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 mans 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.
Keystone (structuring) species
Ophiothrix fragilis, Ophiocomina nigra, Ophiopholis aculeata, Luidia
Importance of habitat for other species
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).
Time for community to reach maturity
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.