Factors affecting the spatial distribution of the brackish water snail Iravadia (Fairbankia) sakaguchii were investigated in the field in the Waka River Estuary (34°10'N, 135°10'E) of central Japan and in the laboratory. The snails occurred under stones partially buried on intertidal muddy flats. A tracking survey of marked snails released beneath stones revealed that many of the snails moved among the stones. Observations in tide-simulating tanks indicated that most of the snails came out from beneath the stones and were active during both daytime and nighttime submergence. Salinity tolerance tests in the laboratory showed that the snails died within 6 days at 0 psu, 7 days at 5 psu, and 12 days at 10 psu, whereas no snails died during 13 days of exposure to higher salinity conditions (20-35 psu). Neither immigration nor retention rates of snails differed significantly between partially buried stones and non-buried stones artificially placed in the field. This indicates that the extent of burial of the stones is not critical for habitat selection by the snails. Preference tests conducted in the laboratory and in the field revealed that the snails prefer mud to sand under the stones. Retention and survival rates were lower for snails released with stones in the lower intertidal zone than for those released with stones in the middle intertidal zone, and this suggests the importance of tidal exposure in habitat selection of I. (F) sakaguchii.

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The influence of water depth on growth rate and size of the Japanese endemic crayfish, Cambaroides japonicus, was examined in both the laboratory and the field. The growth rates of individuals reared at different depths (10, 50, 100mm) were investigated for 5 months, from stage II initially to stage VII or VIII at the end of the study. This experiment clearly showed that the growth rates of individuals were affected by water depth, shallow individuals growing more slowly. Crayfish were collected from 28 brooks and 6 lakes throughout the Shiribeshi district, Hokkaido, Japan, and their sizes were compared. Analysis of variance indicated that crayfish collected from lakes (i. e. deeper habitats) were larger than those from brooks.

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A chemosynthetic ecosystem is recognized as a living community with interrelationships between the organisms and the inorganic compounds used for energy acquisition. The oldest Glade of thermophilic prokaryotes possesses chemosynthetic physiology, and they have been the principal source of bio-production at deep sea hydrothermal vents since the first such ecosystem was established some 3.8 to 3.5 billion years ago. Modern communities based on primary production by chemolithotrophic prokaryotes exist in many kinds of environment, such as deep aquifers, geothermal hot springs, hydrothermal vents, cold seep areas, and the subsurface of the sea floor. Their contribution towards production within the whole marine ecosystem has been disregarded because of its very small estimated production compared to the entire photosynthetic production. However, it has become apparent that the extent of such chemolithotrophic habitats is much greater than previously realized, and it has become necessary to reconsider the role of chemosynthetic production within certain limited areas. Chemosynthetic ecosystems in the deep sea may create a specific ecotone of increased biodiversity and biomass at their boundary.

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Endosymbiotic associations between invertebrates and chemosynthetic bacteria are common in deep-sea hydrothermal vent and cold seep environments. Since characterization of the endosymbionts has been difficult due to their resistance to cultivation once removed from their hosts, the chemotrophic endosymbioses were initially confirmed by electron microscopic observation, stable isotope analysis, lipid analysis, and enzymatic characterization. They were classified into two types: one using sulfide, oxygen, and carbon dioxide as an electron donor, acceptor, and carbon source, respectively; another using methane as an energy and carbon source. Molecular phylogenetic analysis based on 16S ribosomal RNA gene sequences have revealed that most host species contain a single phylotype of endosymbionts belonging to a limited domain of the γ-subdivision of Proteobacteria despite the wide taxonomic range of hosts. Most symbionts are characterized as thioautotrophs or methanotrophs. Symbiont transmission mechanisms have been reported in several host taxa. Vertical transmission has been shown in vesicomyid clams and deep-sea mussels. Evidence is accumulating to suggest environmental transmission in vestimentiferans and lucinid clams. In the case of solemyid clams, several lines of evidence seem to conflict. A hypothesis is suggested that bacterial chemotrophic endosymbionts influence host distributions in deep-sea chemosynthetic ecosystems. The distribution of methanotrophic symbioses between deep-sea mussels and their symbionts correlates with high methane concentrations in deep-sea chemosynthetic ecosystems. A thyasirid clam living in the deepest yet known chemosynthetic environment has an unusual intracellular dual symbiosis, which is likely essential for the survival of host bivalves at great depth in reduced environments. These results indicate that bacterial chemotrophic endosymbionts influence host distributions in deep-sea chemosynthetic ecosystems.

