Stenothoe valida, a small (2–4.5 mm) gammarid amphipod frequently associated with coelenterates, is commensal on millepore hydrocorals in Barbados, West Indies. Juvenile amphipods occur singly on the living surface of host colonies at densities of about 7–10 individuals
100˙cm−2. Mature adults (>2.5 mm) were discovered nestling, in groups of 5–15, in cavities formed by the dead empty shells of the commensal barnacle Megabalanus stultus. Both sexes occurred in the same shell and were vigorous in maintaining their relative
positions in the cavities and in resisting the entrance of juveniles from outside. The similarity in size-frequency distributions of the juveniles in winter and summer and the presence of ovigerous females throughout the sampling period, suggest continuous breeding activity. Because dead barnacle
shells become overgrown by coral tissue, continuous breeding would be an advantage to adults occupying these temporary refuges. Nestling by mature adults may provide the opportunity for sexual activity and the development of special reproductive strategies.
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