Associated Fauna

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). However, beds on softer substrata may contain a rich associated fauna (Warner, 1971; Allain, 1974; Davoult & Gounin, 1995). Allain (1974) provides a list of the species found by various authors in brittlestar beds in the English Channel and Irish Sea. Large suspension-feeders such as the octocoral 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 not smothered by the brittlestars, usually being surrounded by a ‘halo’ of clear space (Brun, 1969; Warner, 1971). Urticina will eat brittlestars, hence their avoidance of it.

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. Warner found that 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 (eg. Downie, 1990).

Where brittlestar beds exist on muddy gravel or sandy substrata, the underlying sediments may contain a diverse infauna. Warner (1971) found that numbers and biomass of sediment-dwelling animals were not significantly reduced under dense brittlestar patches. Shell length of the commonest animal, the bivalve Abra alba, showed no correlation with the presence of brittlestars. There was thus no evidence that brittlestar beds restricted the occurrence or growth of other benthic animals. Deposit-feeding animals might even find conditions more favourable under beds as a result of the increased deposition of organic matter (from brittlestar faeces) in those areas. Similar conclusions were drawn by Allain (1974), and by Davoult & Gounin (1995), who noted that the epifaunal animal community on brittlestar-covered pebble bottoms in the Dover Strait was more diverse than nearby communities in fine sediments experiencing lower current speeds.

The larger animals found in and around brittlestar beds are all found in other coastal benthic biotopes, and there are no known examples of species dependent on the beds or obligately associated with brittlestars. However, it is not impossible that some specialist brittlestar commensals may eventually be recognized amongst the smaller fauna.

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