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There are unique biological communities around deep-sea hydrothermal vents and cold-water seeps throughout the world. Mussels of the genus Bathymodiolus (Bivalvia, Mytilidae) are one of the dominant macroorganisms in these communities. Bathymodiolus mussels live in a reductive environment, depending on energy supplied by bacterial endosymbionts employing chemosynthesis. Although Bathymodiolus mussels are taxonomically classified by conchological traits, their evolution is poorly understood. In this study, phylogenetic relationships of nine Bathymodiolus species obtained from the Western Pacific and Indian Oceans were investigated by comparing partial nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI). These species were divided into two clusters comprising Bathymodiolus japonicus, B. platifrons, B. sp. (short-type, Kuroshima Knoll), and B. sp. (long-type, Kuroshima Knoll) on the one hand (group 1) and B. septemdierum, B. aduloides, B. marisindicus, B. sp. (Mariana Back-Arc Basin), and B. sp. (Manus Basin) on the other (group 2). The former is a lineage containing methanotrophic endosymbionts and the latter is a lineage containing thioautotrophic endosymbionts (no data for B. sp. from the Mariana Back-Arc Basin). Bathymodiolus septemdierum, B. marisindicus, and B. sp. (Mariana Back-Arc Basin) are closely related with small genetic distances among them (cluster A). Divergence time calculated based on the estimated evolutionary rate of COI (0.014 of sequence divergence/million years) was 12 to 14 million years ago between the two groups and 0.3 to 2.9 million years ago among the three species in cluster A.

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The benthic molluscan species hitherto collected from depth deeper than 5, 000 m in the Japan Trench are reviwed on the basis of literature and published documents. The past collections by research vessels of Russia (ex-USSR) and Japan have been mostly identified to species, but some to only generic level. Additional species have been discovered by recent cruises of reserch vessels such as the R/V Hakuho-Maru, and the submersibles, however, these newer taxonomic results have seldom been made public up to now.

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Molecular phylogenetic analyses are powerful techniques for the study of the dispersal and evolutionary processes of animals endemic to chemoautosynthesis-based communities. In the Western Pacific, these techniques have been used with some species and dispersal processes have been hypothesized to explain the different distribution patterns that have been recognized between vent-endemic animals and those inhabiting both vent and seep areas. To understand the dispersal processes of both groups, information on the ecological characteristics of their planktonic larval life history is necessary, but there are few data about the developments and growth of vent and/or seep animals in the Western Pacific. The vent-endemic barnacles, Neoverruca spp., are widely distributed and abundant in the hydrothermal vent fields of the Western Pacific. Preliminary results of molecular phylogenetic analysis based on partial mitochondrial COI sequences indicate that there are genetic differences among four regional populations, namely the Manus Basin, the Mariana Trough, the Okinawa Trough, and the Izu-Ogasawara Ridge. In the laboratory, the adults and larvae can be reared under 1 atm. The larvae have a long planktonic period (about 70 days) even though they are lecithotrophs. Neoverruca barnacles are useful for studies on dispersal and evolution in chemoautosynthesis-based communities.

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Photographs and observations from submersibles have established the presence of deep-sea fish living around hydrothermal vents, but the difficulty in sampling these mobile fish from submersibles has restricted studies about their biology and ecology. About 25 families of deep-sea fish have been observed at hydrothermal vents around the world, with some species appearing to be found only at hydrothermal vents, and others commonly being observed in the deep-sea worldwide. However it is difficult to determine if there are many endemic species at hydrothermal vents. Studies on the food habits of some deep-sea fish species have found that some types of sessile invertebrate in hydrothermal vent communities are important components in their diets. This suggests that there are predator-prey interactions between these fish and the hydrothermal vent fauna, and recent studies have indicated that predation by large mobile predators, such as fish and crabs, can influence the structure of hydrothermal vent communities directly and indirectly. Further studies are needed to clarify the species composition and ecology of fish associated with hydrothermal vents and to examine their influence on sessile chemosynthetic communities.

