Biodiversity & Conservation

The characterizing and other species in this biotope occupy space in the habitat but their presence is most likely primarily determined by the occurrence of a suitable substratum rather by interspecific interactions.

There are however, some interspecific relationships within the biotope. The bivalve Tellimya (=Montacuta) ferruginosa is a commensal of Echinocardium cordatum, and as many as 14 or more of this bivalve have been recorded with a single echinoderm. Adult specimens live freely in the burrow of Echinocardium cordatum, while the young are attached to the spines of the echinoderm by byssus threads (Fish & Fish, 1996). The amphipod crustacean Urothöe marina (Bate) is another common commensal (Hayward & Ryland, 1995).

Predation in the biotope can be an important structuring force. Predators in the biotope include surface predators such as crabs, gastropods and fish; burrowing predators such as some polychaete worms and digging predators like Cancer pagurus. An increase in the numbers of these types of predators can have an influence on the abundance and diversity of species in benthic habitats (Ambrose, 1993; Wilson, 1991). For example, enclosure experiments in a sea loch in Ireland have shown that high densities of swimming crabs such as Liocarcinus depurator, that feed on benthic polychaetes, molluscs, ophiuroids and small crustaceans, led to a significant decline in infaunal organisms (Thrush, 1986).

The hydrodynamic regime, which in turn controls sediment type, is the primary physical environmental factor structuring benthic communities such as IMS.EcorEns. The hydrography affects the water characteristics in terms of salinity, temperature and dissolved oxygen. It is also widely accepted that food availability (see Rosenberg, 1995) and disturbance, such as that created by storms, (see Hall, 1994) are also important factors determining the distribution of species in benthic habitats. The role of biological factors in the structuring of benthic communities is much more complicated than the physical and has proved to be much more difficult to assess experimentally.

One of the key factors affecting benthic habitats is disturbance, which in shallow subtidal habitats will increase 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 (Rees et al., 1976):

• Wave scour washed out some individuals of Ensis ensis although numbers were much lower than for some other fauna.
• The northerly gales threw piles of Echinocardium cordatum on to the strand line and the author suggests these events are not uncommon. Lawrence (1989) also reports that spatangoid echinoderms such as Echinocardium cordatum can be washed out by water currents generated by gales.
Other organisms such as bivalves and brittle stars were also washed out of the sediment.

The numbers of many species in the biotope are likely to show peak abundances at certain times of the year due to seasonality of breeding and larval recruitment. Immature individuals of Liocarcinus depurator, for example, are more frequent in the periods May - September. Breeding of Ensis ensis probably occurs during spring and the veliger larvae has a pelagic life of about a month (Fish & Fish, 1996). Echinocardium cordatum is a long lived species and is unlikely to show significant seasonal changes.

• The biotope has very little structural complexity with most species living in or on the sediment. Macroalgae are largely absent although in some areas sparse cover of seagrass may increase habitat heterogeneity because of the leaves and root rhizomes.
• Some structural complexity is provided by animal burrows although these are generally simple. The burrows of Echinocardium cordatum, for example, provide a habitat for other species such as the small bivalve Tellimya (=Montacuta) ferruginosa. 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 Echinocardium cordatum 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).
• Deposit feeders manipulate, sort and process sediment particles and may result in destabilization and bioturbation of the sediment which inhibits survival of suspension feeders. Large deposit feeders like the lugworm Arenicola marina act like conveyor belts (Rhoads, 1974) ingesting particles from many centimetres below the surface, passing them through their guts and depositing them as faeces on the sediment surface. This often results in a change in the vertical distribution of particles in the sediment that may facilitate vertical stratification of some species with particle size preferences. Vertical stratification of species according to sediment particle size has been observed in some soft-sediment habitats (Peterson, 1977).

Productivity in lower shore and shallow subtidal sediments is often quite low (Elliot et al., 1998). Macroalgae are generally absent and so productivity is mostly secondary, derived from detritus and organic material. Allochthonous organic material is derived from anthropogenic activity (e.g. sewerage) and natural sources (e.g. plankton, detritus). Autochthonous organic material is formed by benthic microalgae (microphytobenthos e.g. diatoms and euglenoids) and heterotrophic micro-organism production. Organic material is degraded by micro-organisms and the nutrients recycled. The high surface area of fine particles provides surface for microflora.

• In Echinocardium cordatum the sexes are separate and fertilization is external, with the development of a pelagic larva (Fish & Fish, 1996). The fact that Echinocardium cordatum is to be found associated with several different bottom communities would indicate that the larvae are not highly selective and discriminatory and it is probable that the degree of discrimination in 'larval choice' becomes diminished with the age of the larvae (Buchanan, 1966). Metamorphosis of larvae takes place within 39 days after fertilization (Kashenko, 1994). On the north-east coast of England a littoral population bred for the first time when three years old. In the warmer waters of the west of Scotland breeding has been recorded at the end of the second year (Fish & Fish, 1996). Buchanan (1967) observed that offshore populations were very slow growing and did not appear to reach sexual maturity so recruitment may be sporadic in places. However, since Buchanan (1967) also found that intertidal populations bred every year recruitment should take place on an annual basis.
• The razor shell Ensis ensis does not appear to breed before they are three years old. Breeding occurs during the summer but larval settlement is not successful every year, and recruitment of juveniles is irregular. Breeding probably occurs during spring and the veliger larvae has a pelagic life of about a month (Fish & Fish, 1996). Studies on razor shells from North Wales showed that individuals of Ensis ensis were mature in July but were spent in August, indicating that spawning had occurred by the middle of the summer (Henderson & Richardson, 1994).
• Most other macrofauna in the biotope breed several times in their life history (iteroparous) and are planktonic spawners producing large numbers of gametes (depending on food availability) with fertilisation in the water column. Dispersal potential is high, although in sheltered bays the larvae may be entrapped. Recruitment is linked to the hydrographic regime for dispersal and small scale eddy's (e.g. over obstacles and inconsistencies in the surface of the substratum) may result in concentration of larvae or propagules. High density of adults, suspension feeders and surface deposit feeders together with epibenthic predators and physical disturbance results in high post settlement mortality rate of larvae and juveniles (Olafsson et al., 1994). The larvae of some species may settle outside usual habitat preferences away from areas dominated by adults. Overall recruitment is likely to be patchy and sporadic, with high spat fall occurring in areas devoid of adults, perhaps lost due to predation or storms and habitats may alternate between being deposit feeder or suspension feeder dominated. Similarly larvae may be concentrated by the hydrographic regime or swept to neighbouring or removed sites.

No evidence on community development was found. However, the two key species Echinocardium cordatum and Ensis ensis are long lived species and take a relatively long time to reach reproductive maturity. Razor shells, for example, do not appear to breed before they are three years old and UK populations of Echinocardium cordatum breed for the first time when two to three years old. Recruitment of Echinocardium cordatum is often sporadic with reports of recruiting in only 3 years over a 10 year period (Buchanan, 1966) although this relates to subtidal populations. Intertidal individuals reproduce more frequently. Many of the other species in the biotope, such as polychaetes and bivalves, are likely to reproduce annually. However, because the key species in the biotope, Ensis ensis and Echinocardium cordatum, are long lived and take several years to reach maturity the time for the overall community to reach maturity is also likely to be several years. Recovery of the benthos after mechanical harvesting in the tidal flats of the Wadden Sea, for example, took several years because of the slow re-establishment of a population of another large, long-lived invertebrate Mya arenaria (Beukema, 1995).

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