Description and significance of a peculiar intradigestive symbiosis between bacteria and a deposit-feeding echinoid

Individuals of the deposit-feeding echinoid Echinocardium cordatum (Pennant) harbour peculiar irregular to round-shaped nodules in their intestinal caecum. The nodules have a well-defined structure being made of a central detrital core originating from the gut content and surrounded by actively growing bacteria. These bacteria consist of chains of rod-shaped, usually sheathed cells. They form a core-embedding stratified coat, the thickness of which allows estimation of the relative age of the nodules.