A QUANTITATIVE STUDY OF BIVALVES BIOERODED BY CLIONAIDAE SPONGES, FROM SAN SALVADOR, BAHAMAS

Cliona sponges are known to play an important role in bioerosion. The sponge excavates a framework of chambers and galleries, so-called Entobia, within calcareous hosts. Most common are: corals, bivalves, and gastropods. This study evaluates the abundance of Entobia in 1063 bivalve shells sampled from coral reefs, seagrass meadows and nearshore sandy bottoms from eight localities around the island of San Salvador, The Bahamas.

A higher proportion of bivalves in coral reefs are bored by Cliona sponges compared to the two other habitats (average of 36% in coral reefs, average of 10% in seagrass meadows and an average of 6% in near shore sandy bottoms). The data suggest that Entobia was comparably frequent in the smaller size classes (bivalve specimens between 8 mm and 48mm in height). The frequency of Entobia increased in larger size classes (100% of bivalves with size above 80mm). The largest species of bivalves were found in coral reefs and seagrass meadows (in total ranging from 8-89 mm) whereas only smaller sized shells were sampled from the sandy bottoms (8-40 mm of which only 7% was fragmented). Specimens from near shore sandy bottom were also bored by Cliona sponges, but the material was sampled along with coral rubble, thus suggesting the bivalve specimens with Entobia were transported from a nearby reef. Focus on distribution of the most common species B. cancellaria it shows tentavily a similar pattern regarding the grade of Entobia for the coral reefs and sandy bottoms. Encrusted bivalves occurred in all three habitats but did not show a tendency of higher frequency of Entobia versus non-encrusted bivalves. High distribution of Entobia in modern coral reefs contributes to the understanding of the bioerosional effect caused by Cliona sponges.