Fluctuations ofOphiothrix fragilisin the western English Channel

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Abstract
The brittle-starOphiothrix fragilisforms 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 ofOphiocomina nigra, which sometimes occurs in mixed populations withOphiothrix, 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 ofOphiothrix.Predation by the starfishLuidia ciliarisandL. sarsiis thought to be a likely cause of the disappearance ofOphiothrix. Luidiaspp. are active predators on ophiuroids and other echinoderms, and numbers ofL. ciliariswere observed in the vicinity of theOphiothrixbeds at the time of the decline. Both species ofLuidiahave 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 aSagitta setosato a 5.elegansplankton community from 1968 on. The distribution of theseLuidiais confined to stratified waters in the western Channel, surviving stocks ofOphiothrixbeing limited to mixed waters in the southern and eastern Channel whereLuidiadoes not occur.

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