COHABITATION OF A CORAL-REEF SPONGE AND A COLONIAL SCYPHOZOAN

Samples from populations of a common Red Sea coral reef sponge, Mycale fistulifera, contained 1.7 to 25% sponges associated with polyps of N ausithoe cf. punctata Kolliker,1853 (Scyphozoa: order Coronata). In th e northern Red Sea this colonial scyphozoan was found exclusively in s ymbiosis with sponges. The proportion of the symbiotic sponges within the population differed significantly between seasons. Polyps were fou nd significantly more frequently on large (>30 cm(2)) sponges than on small (< 30 cm(2)) ones. The polydisc polyps of N. cf. punctata releas ed ephyrae for 1 to 4 mo during the summer, several days before full m oon, mainly during the afternoon and early evening. A colony discharge d up to 1340 ephyrae d(-1), with variations between colonies and years . During the 13.5 mo of monitoring, symbiotic sponges survived better than aposymbiotic ones (43 versus 3%). Few ephyrae survived in the lab oratory until maturity (12% for 30 d). During in situ experiments the starfish Choriaster granulatus avoided symbiotic sponges but fed on ap osymbiotic ones. The echinoid Tripneustes gratilla showed a similar tr end (although statistically P > 0.05), while Diadema setosum fed equal ly or less (P > 0.05) on symbiotic compared with nonsymbiotic sponges. Echinotrix calamaris fed on both symbiotic and nonsymbiotic sponges. This mutual symbiosis seems to be facultative for the sponge and oblig atory for the scyphozoan. Some of the demonstrated benefits of this sy mbiosis are: the sponge enhanced survival, size and relative protectio n; and the absence of free-living scyphozoans indicates the necessity of the association for the survival of N. cf. punctata.