“Farming” of Microbial Mats in the Hindgut of Echinoids

Abstract

The hindgut of several species of spatangoids (burrowing echinoids) harbours a highly contractile and extensile caecum filled up with spherical nodules (De Ridder et al., 1985, De Ridder 1994). A nodule reaches up to 7 mm in diameter and the caecum usually contains numerous nodules of varied size (Figs 1-2). Nodules are kept for days before to be expelled with the feces. Each nodule is made of a central detrital particle, the nucleus, which is coated with a laminated microbial mat (Fig. 1). The nature of the nucleus consists of skeletal remains of animals (e.g., shells, fragments of hydrozoans or bryozoans colonies, pieces of bone), of wood or algae pieces, or of organic aggregates. Sheathed filamentous bacteria form the “frame” of the mat (De Ridder et al., 1985). Previous works have indicated that these bacteria can display a sulfur-oxidative metabolism (Temara et al. 1993). Enzyme-linked immunosorbent assay (ELISA) and indirect imunofluorescence (IIF), have suggested that these bacteria could belong to the genus Thiothrix (Brigmon and De Ridder 1998).