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“Babysitting” symbioses between brittle star species involve juveniles of one species and adults of another. During this phenomenon, reported from many localities in the Indo-Pacific Ocean, juveniles are attached to the disk or lie in the bursa of the adults. The symbiosis between members of Ophiomastix venosa and their host, Ophiocoma scolopendrina, was investigated on the Great Barrier Reef of Toliara (Madagascar) during a 14-month period. The population of O. scolopendrina only occurs on rocky spurs that frequently emerge at low tide, while the population of O. venosa lies in adjacent surge channels that are always immersed. Only juveniles of O. venosa associated with adults of O. scolopendrina may occur on the rocky spurs. Analyses conducted on the populations of the two species showed that (1) the symbiosis is facultative and that symbiotic juveniles migrate into the channels when they reach a disc diameter of 6 mm, and (2) recruitment in the channels occurs in April. Host choice experiments, Y-tube experiments, and experiments assessing the resistance of the ophiuroids to air-drying were conducted in the laboratory. The experiments clearly demonstrated that (1) symbiotic juveniles of O. venosa specifically recognize adults of O. scolopendrina, while free juveniles of the same size do not, and (2) juveniles of O. venosa would not survive air-drying conditions similar to those observed on the spurs at low tides if they were not in symbiosis with adults of O. scolopendrina. [ABSTRACT FROM AUTHOR]