Abstract

Epibiontic Porifera on shells of the bivalve mollusc Plnna nobilis were studied along the coast of Montecristo Island. Italy. Thirty-five species were found; 6 on more than 30 Yb of the examined molluscs, the others sporadic in occurrence. The per cent area covered varied from 0 to 68 % and the number of species per specimen of P. nobhs varied from 0 to 11. The number of epizoic individuals and the number of specles per mollusc was highly correlated with the age of the host showing that the population is still developing throughout the host's Life-span. The shell of P. nobiljs does not seem to be selectively settled by Porifera. The sponge population differs structurally on the 2 valves of the same mollusc specimen. Small shell size and distance from the rocky coast probably limit sponge settlement opportunities. The colonization pattern by sponges on P nobilis shells conforms to the facilitation model in its first stages and later to the inhibition model.

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