Aggregation behaviour of the brittle-star Ophiothrix fragilis

Abstract

Dense aggregations of echinoderms on the sea bed have been described from various parts of the world. Reese (1966) considered that such an aggregation ‘is a response to one or more essential environmental factors such as food or substratum, and does not reflect social behaviour per se’ Observations by Warner (1969, 1971) and by Brun (1969) suggested that, in aggregations of the brittle-star Ophiothrix fragilis, individuals may respond to conspecifics rather than solely to food or the substratum. On an apparently uniform substratum, some areas are covered with a dense bed of brittle-stars while other adjacent areas are devoid of brittle-stars. Warner (1971) pointed out that the aggregations are advantageous to the species so it seems possible that the animals might aggregate actively rather than coming to a suitable feeding area by chance. The aims in this study were to investigate the responses shown by Ophiothrix fragilis to its physical surroundings, to some of the other organisms living near it and, especially, to conspecifics. The animals studied are in dense aggregations off Berry Head in Southern England and all observations were made underwater in the natural habitat.