Symbiotic relationships between the burrowing echinoid Echinocardium cordatum and its intradigestive bacteria*

The burrowing echinoid E. cordatum harbours in its intestinal caecum, nodules built by symbiotic filamentous bacteria. The morphological study of the bacteria shows their affinity with Thiothrix, a genus gathering sulfo-oxidizing species. Sulfo-oxidation has been brought forward by re spirometry and spectrophotometry analysis: the bacteria are able to oxidize reduced sulfur compounds (S2^-, S₂O₃ ^2-), to store elemental sulfur in their cells and to metabolize it in the absence of reduced sulfur compounds.