Abundance and biomass of the gut-living microorganisms (bacteria, protozoa and fungi) in the irregular sea urchin Echinocardium cordatum (Spatangoida: Echinodermata)

Abstract

The microorganisms associated with the gut contents of the irregular sea urchin Echinocardium cordatum (Pennant) include non-filamentous and filamentous bacteria, zoosporic fungi and protozoa. The numbers of microorganisms decrease through the gut segments, except for the two caeca. In the anoxic anterior caecum, the microbial biomass increases to 5% of the total particulate organic carbon (POC) compared to 2% of the POC in the preceding anterior gut segment. The microbial biomass in the anterior caecum consists mainly of non-motile cells interpreted as chytrid fungi, zoospores and protozoa, especially flagellates. A larger increase of the microbial biomass (13%/POC) is seen in the intestinal caecum. The biomass here is associated with the detrital nodules, and consists of both non-filamentous bacteria and filamentous white sulphur bacteria. Chytrid zoospores were observed also in the coelomic fluid. The digestive strategy of E. cordatum is still insufficiently described, but the microbial activity in the gut and the presumed prolonged residence time in the two caeca, particularly in the intestinal caecum, suggest a digestive strategy to exploit refractile organic matter through microbial degradation.