Fabulina fabula and Magelona mirabilis with venerid bivalves in infralittoral compacted fine sand
Ecological and functional relationships
The hydrodynamic regime, sediment composition and interaction between the two are probably the most significant factors structuring the community of the biotope rather than the biological interactions (Tyler, 1977; Warwick & Uncles, 1980; Elliott et al., 1998).
The fine sand habitat supports high abundances of suspension feeders (e.g. the bivalves Chamelea gallina, Gari fervensis and Donax vittatus), deposit feeders (e.g. the polychaetes Magelona mirabilis, Spiophanes bombyx and Chaetozone setosa) and species with flexible feeding methods (e.g. the bivalve Fabulina fabula and the tube building polychaete Lanice conchilega).
The amphipods (e.g. Bathyporeia guilliamsoniana, Bathyporeia elegans and Ampelisca brevicornis) and the infaunal annelid species in the biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows and tubes, while high densities of amphipods can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986).
Spatial competition probably occurs between the infaunal suspension feeders and deposit feeders. Reworking of sediment by deposit feeders makes the substratum less stable, increases the suspended sediment and makes the environment less suitable for suspension feeders (Rhoads & Young, 1970). Tube building by amphipods and polychaetes stabilizes the sediment and arrests the shift towards a community consisting entirely of deposit feeders.
Amphipods are predated chiefly by nemertean worms (e.g. McDermott, 1984) and demersal fish (Costa & Elliott, 1991).
The abundant infauna are preyed upon by carnivorous polychaetes, e.g. Nephtys hombergii.
The echinoderms, Astropecten irregularis and Asterias rubens, predate the bivalves (Aberkali & Trueman, 1985; Elliott et al., 1998).
Crabs, particularly Liocarcinus depurator, are scavengers and predators of molluscs and annelids (Thrush, 1986; Elliott et al., 1998).
Seasonal and longer term change
- Temporal changes are likely to occur in the community due to seasonal recruitment processes. In the German Bight, peak abundance of Fabulina fabula (ca 2000 individuals/m²) occurred in September following the main period of spatfall and then decreased to a minimum in February (ca 500 individuals/m²), at which point settlement began to occur again (Salzwedel, 1979). Similarly, temporal evolution of Fabulina fabula in NW Spain showed well marked annual peaks in autumn (Lopez-Jamar et al., 1995). In the German Bight, spatfall for Magelona mirabilis was heaviest in August/September, and for Echinocardium cordatum spatfall was heaviest in August (Bosselmann, 1989).
- Temporal variations in species richness and abundance are likely to occur due to seasonal patterns of disturbance, such as storms, harsh winters and oxygen deficiencies (Bosselmann, 1989; Lopez-Jamar et al., 1995). The biotope may also be liable to severe substratum disturbance, such as one in 25 year or one in 50 year storms, which can turn over sediment and completely disrupt the community (Elliott et al., 1998).
- The water temperature in subtidal sandy habitats may vary over 5-10°C through the year in British coastal waters depending on depth. The variation may have short term but significant effects on species diversity (Buchanan & Moore, 1986).
- There may be a spring-neap and winter-summer cycle of erosion and deposition of sediment, altering the biotope extent and reflecting changes in hydrodynamic energy (Dyer, 1998).
- 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.
Habitat structure and complexity
- The spatial extent of the biotope is dictated by the physical conditions, especially the physiography and underlying geology coupled with the hydrodynamic regime, which dictates where and how much sediment will be deposited (Elliott et al., 1998).
- By definition the biotope is fairly homogeneous, although it may occur under variable environmental conditions (Elliott et al., 1998). The uniform fine sand is unlikely to be very sculpted as the biotope occurs in areas of weak tidal streams (Connor et al., 1997a).
- The sediment itself is relatively fluid, but is probably stabilized by the biota. For example, the microphytobenthos in the interstices of the sand grains produce mucilaginous secretions which stabilize fine substrata (Tait & Dipper, 1998), and tube building polychaetes, e.g. Lanice conchilega and Owenia fusiformis, stabilize the sediment and enhance deposition.
