Biodiversity & Conservation

The biotopes represented by MCR.ErSEun are sponge and soft coral dominated. Sponges are noted as being inhabited by a wide diversity of invertebrates. Sponges can provide hard substrata for attachment, refugia and shelter, an enhanced food supply in feeding currents and a potential food source themselves (Klitgaard, 1995; Koukouras et al., 1996.)

The fauna associated with sponges in temperate to cold waters is considered to be facultative rather than obligate and reflects the fauna of the local geographic area (Klitgaard, 1995)

Predation levels of the characterizing species in the biotope are poorly understood. Eunicella verrucosa is preyed upon by the sea slug Tritonia nilsodhneri and Alcyonium digitatum by Tritonia plebeia. Alcyonium digitatum and Alcyonium glomeratum are preyed upon by the prosobranch Simnia patula Grazing by the sea urchin Echinus esculentus may modify faunal abundance and distribution. Some species of temperate sponge contain chemicals that can inhibit sea urchin feeding (Wright et al., 1997)

Large colonies of Pentapora fascialis with their complex laminar structure are noted as potentially sheltering thousands of other animals. Pentapora fascialis in the Mediterranean supports various epibiotic species, some of which may cause partial mortality of colonies (Cocito et al., 1998(a)).

The various mobile echinoderms characteristic of the biotope (e.g. Luidia ciliaris, Henricia oculata, Asterias rubens) may have a role in modifying other benthic populations through predation.

Eunicella verrucosa provides a habitat for the nationally rare sea anemone Amphianthus dohrnii.

Where the deposit feeding sea cucumber, the cotton spinner Holothuria forskali occurs, it may be important in removing silt and enabling settlement of other benthic species.

Annual species in the biotope such as Nemertesia ramosa will increase and decrease through the seasons. Other species such as Alcyonium digitatum have seasonal stages, retracting their polyps and not feeding from about July to November, during which time the surface of the colony becomes covered with encrusting algae and hydroids (Fish & Fish, 1996). When the colony recommences feeding in December the surface film, together with the surface epithelium, is shed. The main species used to represent the biotope, Eunicella verrucosa, Axinella dissimilis, & Pentapora fascialis are typically long-lived perennials. Where the biotope occurs in the lower infralittoral or upper circalittoral, extensive growth of annual algae may occur, especially in years when the water is clear.

Many of the species characteristic of this community add considerable physical complexity to the biotope. There are upright branching and cup sponges, sea fans, colonies of dead mans fingers and erect bryozoans. All of these species add depth and a three dimensional structure to the substratum. The biotope occurs on bedrock and boulders which may provide overhangs, crevices and shelter where crevice dwelling species such as sea cucumbers (Aslia lefevrei), squat lobsters and wrasse (mainly Centrolabrus exoletus) may live. Complex upright bryozoans as well as many sponges are recorded as providing substratum and shelter for other species . Sponge morphology is important in determining the number and abundance of inhabitant species. Sponges with a spicule 'fur' have more associated taxa than sponges without (Klitgaard, 1995). For example, Axinella species have a spicule 'fur' (Moss & Ackers, 1982). Hayward & Ryland, (1979) record large colonies of Pentapora foliacea as potentially sheltering thousands of other animals.

• The biotope MCR.PhaAxi has a similar sponge component to MCR.ErSEun but has different associated fauna and occurs in deeper water with greater wave exposure.
• MCR.ErSPbolSH is again a sponge dominated biotope with an understorey of hydroids and bryozoans. Although still on fairly stable substrata some of the species present are associated with more ephemeral or disturbed biotopes.

No photosynthetic species are listed as characterizing species in MCR.ErSEun, a circalittoral biotope. Consequently, primary production is not regarded as a major component of productivity. Nevertheless, some characteristically deep water species of algae are often present and near to the infralittoral algae may sometimes be abundant. The biotopes MCR.ErSPbolSH and MCR.ErSSwi may have a small algal component. The biotopes are often species rich and may contain quite high animal densities and biomass. Specific information about the productivity of characterizing species or about the biotopes in general are not available.

Most of the characterizing species in the biotope are sessile 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. Other species such as Pentapora foliacea have larvae that typically exist for only a short time and will settle in the proximity of the parent (Cocito et al., 1998b). Recruitment of the mobile predators and grazers may be through immigration of adults or via a larval dispersal phase.

Some species within the biotope community are annuals and recruit each year (e.g. Nemertesia ramosa). Other species are potentially very slow growing and long lived such as Eunicella verrucosa which may live as long as 50 years (K. Hiscock pers. comm.).

The main trophic group in the biotope is suspension feeders although there may be several species of fish and echinoderm predators or grazers present.