The brittle-star Ophiothrix fragilis forms dense aggregations on the sea bed off Plymouth, as well as in other parts of the English Channel. From about 1970 there was a marked decline in populations off Plymouth, and study of previous records suggests that there have from time to time been considerable fluctuations in abundance of this species — since beds were first reported by E. J. Allen in the 1890s. The decline did not extend to populations in the southern and eastern parts of the Channel. The distribution of Ophiocomina nigra, which sometimes occurs in mixed populations with Ophiothrix, is also described. This species is more or less limited to the western Channel, and populations, which are nowhere very dense, showed no decline comparable to that of Ophiothrix.Predation by the starfish Luidia ciliaris and L. sarsi is thought to be a likely cause of the disappearance of Ophiothrix. Luidia spp. are active predators on ophiuroids and other echinoderms, and numbers of L. ciliaris were observed in the vicinity of the Ophiothrix beds at the time of the decline. Both species of Luidia have a ’western’ distribution in the Channel, and showed an increase in density off Plymouth after the late 1960s, which is probably related to a changeover from a Sagitta setosa to a 5. elegans plankton community from 1968 on. The distribution of these Luidia is confined to stratified waters in the western Channel, surviving stocks of Ophiothrix being limited to mixed waters in the southern and eastern Channel where Luidia does not occur.
