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Abstract: We investigated the genus diversity and community composition of harpacticoid copepods and their relationship with environmental factors around the Kuril Trench (490–7090m). Harpacticoid densities did not decrease with water depth and were highest at 1000m water depth. Diversity values based on the number of genera, Shannon diversity and the expected number of genera (rarefaction) indicated unimodal patterns with water depth, with peaks at intermediate depth; genus evenness increased with water depth and slightly decreased at hadal depths. This result suggested that the general relationship between water depth and diversity described for macrofauna and megafauna could be extended to meiofauna across all depth ranges. However, the regulating factor that affects harpacticoid diversity could not be identified. The community composition of harpacticoids gradually changed with water depth (from bathyal to hadal depths). In addition, comparison of assemblages between the trench slope, trench floor and abyssal plain suggested that the community found at hadal depth was largely different from those found on the trench slope and abyssal plain. Multivariate analyses suggested that water depth, or certain factors associated with water depth, affects harpacticoid assemblages around the Kuril Trench. [Copyright &y& Elsevier]

Genetic deviation between two deep-sea snailfishes, Careproctus pellucidus from the Pacific Ocean and Careproctus rastrinus from the Okhotsk Sea, of the Careproctus rastrinus species complex was analyzed, based on the nucleotide sequences of the mitochondrial cytochrome b ( Cytb ) gene. Our sampling revealed the occurrence of individuals with C. rastrinus -type Cytb genes off northeastern Japan, in the northwestern Pacific. Most of these individuals were collected from an area off Miyagi Prefecture, while few individuals were collected from areas to its north (off Iwate Prefecture) and south (off Fukushima and Ibaraki Prefectures). Phylogeographic analyses based on nucleotide sequences of the first intron region of the nuclear S7 ribosomal protein gene ( S7 ) and 11 microsatellite loci indicated little genetic deviation between individuals with C. pellucidus -type and C. rastrinus -type Cytb genes in the area off Miyagi Prefecture. Significant genetic differences between the Pacific Ocean and the Okhotsk Sea populations may be attributed to unidirectional migration due to a greater current from the latter to the former. In addition, peritoneum and stomach colors of 10 and 17 specimens collected from the Pacific Ocean and the Okhotsk Sea, respectively, were evaluated quantitatively. The colors were significantly different for the two populations; however, they appeared to overlap. Although only one Pacific individual with a C. rastrinus -type Cytb gene was available for color evaluation, it was more similar to Pacific individuals than to Okhotsk Sea individuals. These results suggest inbreeding between C. pellucidus and C. rastrinus in the Pacific Ocean, off Miyagi Prefecture.

Deep-sea demersal fishes of the Bothrocara hollandi species complex are distributed in the Japan Sea, the Okhotsk Sea, and the northwestern Pacific Ocean. Based on the nucleotide sequences of the nuclear Internal Transcribed Spacer 1 (ITS1) region and the microsatellite analysis, cryptic speciation resulting in the existence of two species (sp. 1 and sp. 2) in the Japan Sea was indicated for this species complex. In the Japan Sea off the San-in district, the westernmost part of the Japanese mainland, the frequency of sp. 2 individuals was highest at a depth of ∼400 m and decreased at both greater and lesser depths. Complete genetic deviation was observed between the individuals of the Japan Sea and the other sea areas, with the exception of a single sp. 2 individual, which shared an ITS1 sequence with an individual from the Pacific Ocean. Furthermore, a microsatellite analysis showed that the individuals of the other sea areas were more closely related to sp. 2 individuals. Two species were thought to have deviated from each other after their isolation from the individuals of the sea areas outside of the Japan Sea, through the occurrence of habitat fragmentation and bottleneck events in the Japan Sea during the glacial periods. Group A, one of two mitochondrial haplotype groups that were reported for the Japan Sea individuals in the previous studies, may have evolved within the lineages of sp. 2.

A new vestimentiferan tubeworm species of the genus Lamellibrachia Webb, 1969 is described. It was collected from cold seep areas off Hatsushima in Sagami Bay and at the Daini Tenryu Knoll in the Nankai Trough (606-1170 m depth). Lamellibrachia sagami sp. nov. differs from seven congeneric species in the following character states; showing a wider range of diameter of vestimental and trunk plaques than L. barhami, L. luymesi, L. satsuma and L. anaximandri; and having more numerous sheath lamellae (3-6 pairs) than L. juni (2-3 pairs) but fewer than L. victori (7 pairs) and L. columna (8-16 pairs).

