Bed-forming brittlestars may use sessile organisms as convenient
supports for elevated suspension-feeding, but they are not dependent on them. Predation is
the component of the biotic environment that has been most frequently implicated as a
factor limiting the occurrence of brittlestar aggregations. The influence of predation
over geological time has been discussed elsewhere. However, it may also operate at a
smaller scale to limit the spatial distribution of beds in modern benthic environments.
Aronson (1987a,b, 1989, 1992) has been the chief proponent of the
hypothesis that low predation pressure is a necessary condition for the existence of
present-day brittlestar beds. The hypothesis has been tested by field experiments in the
Isle of Man and the Clyde Sea, involving measurement of the mortality rates of tethered
brittlestars in natural aggregations, and others transported to nearby rocky reefs where
beds are absent. In both the Isle of Man and the Clyde, mortality of tethered brittlestars
was much higher on rocky reefs than in the brittlestar beds. Predators attacked almost
four times as many tethered brittlestars in the reef habitats as they did in the beds.
Fish (mainly wrasse) and crabs were the main predators on reefs, whereas starfish (Asterias
rubens) were the main agents of mortality in the beds. Starfish were common both on
reefs and in brittlestar beds, while fish and crabs were rare outside the reef habitats.
The differences in predation pressure between the two habitats were therefore due largely
to differences in fish and crab abundance. Aronson noted that in situations where an Ophiothrix
bed adjoins a rocky reef, the two are often separated by a halo of several
metres of bare substratum. This may represent the foraging radius of rock-dwelling fish
and crabs, which are inhibited by their own predators (larger fish such as cod, Gadus
morhua) from moving more than a short distance from shelter.
Similar experiments carried out in the Bahamas provided further
evidence in support of the predator-limitation hypothesis (Aronson & Harms, 1985).
Specimens of Ophiothrix oerstedi transplanted from the Sweetings Pond population to
nearby coastal reefs were rapidly eaten by fish. Brittlestars occurred naturally in the
reef habitat but at much lower densities than in Sweetings Pond, and lived cryptically
among corals rather than out in the open. Predatory fish were absent from the pond, where
the only potential brittlestar predators were slow-moving invertebrates.
There is evidence that fluctuations in Ophiothrix fragilis populations
in the western English Channel over a scale of decades may be related to changes in the
abundance of the large predatory starfish Luidia ciliaris (Holme, 1984, Aronson,
1992). This phenomenon, and other natural events affecting brittlestar beds, will be
reviewed in Chapter V.
Predation is less likely to be a controlling factor in the distribution
of Ophiocomina nigra beds, as this species appears to secrete a distasteful mucus
which offers at least partial protection from predators (Fontaine, 1964; Wilson et al.,
1977). Ophiocomina is absent from the stomachs of most fish species caught in the
vicinity of brittlestar beds (B. Ball, personal communication).