The Aggregation Behavior of Diadema (Echinodermata, Echinoidea)

Pearse, J.; Arch, S.

Many species of echinoids occur together in aggregations of both single and multiple species composition. Some short term aggregations may be associated with reproductive activities, but in most cases the aggregations seem to be responses to localized environmental factors and involve little "social" interaction among the different individuals (Reese, 1966). Populations of tropical diademid echinoids, however, may form aggregations involving true social behavior. Large "flocks," "families." "colonies" or "social clusters" of Diadema setosum or D. savignyi have been noted on the Great Barrier Reef (Saville-Kent, 1893; Stephenson, et al., 1931; H. L. Clark, 1921, 1946; Roughley, 1951; Dakin & Bennett, 1963), at Suez (Fox, 1926) and off central Japan (Onada, 1936). Similar aggregations have been observed of D. antillarum and Astropyga magnifica in the Caribbean (Randall, et al., 1964; Kier and Grant, 1965) and D. mexicanum in the Eastern Tropical Pacific (Pearse, 1968a). Such aggregations do not seem to be simple passive responses to environmental limitations because adjacent areas of apparently identical substratum and feeding conditions are not occupied. We were able to make intermittent observations of populations of D. setosum and D. savignyi while snorkel diving off Singapore, northeast Borneo (Darvel Bay), New Britain (Rabaul), Bougainville (Kieta), Guadacanal (Honiara), the New Hebrides (Port Vila) and New Caledonia (Noumea). Scattered solitary individuals were found hidden in the reefs of all these areas. Small aggregations of 20 to 30 animals, similar to those described by Magnus (1967) in the Red Sea, were seen around crevices in the reefs and rocks at Rabaul (dock area) and Honiara. Aggregations of up to several hundred animals on open reef flats occurred at Rabaul (Dawapia Rocks), Kieta and Port Vila. Observations made over a period of two days at Rabaul and seven days at Kieta provided data on the stability and social nature of the aggregations.

The Aggregation Behavior of Diadema (Echinodermata, Echinoidea)

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