Composition and biostratinomy of sponge-rich biogenic crusts in submarine caves (Aegean Sea, Eastern Mediterranean)

Submarine caves constitute “natural laboratories” for studying biodiversity, community structure and inter-specific relationship in cryptic habitats. These unique systems are of strategic importance to reconstruct the complex biotic processes developed in cavities of skeletal frameworks through geologic time. In this study, the biogenic crusts in two submarine caves of the Aegean Sea were characterized with regards to the organisms involved in their formation. The crusts were examined with the aim to investigate the possible influence of environmental parameters and/or biotic associations on the framework. The walls and ceilings of the studied Fara and Agios Vasilios Caves (Lesvos Island, Greece) are covered mainly with sponges, coralline algae, scleractinian corals, serpulid polychaetes, and bryozoans. Skeletons of these organisms are often cemented together and form extensive crusts of variable thickness. Optical and electron microscopy were utilized to detect quantitative relationships among skeletal components inside the crusts. Coralline algae and corals dominate close to the opening of the caves, whereas serpulids, bryozoans and sponges are the main crust builders in the innermost cave sectors. Among skeletal components, the micrite (microcrystalline calcite) is a minor component of the crusts and consists mainly of an autochthonous fraction, mineralized directly inside the crust through organomineralization processes. Crusts show abundance of endolithic and insinuating sponges, whose organic tissue decay leaves borings filled with spicules. The abundance of sponges competing with carbonatogenetic bacteria for the same living cryptic spaces prevented the development of microbialites. This competition explains the morphological differences between the studied biogenic crusts and the large biostalactites, which are common in other Mediterranean caves (i.e. Sicily, Apulia and Cyprus) and are characterized by only a few sponges but abundant microbialites.