Infestation by excavating sponges on the oyster (Ostrea edulis) populations of the Blanes littoral zone (north-western Mediterranean Sea)

In order to estimate the degree of implantation of excavating sponges in th e oyster populations from the littoral zone at Blanes (north-western Medite rranean Sea), a representative sample of oysters (Ostrea edulis) was collec ted from depths between 0.5 and 4 m in 1986. All oysters were infested by t he excavating sponges Cliona viridis and Cliona celata. Infestation affecte d always and exclusively the lower valve always reaching the inner side. Th e degree of infestation proved to be independent of oyster size and weight. Colonization by contact was considered the main means of dispersion of the se sponges within the oyster population. Cliona viridis and C. celata emerg ed as the single excavating species able to effectively compete with other non-excavating or excavating organisms when trying to colonize the particul ar calcareous substrate provided by oyster shells in the north-western Medi terranean Sea.