Aggregation behaviour of the brittle-star ophiothrix fragilis


The role of behaviour in the formation and maintenance of dense aggregations of Ophiothrix fragilis has been studied by experiments carried out on the sea-bed. Individual animals were removed from extensive beds of brittle-stars with a density of 1000-2000/m-2 and placed nearby on the muddy gravel bottom. At current strengths of 4–8 cm/s, the brittle-stars paused for about 20s then started walking across current. A series of walks and turns continued unless other brittle-stars were encountered. In this case the animal usually climbed on top of the others and stopped walking. Walking was not terminated if the animals encountered stones, weed, hydroids, or Alcyonium colonies. Large anemones and starfish were avoided. When groups of animals were placed together on the bottom, the minimum group size which survived for one day on open muddy gravel was 35 and it seemed that at least 100 were needed for the continued survival of a group. Smaller groups could survive only if protection was afforded by hydroids, Alcyonium or inanimate objects. The experiments demonstrate that these brittle-stars are able to recognize and respond to conspecifics and that these responses, together with responses to currents, are important in the maintenance of aggregations and hence the preservation of the optimum situation for suspension feeding. © 1975, Marine Biological Association of the United Kingdom. All rights reserved.