Gutless vestimentiferan tubeworms are known to harbor endosymbiotic bacteria in a specialized tissue, the trophosome, which consists of lobules. The endosymbionts of vestimentiferans inhabiting sulfide-rich hydrothermal vents are monospecific for their host. In contrast, previous studies suggest that vestimentiferas of methane-rich seeps may host multispecific symbionts. Phylogenetic analysis and dot-blot hybridization of 16S ribosomal RNA genes (16S rDNA) detected 4 operational taxonomic units (OTUs) in the trophosome of the vestimentifera Lamellibrachia species from a bathyal methane-seep. The OTUs were closely related to 16S rDNA of the species belonging to alpha -Proteobacteria ( Sulfitobacter), beta- Proteobacteria ( Janthinobacterium), and gamma -Proteobacteria ( Acinetobacter and Pseudomonas). Localizations of the 4 OTUs within the trophosome were confirmed by in situ hybridization (ISH). ISH signals of the alpha-proteobacterial OTU were observed in the innermost zone of the trophosome lobules. In contrast, ISH signals of the beta- and gamma-proteobacterial OTUs were observed at the periphery of the lobules; however, whether they occur inside or outside the lobules remains unclear. These results support the possibility that the studied methane-seep tubeworm has a microflora composed of multispecific endosymbionts.

A gutless polychaete of the family Siboglinidae, Oligobrachia mashikoi, known in the past as a beard worm of the group Pogonophora, inhabits Tsukumo Bay of the Noto Peninsula in the Sea of Japan. Photographs were taken of this polychaete projecting about one third of the length of its tentacles outside of its tube. The tube protruded several mm from the sea bottom. These are the first field photographs of beard worms. The trophosome of this beard worm harbors sulfur-oxidizing bacteria. In fact, the muddy sediment where this worm inhabits smells slightly of hydrogen sulfide. Total sulfide levels, which can be an indicator of the generation of hydrogen sulfide gas, were measured at 10 locations in the bay. Furthermore, at the location which this species inhabits, the total sulfide levels in the vertical direction were determined. In addition, the total nitrogen levels, which can indicate the quantity of organic substances, were measured. The sediment inhabited by this worm was determined to have total sulfide levels of 0.24-0.39 mg/g dry mud, measured in the form of acid-volatile sulfide-sulfur. The total nitrogen levels were 1.0-1.5 μg/mg dry mud. These values suggest that the bottom of Tsukumo Bay has not been deteriorated by eutrophication. The levels were, however, highest in the surface layer of the sediment. These results suggest that hydrogen sulfide is generated in the surface of the sediment by sulfate-reducing bacteria, and that O. mashikoi appears to able to live in an environment that contains a slight amount of sulfide.