Biodiversity & Conservation

Ecological relationships within the biotope are not particularly complex and the main functional groups are those that are dependant on high levels of suspended particles, the suspension and deposit feeders. Although abundance of the key functional species, the common piddock Pholas dactylus, and the tube worm Polydora ciliata may be high, other fauna are relatively sparse so competition for space is not likely to be a factor structuring the biotope. In relatively unstable areas like soft chalk or clay, there is an opportunity for short-lived species to take up residence. These species, such as the sea-squirt Molgula manhattensis, settle out from the plankton at various times of the year and thrive where there is least competition from well established species.

There are few species that prey on other members of the community although feeding by fish and predatory crabs probably occurs. The dahlia anemone Urticina felina is a passive carnivore, waiting to trap animals that stumble into its tentacles.

Crabs, such as Pisidia longicornis are the predominant mobile species in the biotope, travelling through as they scavenge for food.

The abundance of filter feeding organisms such as sponges, bryozoans and tunicates within the biotope indicates the importance of planktonic input to the benthic community. Piddocks and the tube worm Polydora ciliata contribute to the creation of a relatively high silt environment through burrowing activities.

Some of the characterizing species in the biotope, such as piddocks, the sponge Halichondria panicea and the anemone Urticina felina have a longevity of several years and may not show great seasonal changes. Abundance of the polychaete Polydora ciliata is highly seasonal with numbers of individuals dropping off significantly in winter months before the reproductive period begins in the spring. Annual species in the biotope such as the hydroids Tubularia indivisa and Nemertesia antennina will increase and decrease through the seasons. Other species such as Alcyonium digitatum have seasonal stages, 'shutting up shop' during the winter months.

Chalk or clay platforms are not particularly structurally complex habitats. However, piddock burrowing forms a generally uneven surface on a small scale (5-15 cm) creating habitats for other animals that inhabit vacant burrows and crevices in the rocks. Empty burrows are colonized by various animals, including sponges, anemones such as Sagartia elegans, bristleworms like Pomatoceros triqueter, crabs and bryozoan sea mats (Pinn et al., in press). The empty shells protruding from the eroded surface are also an important settlement surface within this habitat. In addition to piddock borings, the top centimetre or so of the chalk is often riddled with large numbers of tiny U-shaped burrows of the bristleworm Polydora ciliata and in silty habitats may be associated with the amphipod Jassa falcata. Scattered on the chalk platforms, small rounded chalk pebbles and larger more angular cobbles on the surface may support sparse small hydroids on upper surfaces and occasional red algae. Where massive growths of the sponge Halichondria panicea occur, they may provide a significant habitat for other species especially amphipods.

No algal species are listed as characterizing species in MCR.Pid, although some red algae may be present attached to cobbles, so primary production is not a major component of productivity. Specific information about the productivity of characterizing species or about the biotopes in general are not available. However, many of the species that are present are either suspension or deposit feeders so productivity of the biotope will be largely dependent on detrital input.

Most of the characterizing species in the biotope are sessile or sedentary suspension feeders. Recruitment of adults of these species to the biotope by immigration is unlikely. Consequently, recruitment must occur primarily through dispersive larval stages. Some species have larvae that can disperse widely and these may arrive from distant locations.

• Pholas dactylus usually spawns between May and September. The larvae are pelagic, with settlement and recruitment of juvenile piddocks occuring between November and February (Pinn et al., 2005).
• The spawning period for Polydora ciliata is from February until June in northern England. Larvae are substrate specific selecting rocks or sediment according to their physical properties settling preferentially on substrates covered with mud.
• Among sessile organisms, patterns fixed at settlement, though potentially altered by post settlement mortality, obviously cannot be influenced by dispersal of juveniles or adults.
• Some of the species in the biotope do not have pelagic larvae, but instead have direct development of larvae producing their offspring as 'miniature adults'.

The time for biotope MCR.Pid to reach maturity is unknown. However, most characterizing species have a planktonic larva and so colonization should be fairly rapid. Colonization time by the key structuring species, Pholas dactylus, is unknown but it is expected that recruitment should be fairly rapid as the species has pelagic larvae and spawns for several months in the summer. Polydora ciliata, for example, can recolonize areas within a few months and is able to disperse over large distances. Also, although the recruitment of Pholas dactylus is annual, very few of the individuals get beyond their first year (Pinn et al., 2005). Some species, such as the sponge Halichondria panicea, are fast growing and mobile species within the biotope, predominantly crabs, can migrate into the area. The anemone Urticina felina, however, is noted as having poor dispersal (Sol-Cava et al., 1994) and being slow growing and living for several years (Chia & Spaulding, 1972). Therefore, although many species can colonize a suitable area fairly rapidly it is expected that the community as a whole would take longer to reach maturity, probably within five years. How