The Osedax trophosome: organization and ultrastructure.

The polychaete family Siboglinidae, which is currently construed as comprising the Frenulata, Monilifera (composed of Sclerolinum), Vestimentifera, and Osedax, has become known for its specialized symbiont-housing organ called the trophosome. This organ replaced the digestive system of the worms and is located in the elongated trunk region in Frenulata, Sclerolinum, and Vestimentifera. Currently two types of trophosomes have been described: in the taxa Frenulata and Sclerolinum the bacteriocytes originate from endoderm, and in Vestimentifera they originate from mesoderm. In Osedax, a trophosome was described as lacking (Rouse et al., 2004), but bacteriocytes are located in Osedax's characteristic root tissue. Here, we argue for a consistent name for the symbiont-housing tissue, namely trophosome, as in other siboglinids. In this study we provide morphological evidence that in Osedax the bacteriocytes are derived from somatic mesoderm. We show that the trophosome in Osedax is an apolar tissue composed of bacteriocytes and nonsymbiotic cells. As in vestimentiferans, a specific cell cycle was identified; however, in this case it is directed from the posterior to the anterior end of the worms instead of from the center toward the periphery. Comparison of all siboglinid trophosomes and re-evaluation of their body regions allows us to discuss whether the trophosomes are homologous and to hypothesize about the organization of the last common ancestor of Siboglinidae.