and summer, increasing food availability for grazers, deposit feeders and suspension feeders.
One of the key factors affecting benthic habitats is disturbance, which in shallow subtidal habitats increases in winter due to weather conditions. Storms may cause dramatic changes in distribution of macro-infauna by washing out dominant species, opening the sediment to recolonization by adults and/or available spat/larvae (Eagle, 1975; Rees et al.
, 1977; Hall, 1994) and by reducing success of recruitment by newly settled spat or larvae (see Hall 1994 for review). For example, during winter gales along the North Wales coast, large numbers of Abra alba
were cast ashore and over winter survival rate was as low as 7% in the more exposed locations. The survival rates of the bivalve, Mysella bidentata
, and the polychaete, Notomastus latericeus
, were 50% and 12% respectively (Rees et al.
, 1977). Sediment transport and the risk of smothering also occurs. A storm event at a silt/sand substratum site in Long Island Sound resulted in the deposition of a 1cm layer of shell fragments and quartz grains (McCall, 1977).
Habitat structure and complexity
- The mixed sediment in this biotope is the important structural component, providing the complexity required by the associated community. Epifauna and algae are attached to the gravel and pebbles and infauna burrow in the soft underlying sediment. Sediment deposition, and therefore the spatial extent of the biotope, is dictated by the physiography and underlying geology coupled with the hydrodynamic regime (Elliot et al., 1998).
- There is a traditional view that the distribution of infaunal invertebrates is correlated solely with sediment grain size. In reality, and in this biotope, it is likely that a number of additional factors, including organic content, microbial content, food supply and trophic interactions, interact to determine the distribution of the infauna (Snelgrove & Butman, 1994).
- Reworking of sediments by deposit feeders, such as Arenicola marina, increases bioturbation and potentially causes a change in the substratum characteristics and the associated community (e.g. Rhoads & Young, 1970). The presence of tube builders, such as Lanice conchilega, stabilizes the sediment and provides additional structural complexity.
- The presence of macroalgae, such as Fucus serratus and Osmundea pinnatifida, increases structural complexity in the biotope, providing shelter and cover for mobile fauna. The fronds increase the area available for attachment of epifauna and epiphytes.
Primary production in this biotope comes predominantly from benthic microalgae (microphytobenthos e.g. diatoms, flagellates and euglenoides) and water column phytoplankton. Macroalgae, although not very abundant in the biotope also contribute to primary production. They exude considerable amounts of dissolved organic carbon which are taken up readily by bacteria and possibly by some larger invertebrates. Only about 10% of the primary production on rocky shores is directly cropped by herbivores (Raffaelli & Hawkins, 1999) and the figure is likely to be similar or less in this biotope. Photosynthetic processes may be light limited due to the turbidity of the water (Elliot et al., 1998) and in situ primary production overall is likely to be low. Large allochthonous inputs of nutrients, sediment and organic matter come from river water and the sea, containing both naturally derived nutrients and anthropogenic nutrients (e.g. sewage) (Elliot et al., 1998). The allochthonous nutrient input results in enriched sediments and explains the high biomass of detritivores and deposit feeders.
Characteristic and other species in the biotope recruit as larvae and spores from the plankton. More detailed information is given for dominant and characteristic species below.
- Venerupis senegalensis is a long lived, fast growing species that reaches maturity within one year and spawns several times in one season (Johannessen, 1973; Perez Camacho, 1980). No information was found concerning number of gametes produced, but the number is likely to be high as with other bivalves exhibiting planktotrophic development (Olafsson et al., 1994). The larvae remain in the plankton for up to 30 days (Fish & Fish, 1996) and hence have a high potential for dispersal. The species exhibits pronounced year class variability in abundance (Johannessen, 1973; Perez Camacho, 1980) 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, for Venerupis senegalensis, annual predictable recruitment is unlikely to occur.
- Recruitment of shallow burrowing infaunal species can depend on adult movement by bedload sediment transport and not just spat settlement. Emerson & Grant (1991) investigated recruitment in 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 infaunal polychaetes Arenicola marina and Aphelochaeta marioni have high fecundity and the eggs develop lecithotrophically within the sediment or at the sediment surface (Farke, 1979; Beukema & de Vlas, 1979). There is no pelagic larval phase and the juveniles disperse by burrowing. Recruitment must 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 patchy and sporadic otherwise.
- The epifaunal gastropods in the biotope, such as Littorina littorea, are iteroparous, highly fecund and disperse via a lengthy pelagic larval phase. Recruitment is probably sporadic and opportunistic, large spat fall occurring when a suitable substratum and food supply becomes available.
- Recruitment of Fucus serratus from minute pelagic sporelings takes place from late spring until October. There is a reproductive peak in the period August - October and dispersal may occur over long distances (up to 10 km). However, weak tidal streams may result in a smaller supply of pelagic sporelings and most recruitment probably comes from local populations.
Time for community to reach maturity
is the important characterizing species in the biotope. It is highly fecund and fast growing (Johannessen, 1973; Perez Camacho, 1980; Olafsson et al.
, 1994) and therefore is likely to attain high numbers in the community rapidly. The same is true for the majority of other infauna, epifauna and flora in the biotope. It is predicted therefore that the community will reach maturity in less than 5 years.
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This review can be cited as follows:
Venerupis senegalensis and Mya truncata in lower shore or infralittoral muddy 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=354&code=1997>