Cohabitation of a coral reef sponge and a colonial scyphozoan

Abstract

Samples from populations of a common Red Sea coral reef sponge, Mycale fistulifera, contained 1.7 to 25% sponges associated with polyps of Nausithoë cf. punctata Kölliker, 1853 (Scyphozoa: order Coronata). In the northern Red Sea this colonial scyphozoan was found exclusively in symbiosis with sponges. The proportion of the symbiotic sponges within the population differed significantly between seasons. Polyps were found significantly more frequently on large (>30 cm²) sponges than on small (<30 cm²) ones. The polydisc polyps of N. cf. punctata released ephyrae for 1 to 4 mo during the summer, several days before full moon, mainly during the afternoon and early evening. A colony discharged 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 laboratory until maturity (12% for 30 d). During in situ experiments the starfish Choriaster granulatus avoided symbiotic sponges but fed on aposymbiotic ones. The echinoid Tripneustes gratilla showed a similar trend (although statistically P>0.005), while Diadema setosum fed equally or less (P>0.05) on symbiotic compared with nonsymbiotic sponges. Echinotrix colamaris fed on both symbiotic and nonsymbiotic sponges. This mutual symbiosis seems to be facultative for the sponge and obligatory for the scyphozoan. Some of the demonstrated benefits of this symbiosis are: the sponge enhanced survival, size and relative protection; and the absence of free-living scyphozoans indicates the necessity of the association for the survival of N. cf. punctata.