Association between calcareous Clathrina cerebrum (Haeckel) and bacteria: electron microscope study

A homogeneous population of gram-negative elongated bacteria grows within the canal system of the calcareous sponge Clathrina cerebrum (Haeckel). The location of symbionts is probably related to the ascon organization of the host sponge. Bacteria have outer and inner membranes in the cell envelope and a number of lamellar mesosomes in the cytoplasm. They show a rough surface and peduncles at both extremities, due to a complex folding of the outer membrane. Most bacteria live in close association with the choanocytes, cells lining sponge canals. After in vitro sponge dissociation, bacteria still embrace isolated sponge cells, and show gliding motility and flexibility. Owing to motile properties and surface topography, these bacteria are tentatively referred to the genus Cytophaga, a group related to flexibacteria. A survey of three Clathrina species, C. cerebrum, C. coriacea (Montagu), and C. clathrus (Schmidt), living in the same or in neighbouring habitats, shows that only C. cerebrum can host a well-developed population of bacterial symbionts. This association is probably based on some species-specific mechanism, and shows distinctive features with respect to those which have been described to date in sponges.