Recent studies have revealed the impact of the drastic climate change during the last glacial period on coastal marine and anadromous species in the marginal seas of the northwestern Pacific Ocean; however, its influence on deep-sea species remains poorly understood. To compare the effects of the last glacial period on populations from the Sea of Japan and the Sea of Okhotsk, we examined the mitochondrial control region and cytochrome b gene sequences of Lycodes matsubarai, a deepsea demersal fish that inhabits these two seas. Our results showed clear genetic differentiation of populations between the two seas. The populations may have diverged during the last glacial period, probably as a result of vicariance due to the drastic sea level change. The population in the Sea of Okhotsk was larger than that in the Sea of Japan, but suddenly decreased after the last glacial period. However, the Sea of Japan population expanded after the last glacial period, coincident with high levels of oxygenation in deep-sea areas. These results elucidate regional-scale impacts of climate change on deep-sea organisms.

Hydrothermal vent gastropods of the genus Alviniconcha are unique among metazoans in their ability to derive their nutrition from chemoautotrophic gamma- and epsilon-proteobacterial endosymbionts. Although host-symbiont relationships in Alviniconcha gastropods from the Central Indian Ridge in the Indian Ocean and the Mariana Trough in the Western Pacific have been studied extensively, host-symbiont relationships in Alviniconcha gastropods from the Southwest Pacific remain largely unknown. Phylogenetic analysis using mitochondrial cytochrome c oxidase subunit I gene sequences of host gastropods from the Manus, North Fiji, and Lau Back-Arc Basins in the Southwest Pacific has revealed a new host lineage in a Alviniconcha gastropod from the Lau Basin and the occurrence of the host lineage Alviniconcha sp. type 2 in the Manus Basin. Based on 16S rRNA gene sequences of bacterial endosymbionts, two gamma-proteobacterial lineages and one epsilon-proteobacterial lineage were identified in the present study. The carbon isotopic compositions of the biomass and fatty acids of the gastropod tissues suggest that the gamma- and epsilon-proteobacterial endosymbionts mediate the Calvin-Benson cycle and the reductive tricarboxylic acid cycle, respectively, for their chemoautotrophic growth. Coupling of the host and symbiont lineages from the three Southwest Pacific basins revealed that each of the Alviniconcha lineages harbors different bacterial endosymbionts belonging to either the gamma- or epsilon-Proteobacteria. The host specificity exhibited in symbiont selection provides support for the recognition of each of the host lineages as a distinct species. The results from the present study also suggest the possibility that Alviniconcha sp. types 1 and 2 separately inhabit hydrothermal vent sites approximately 120 m apart in the North Fiji Basin and 500 m apart in the Manus Basin.

The discovery of new hydrothermal vent systems in the back-arc basins of the Western Pacific revealed chemosynthesis-based faunal communities distinct from those of other vents. These vents are dominated by two related gastropods (Alviniconcha spp. and Ifremeria nautilei) that harbour symbiotic bacteria in their gills. We used comparative 16S ribosomal RNA (rRNA) gene sequencing and in situ hybridization with rRNA-targeted probes to characterize the bacterial symbionts of Alviniconcha sp. and I. nautilei from the Manus Basin in the Western Pacific. The analyses revealed that these two gastropod species, although affiliated with the same family, harbour phylogenetically distant chemosymbionts, suggesting independent origins of these endosymbioses. The I. nautilei endosymbiont clusters with sulfur-oxidizing bacteria within the gamma-Proteobacteria, as is the case for all previously characterized endosymbionts from a wide diversity of host taxa harbouring thioautotrophic prokaryotes. In contrast, the Alviniconcha endosymbiont is affiliated with sulfur-oxidizing bacteria within the epsilon-Proteobacteria. These results show that bacteria from the epsilon-Proteobacteria are also capable of forming endosymbiotic associations with marine invertebrates from chemosynthetic environments. More generally, the endosymbiotic lifestyle is now shown to be distributed throughout all recognized classes of the Proteobacteria.