Environmental factors determine which circalittoral rock species can inhabit a given location and this will depend on the biological characteristics of each species – such as size, habit, feeding method and reproductive mode. However, not every species occurs throughout its potential range – its realised distribution is moderated by biological interactions such as competition, grazing and predation. Circalittoral communities have been poorly studied in respect to biological interactions because in order to determine the real role of species in a community experimental manipulation in the field is required. This has been carried out extensively in the intertidal and infralittoral but rarely in the circalittoral owing to the logistics of working at such depths.

A great majority of prominent circalittoral rock species are sessile. However, under conditions of shelter a number of the circalittoral faunal turf species may be mobile. Such species consist mainly of decapod crustaceans, gastropod molluscs and echinoderms, and as grazers or predators these must be able to move to locate further food supplies. Even so, many of them are very well attached to the rocks, such as starfish and sea urchins with their many sucker-like tube feet.

Whilst most of the circalittoral rock species spend their larval life in the plankton, there are a few planktonic species which spend their early stages within the circalittoral rock biotopes. This is true, in rather different degrees, of the hydroids and the jellyfish. Hydroids are common and conspicuous members of circalittoral faunal turf biotopes, but the attached hydroids are only the juvenile stages. The sexually reproducing mature stages are small medusae which are released into the plankton, where they reproduce to produce larvae which settle again. In contrast, for jellyfish the large adult medusae in the plankton are the prominent phase. The juvenile stages live attached to rocks as an inconspicuous scyphistoma stage in which the jellyfish overwinters. In spring this buds off a series of juvenile medusae, or ephyrae, which grow rapidly in the plankton to form the adult.

Although not primary producers circalittoral communities are important secondary producers. They accumulate and concentrate the primary production from a large water mass, and make this readily available to higher trophic levels.

Sheltered circalittoral rock biotopes typically are not dominated by single species, but support a mosaic of species.

Circalittoral rock communities interact with others by the provision of food and /or temporary shelter to mobile species which are not permanent members of the community. Shelter is important to juvenile fish, which can find refuge (and food) amongst the dense turf of sessile species. A food source is provided to large mobile crustaceans and fish which are attracted by the rich and stationary food supply available on circalittoral rock.

Sheltered circalittoral rock biotopes tend to be relatively stable in nature when compared with semi-exposed circalittoral communities.

Information is restricted, but it is clear that a number of the more prominent members of the circalittoral rock communities are relatively long lived, and fairly slow growing, some with life spans ranging from 6-100 years. The soft coral Alcyonium digitatum is a prominent member of the circalittoral rock community and observations have shown that colonies of 10-15 cm in height are between five and ten years old (Hartnoll, unpubl.).

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