Aggregations of Ophiothrix fragilis result from the active association of animals with their conspecifics (i.e. true social behaviour is displayed), rather than simply from the individual responses of the brittlestars to features of their physical environment. Ophiocomina nigra is less tolerant than Ophiothrix fragilis of close contact with conspecifics. Individuals of this species often show a dispersed, non-random spatial distribution, this pattern only breaking down at very high local population densities. Individuals of Ophiocomina nigra will maintain a dispersed distribution from each other even when mixed with much larger numbers of Ophiothrix.

Large mobile animals commonly found on Ophiothrix beds include the starfish Asterias rubens, Crossaster papposus and Luidia ciliaris, the urchins Echinus esculentus and Psammechinus miliaris, edible crabs Cancer pagurus, swimming crabs Necora puber, Liocarcinus spp., and hermit crabs Pagurus bernhardus. Brittlestar beds are not a major habitat for fish, although Warner (1971) recorded poor cod Trisopterus minutus shoaling over the beds in Torbay.

There is evidence to suggest that massive aggregations of suspension-feeding brittlestars can have a favourable effect on water quality in coastal environments and may even help counteract some of the potentially harmful effects of eutrophication.

Brittlestar beds may appear at first glance to support few animals besides the brittlestars themselves. Where dense Ophiothrix aggregations are found on bedrock surfaces they may monopolize the substratum, virtually to the exclusion of other epifauna (Ball et al. 1995). In comparison, beds on softer substrata may contain a rich associated fauna (Warner 1971; Allain 1974; Davoult & Gounin 1995). Allain (1974) provided a list of species found by various authors in brittlestar beds in the English Channel and Irish Sea. Large suspension-feeders such as dead man’s fingers Alcyonium digitatum, the anemone Metridium senile and the hydroid Nemertesia antennina are present mainly on rock outcrops or boulders protruding above the brittlestar-covered substratum. The large anemone Urticina felina may be quite common. This species lives half-buried in the substratum but is smothered by the brittlestars, usually being surrounded by a ‘halo’ of cleared space (Brun 1969; Warner 1971). Utricina will eat brittlestars, hence their avoidance of it.

Several species of large, mobile crustaceans and echinoderms can be found on brittlestar beds although it is unclear whether the juvenile forms of these animals make use of the habitat as a nursery area.

Brittlestar beds represent major concentrations of benthic biomass and may play an important role in the functioning of their local ecosystems. It is thought that dense Ophiothrix beds may play an important role in local nutrient cycles by filtration and concentration of suspended particulate matter, and by the excretion of nitrogenous waste.

Ophiothrix fragilis, Ophiocomina nigra, Ophiopholis aculeata, Luidia ciliaris.

Warner (1971) found the Ophiothrix was preyed upon by crabs, dragonets Callionymus lyra and plaice Pleuronectes platessa, but did not seem to be a major food item for any of them. The large starfish Asterias rubens and (especially) Luidia ciliaris are also brittlestar predators, and are usually actively avoided by them. A starfish moving through an Ophiothrix bed is preceded by a ‘bow-wave’ of brittlestars moving out of the way.

Brittlestars of the genus Ophiura are known to be a common prey for flatfish such as plaice (e.g. Downie 1990).

In the Plymouth area, dense Ophiothrix beds were recorded at the turn of the century, but were apparently absent during the 1920s and 30s. Beds were recorded again from the early 1950s onwards, and persisted until the late 1960s. From about 1970 onwards, the extent and density of Ophiothrix populations declined rapidly and only scattered individuals were present by the end of the decade. The 1970s decline of Ophiothrix was associated with an increased abundance of the predatory starfish Luidia ciliaris in the Plymouth area. There are suggestions that the cyclic changes earlier in the century were also related to the abundance of Luidia (Holme 1984).

Records from several areas suggest that brittlestar beds can persist for years or decades. The life span of Ophiothrix individuals is probably 2 – 8 years. Ophiocomina nigra grows slowly and lives for upto 14 years.

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