ABSTRACT
Symbiotic sulfur-oxidizing Thiothrix-like bacteria are found in some spatangoids that feed on fine- to medium-grained sediments. The bacteria are living within an intestinal caecum that opens at the intestine-rectum junction, a portion of the digestive tube where hydrogen sulfide regularly forms as a result of the nature of the ingested sediment and of the digestive mechanics of the echinoid. Through their particular location and their sulfur-oxidative metabolism, these bacteria are suspected to prevent critical accumulation of hydrogen sulfide within the intestine of their host. The symbiosis has been observed in 8 of the 36 species of spatangoids that have been investigated. In all the symbiotic spatangoids, the symbiosis shows similar features: it involves Thiothrix-like bacteria that build typically structured nodules and that are located within an intestinal caecum. The symbiosis appears as a derived character state. It occurs in phylogenetically dispersed families.
