The Role of Sponge Bioerosion in Mediterranean Coralligenous Accretion

Abstract

Coralligenous biocoenoses developed almost 10,000 years ago in the Mediterranean Sea, forming environments characterized by high levels of biodiversity. They are complex biogenic structures growing through the continuous overlapping of organogenic layers, between 20 and 130 m depth. The coralligenous substrate, for the concurrent presence of bio-builders (algae, serpulid worms, bryozoans, and scleractinians) and destroying elements (clionid sponges, bivalves) is subject to dynamic evolution. Studies conducted on sections of coralligenous blocks show that algae are the main component responsible for the building of these concretions. Calcareous remains of benthic animals lack in mineral structures and are one of the most important detritus-forming elements at the bottom of vertical cliffs.

This seems to be due to the attack of clionids which selectively bore animal rather than plant carbonates. A study on the population dynamics of some anthozoans with a calcareous skeleton has shown that about 100% of the newly settled specimens are probably removed by clionids. In this way, the substratum is cyclically renewed by the continuous boring action of clionids. It is a sort of intermediate disturbance which controls the evolution and the structure of skiophilous zoocoenosis, through primary space regeneration. A high specific richness, typical of coralligenous biocoenosis, is developed by this action. From a geological point of view, it forms a kind of limestone characteristic of biogenic constructions and detritic bottoms under the cliff.