Biodiversity & Conservation

In this circalittoral biotope there are few algal species and limited primary production. The fauna is dominated by attached species such as hydroids, solitary ascidians and sponges. The fauna consists predominantly of suspension feeders.

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

However, there are some organisms known to be associated with Antedon bifida such as the rare caridean crustacean Hippolyte huntii (Gosse) and the more common Hippolyte varians Leach and Thoralus cranchi (Leach). These species are cryptically concealed on the feather-star by their colouration. The amphipod Apherusa bispinosa (Bate) and a little known scale worm Adyte pellucida have also been found with Antedon spp. (Leonard & Jeal, 1984). The myzostome Myzostoma cirriferum Leuckart is a well known ectoparasite on the arms and disc of Antedon spp. Level of infestation, however, varies from location to location (Leonard & Jeal, 1984). Sponges and solitary ascidians can provide substratum for small epifaunal species such as hydroids.

There are few predators in the biotope and predation levels are unknown. However, the various mobile echinoderms found in the biotope (e.g. Asterias rubens, Crossaster papposus and Solaster endeca) may have a role in modifying benthic populations through predation. Echinus esculentus is an omnivorous grazer that feeds on hydroids, barnacles and other encrusting invertebrates and the species may also modify faunal abundance and distribution. Hydroids are often eaten by nudibranchs. Nemertesia ramosa, for example, is fed on by a variety of sea slugs including Doto fragilis, Doto cuspidata, Lomanotus genei, and by the sea spider Endeis spinosa.

There will be some temporal changes in the coverage and shelter provided by the species in the biotope. For example, Ciona intestinalis is typically an annual species, although reproduction can occur throughout the year so overall population numbers are probably quite constant over the period of a year. There is much seasonal variability in abundance of individual hydroids and experiments have shown that colony growth is highest over a defined temperature range (Gili & Hughes, 1995). However, there is much interspecific variability in temporal abundance, where different species are found at different times of the year so total hydroid coverage in the biotope may remain constant throughout the year. Seasonal cycles of hydroids are much more pronounced in shallow waters than those at greater depths, probably because of the greater seasonal differences in temperature. Other species such as the anthozoan Alcyonium digitatum have seasonal stages, 'shutting up shop' during the winter months. Otherwise, it is unlikely that there will be any significant seasonal or other temporal changes in the biotope.

The biotope habitat is not particularly complex as fauna are sparsely distributed and there may be areas of unencrusted rock. Some microhabitats may be provided by cracks and crevices in the rock. Some of the larger species such as sponges and solitary ascidians can provide substrata for attachment, refugia and shelter of a number of animals.

• The only photosynthetic species listed as characterizing in the biotope are algal crusts. Consequently, primary production is not a major component of productivity. High densities of encrusting or attached organisms, solitary ascidians, feather stars and tube worms can result in quite high secondary productivity through suspension feeding. However, the fauna in SCR.AntAsH are relatively sparse so secondary production in the biotope is probably not very high.
• Antedon bifida may lose up to half their pinnules through predation, largely, if not entirely by the corkwing wrasse (Nichols, 1994). Feather-stars can replace the loss of one fifth of the disc and the corresponding arm and can sustain the simultaneous loss of four of the five pairs of arms without death (Barnes, 1980).
• An abundance of hydroids together with high food capture rates often recorded and the fact that they are consumed by many different benthic invertebrates indicates that they may have an important, but little known, role in marine food webs by transferring energy from the plankton to the benthos. However, abundance of fauna is not very high so the biotope is not likely to represent an important food resource for a number of predatory species, such as starfish, decapod crustaceans and fish.

Most of the species in the biotope have pelagic larvae so recruitment will be largely from the water column. In sheltered sea lochs tidal flows are not very strong so there may not be a high supply of planktonic larvae so that many species may take a long time to recruit from outside if local populations are lost. However, many of the characterizing species in the biotope, particularly the hydroids and some ascidians have short lived planktonic larvae so dispersal distances are short and recruitment will largely be from local populations. For example, intense local recruitment can occur with Ciona intestinalis where sticky mucus strings containing eggs and larvae are trapped round nearby adults or other objects. Other species such as the anthozoan, Alcyonium digitatum, have long lived and potentially dispersive pelagic larvae. Where neighbouring populations are present recruitment may be rapid but recruitment from distant populations may take a long time.

There is no information available on the time it takes for the biotope to reach maturity. The lifespan of Antedon bifida is unknown although since sexual maturity is not reached until individuals are over 1 year old. Longevity is therefore, likely to be several years. There is much interspecific variability in the lifespan of hydroids. Several species are known to live for 1 year or less (e.g. Tubularia indivisa and some with several generations per year (e.g. Nemertesia antennina). In the case of hydroid colonies it is difficult to determine the life span unless all the hydroids die at the same time so that a distinct change of generations is identifiable. In some cases it appears that old colonies die before the summer and new ones that grew around their bases in the spring, grow in their place in the autumn. Therefore, it appears that many hydroids live for many years. Large sponges may have considerably greater longevity and slower growth. Therefore, it may take several years for the community to reach maturity.