Abstract
Oligobrachia mashikoi, a frenulata, is a marine invertebrate living in an unusual habitat in Tsukumo Bay (20-25 m deep), Japan. It lacks a mouth and a gut, and instead possesses specialized cells called bacteriocytes, in which endosymbiotic bacterial cells are kept. The endosymbiosis involves specific interactions and obligatory metabolic exchanges between the host and endosymbiotic bacterial cells. Analyses of 16S rRNA genes from the endosymbiotic cells indicated that there are at least seven phylotypes of endosymbionts in O. mashikoi, and these phylotypes are closely related Gammaproteobacteria among which sequence homology was more than 97.6%. The analyses suggested that an adult worm predominantly accommodates one of the seven phylotypes. In situ hybridization targeting 16S rRNA demonstrated that the distribution patterns of the bacterial cells in adult worms were very similar among at least two phylotypes of the endosymbionts. The findings suggest that O. mashikoi has a strict selection mechanism for its endosymbiont.
