Sponges (Porifera) In Submarine Caves

Abstract

Sponges (phylum Porifera) have the most simple and probably the most ancient body plan of the multicellular animals. They are, however, very successful in most benthic ecosystems. They are dominant in submarine caves, where their study has provided unexpected results. Submarine caves share several ecological features with deep-sea habitats. The communities living in these habitats depend on allochthonous organic input, and some bathyal species are able to colonize the darkest parts of littoral caves. Some examples will be given, including calcified sponges,«living fossils» of ancient reef builders. Faunal and environmental similarities, however, are limited by the obvious differences in pressure, temperature, habitat size, and by the dispersal abilities of deep water organisms. A recently discovered Mediterranean cave more closely approximates deep-sea conditions, due to the entrapment of a cold water mass resulting in stable temperature conditions throughout the year. Easily accessible to scuba divers, this «bathyal island» in the littoral zone is a natural mesocosm of the deep Mediterranean which offers exceptional opportunities for deep-sea biology. Preliminary results have already provided unexpected insights, which are illustrated. A large population of Oopsacas minuta, a representative of the bathyo-abyssal hexactinellid sponges, reproduces here year round - making possible the first observations of larval behaviour and ultrastructure to be carried out on this phylogenetically important group of invertebrates, and opening up the poorly known area of larval ecology of these deep-sea sponges. The presence of a species of the deepest known genus of sponges, Asbestopluma (8 840 m in the Central Pacific) is a fascinating opportunity to investigate the biology of the strange deep-sea cladorhizid sponges, which may live in the most oligotrophic abyssal basins. A highly unexpected result is that they are non-filter-feeding sponges with a carnivorous feeding habit. They capture and digest small crustaceans by means of filaments provided with minute hook-shaped spicules. The biology and peculiarities of this carnivorous sponge are illustrated by a video.