Production in the biotope is mostly secondary, derived from detritus and organic material. Some primary production comes from benthic microalgae and water column phytoplankton. The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow in the upper several millimetres of illuminated sediments, typically appearing only as a subtle brown or green shading (Elliott et al.
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 reaching the benthos 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.
Productivity in the biotope is expected to be relatively high. The amphipods have a short life span, grow to maturity quickly and have multiple generations per year. The biotope also contains many species that occur in early successional stages and have rapid turnovers, for example Magelona mirabilis
The sediment in the biotope may be nutrient enriched due to proximity to anthropogenic nutrient sources such as sewage outfalls or eutrophicated rivers.
Warwick et al
. (1978) studied the productivity of the fine sand Venus
community in Carmarthen Bay, Wales. Total production by the macrofauna in the community was 25.8 g/m² per year. Fabulina fabula
was responsible for 340 mg/m² of this production, with a production:biomass ratio of 0.90. This ratio is relatively low because the species is long lived and has a low turnover. For comparison, the production:biomass ratio of Magelona mirabilis
- Sandy areas are usually dependent on an input of colonizing organisms and have few species with benthic reproduction. Hence, recruitment is sensitive to changes in the hydrodynamic regime. Sandbanks in particular may be recruitment sinks as they often occur at the centre of hydrographic gyres (Elliott et al., 1998). Of the characterizing species in the biotope, only Astropecten irregularis is likely to migrate into the area as adults.
- The bivalves in the biotope are gonochoristic broadcast spawners with pelagic larval dispersal. They therefore have the potential to recruit both locally and remotely. However, bivalve populations typically show considerable pluriannual variations in recruitment, suggesting that recruitment is patchy and/or post settlement processes are highly variable (e.g. Dauvin, 1985). 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 dominant polychaetes in the biotope, Magelona mirabilis and Nephtys hombergii, and the tube building polychaetes, Lanice conchilega and Owenia fusiformis, disperse via a pelagic larval stage lasting up to 2 months and therefore recruitment may occur from distant populations. However, dispersal of 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.
- Both Fabulina fabula and Magelona mirabilis have heavy spatfalls and the potential for mass development, enabling the population to recover rapidly from disturbance. Spatfall is usually followed by heavy post settlement mortality (Bosselmann, 1989).
- Magelona mirabilis appears to settle preferentially in areas already occupied by adults (Bosselmann, 1989).
- The mating system of amphipods is polygynous and several broods of offspring are produced, each potentially fertilised by a different male. There is no larval stage and embryos are brooded in a marsupium, beneath the thorax. Embryos are released as subjuveniles with incompletely developed eighth thoracopods and certain differences in body proportions and pigmentation. Dispersal is limited to local movements of the subjuveniles and migration of the adults and hence recruitment is limited by the presence of local, unperturbed source populations (Poggiale & Dauvin, 2001). Dispersal of subjuveniles may be enhanced by the brooding females leaving their tubes and swimming to uncolonized areas of substratum before the eggs hatch (Mills, 1967).
Time for community to reach maturity
Following a disaster event or exposure of a new substratum, succession in the biotope is likely to follow a predictable sequence (Diaz-Castaneda et al.
, 1989). Pioneer species, such as Capitella capitata
, Magelona mirabilis
and Scoloplos armiger
, dominate the early successional stages. These species are then partly replaced by regular seasonal species that breed at the same time every year, i.e. the relatively opportunistic bivalves such as Abra alba
. Finally, the equilibrium species become established, e.g. Fabulina fabula, Chamelea gallina
and Nephtys hombergii
, characterized by long life spans and irregular spatfall (Diaz-Castaneda et al.,
The climax species are relatively quick to mature and it is likely that the community would reach maturity within 2-3 years. However, the biotope is vulnerable to physical disturbance and would probably be frequently perturbed, restarting the succession process.
This review can be cited as follows:
Fabulina fabula and Magelona mirabilis with venerid bivalves in infralittoral compacted fine sand.
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=142&code=1997>