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.

Circalittoral rock provides a firm attachment in areas of moderate wave action or tidal currents such that the sessile habit of many species may be advantageous. Firm attachment will prevent the species from being swept away to what might be unfavourable conditions, and will prevent them from being damaged by impact on rocks. Under conditions of moderate exposure the taller erect forms such as sea fans, soft corals and the like are prominent. These species still tolerate considerable exposure though they have a tough yet flexible structure, which enables them to withstand turbulence and strong currents without damage and also allows individuals to maximise food intake.

Whilst most moderately exposed 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 rock 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 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 producer’s circalittoral rock 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.

Circalittoral rock biotopes in moderately exposed conditions typically are not dominated by single species, but support a diverse mosaic of species. The biotopes where single species dominate (e.g. Modiolus, Sabellaria and brittlestars) are described separately.

Circalittoral rock communities interact with others by the provision of food and /or temporary shelter to mobile species which are not permanent faunal turf fauna. 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.

One of the features of circalittoral faunal turf communities is their fine-scale spatial variation, which tends to be very patchy. Whilst the infralittoral tends to be more predictable, circalittoral rock tends to be a mosaic of different species patches such mosaicing is particularly pronounced in moderately exposed circalittoral communities compared with exposed and sheltered communities. The different assemblages may represent ‘alternate stable states’ (Sutherland 1974; Sebens 1985a, b). In most of these biotopes substratum space is very fully occupied and the availability of space is a controlling resource for the settlement and growth of species. According to when free space is made available, and on which species are recruiting at that time, different assemblages of species may develop under the same physio-chemical conditions. Once established, often following a successional sequence (Hextall 1994), these assemblages are stable for long periods and different assemblages may co-exist in close proximity.

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 very prominent member of the circalittoral rock community and observations have shown that colonies of 10-15cm in height are between five and ten years old (Hartnoll unpubl.).

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