Little is known about the consistency or phylogenetic affiliation of accociated intra- or extracellular bacterial populations in Echinodermata. Because certain taxa harbour bacteria and other not, these associations are presumably originated by coevolution and not by ecological circumstances. The intestine of echinodermata is populated by huge amounts of bacteria. Due to the different feeding strategy of echinoderms it is controversly discussed whether these bacteria are passively taken up or if they are permanently present. Hence it will be possible to elucidate if vertical transmission occurs or bacteria are recruted. With the knowledge of phylogenetic affiliations of microbial symbionts and their distribution (or localization) in different hosts, the physiological/biochemical status of the association will be investigated. The main emphasis will be the characterization of the in situ situation by adequate histological techniques (crysectioning) and “passive” (FT-IR) or “active” chemical imaging (confocal imaging, using fluorescent enzyme substrates or physiological dyes). The main experimental work in this WP bases on the creation of 16S-rDNA sequence libraries of echinoderrm associated bacteria (SCB & intestinal). Signature sequences will be analyzed and specific gene probes will be designed and applied.