Distribution and Population of Free-Living Cells Related to Endosymbiont A Harbored in Oligobrachia mashikoi (a Siboglinid Polychaete) Inhabiting Tsukumo Bay.

Beard worms (Siboglinidae, Polychaeta), which lack a mouth and a digestive tract, harbor thioautotrophic or methanotrophic bacteria in special cells called bacteriocytes. These endosymbionts have been considered to be trapped at a specific larval stage from the environment. Although many species of beard worms have been discovered in various abyssal seas, Oligobrachia mashikoi inhabits Tsukumo Bay which is only 25 m deep. At least seven types of endosymbionts (endosymbiont A-G) have been distinguished in O. mashikoi. In this study, we investigated the distribution pattern of free-living cells related to the major endosymbiont (endosymbiont A) in Tsukumo Bay by quantitative PCR targeting the 16S rRNA gene. The endosymbiont A-related phylotype was detected in almost all sediment samples collected from 23 points in Tsukumo Bay, ranging in copy number of the 16S rRNA gene from 2.22×10(4) to 1.42×10(6) copies per gram of dry-sediment. Furthermore, the free-living cells made up less than 9% of the total eubacterial population, suggesting that the O. mashikoi larvae precisely select candidates for their endosymbiont from bacterial flora in the environment. This is the first report on the ecological characterization of a free-living bacterium related to the endosymbiont of the siboglinid polychaete, O. mashikoi.

Relationship between the lifestyle of a Siboglinid (Pogonophoran) polychaete, Oligobrachia mashikoi, and the total sulfide and nitrogen levels in its habitat.

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 microg/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.

A histological investigation of the maturation of the acorn worm, an inhabitant of the Sea of Japan, and a suggestion about the relationship between synchronized spawning/spermiation and the tidal level.

One species of Hemichordata, Balanoglossus misakiensis, is then acorn worm originally reported from the intertidal zone of the Miura Peninsula on the Pacific Ocean side of Japan. We histologically examined the reproductive cycle of the population of this species, which inhabits only the sublittoral zone in the Sea of Japan. Testes and ovaries began to develop at the beginning of May 2003 and were almost mature in the latter half of June in males and in the first half of July in females in the same year. Subsequently, spermiation and spawning followed in the latter half of July in males and in the first half of August in females. Progress in maturation appeared to be related to increases in the water temperature. Although some experiments were conducted in aquariums to identify the conditions responsible for the synchronization of the occurrence of spontaneous spawning/spermiation, no clues were obtained. During the experiments, however, 11, 2, and 4 individuals out of the 67 used achieved spawning/spermiation on separate days. The occurrence of spawning/spermiation in the laboratory corresponded to the latter half of the switch from high tide to low tide on those days. Also in the field, it was known that they released the gametes according to this specific schedule. Therefore, it was suggested that, in the Japan Sea population of this species, the tide level may be a condition for synchronized spawning/spermiation.

External morphology of the posterior end, the "opisthosoma", of the beard worm Oligobrachia mashikoi (Pogonophora).

The entire length of the beard worm, Oligobrachia mashikoi (Pogonophora), including the posterior end, the "opisthosoma" was collected successfully. This species is exclusive to Tsukumo Bay in Ishikawa Prefecture, Japan. Although the portion preceding the opisthosoma was similar to a fine filament, it abruptly assumed a shape similar to a shovel and appeared to be composed of many segmental structures. The number of segments exceeded 50. The dorsal side of the opisthosoma differed from that of the ventral side in morphology. The opisthosoma was equipped with 4 lines of setae arranged longitudinally and a sucker on the tip. When considering the fact that the Family Oligobrachiidae is the most primitive group of pogonophores, the external morphology of the opisthosoma is interesting as it may be reminiscent of the ancestral condition. This is the first report of the opisthosoma in Oligobrachiidae.