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Benthic animals that rely on endosymbiotic bacteria for their nutrition are ubiquitous around deep-sea hydrothermal vents and cold seeps where hydrogen sulfide together with methane is issuing. To elucidate the strategic adaptation of these animals to these environments, relevant analytical data on the carbon, nitrogen, and sulfur isotopic signatures of the soft body were compiled and evaluated in relation to those of hydrogen sulfide, methane, and ammonia in the habitat. The soft body part carbon isotopic compositions (δ¹³C) of pogonophorans, vesicomyid and solemyid clams, gastropods, and Bathymodiolus spp. in which sulfur-oxidizing bacteria are the sole endosymbiont, varied within a relatively narrow range of 35±5‰, which indicates catalytic fixation of dissolved bicarbonate by ribulose-1, 5-biphosphate carboxylase/oxygenase. In contrast, benthic animals habouring methanotrophic endosymbionts showed a wider variation in ranges of carbon isotopic composition, from extremely light (δ¹³C=-70‰) to heavy (δ¹³C=-19‰). The variation results from the diverse sources of the substrate methane and associated specific metabolic pathways. Soft body part sulfur isotopic composition (δ³⁴S) of benthic animals harbouring methanotrophic bacteria showed a narrow range of+13 to+16‰, indicating incorporation and assimilation of seawater sulfate (δ³⁴S= +21‰) under a limited kinetic, sulfur isotopic fractionation (up to 6‰). The benthic animals harbouring sulfur-oxidizing bacteria showed ³⁴S values from light (down to -20‰) to near 0, in accordance with the biogenic and magmatic origins of the sulfur, respectively. Two samples of Bathymodiolus spp. with dual symbiosis from the South Chamorro Seamount and Gulf of Mexico showed intermediate δ³⁴S values, which indicate use of both heavy seawater sulfate-sulfur and light sulfide-sulfur in their nutrition. Animals that rely on chemosynthesis for nutrition are characterized by their unique soft body part nitrogen isotopic signatures with fairly light values down to -10‰, values that are seldom observed in terrestrial and ordinary marine ecosystems. Such light values may be inherited from the source inorganic nitrogen or they may arise from large-scale nitrogen isotopic fractionation in the course of metabolic pathways. Data concerning the nitrogen isotopic composition of the dissolved ammonia associated with chemosynthesis-based animal community is needed.

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Relationships between polychaetes living in the mantle cavity of bivalves and their hosts were examined in a hydrothermal vent and a cold seep in the Ryukyu Islands region, and also cold seeps in the Japan Trench. The polynoid polychaete Branch ipolynoe pettibonae inhabited the mantle cavity of more than 85% of individuals of the mytilid bivalve Bathymodiolus platifrons from the Hatoma Knoll of the former area. One polychaete individual was observed in each bivalve. There was a tendency for the size of the polynoid polychaetes to increase with the size of the host bivalve. The same species was observed to inhabit the mytilid bivalve Bathymodiolus sp. short-type from the Kuroshima Knolls of the Ryukyu Islands region, and this is the first record of this polychate living in a cold seep. The nautiliniellid polychaete Nautiliniella calyptogenicola inhabited the mantle cavity of more than 80% of individuals of the vesicomyid bivalve Calyptogena phaseoliformis from the Japan Trench. One to eight polychaetes were observed to live in one bivalve. There was no relationship between the size of this nautiliniellid polychaete and the size of its host bivalve. More than half of the individuals were incomplete and more than 95% of all individuals had oocytes in the coelom. The thyasirid bivalve Maorithyas hadalis were collected from the Japan Trench, but no worms were found from them.

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Bivalves belonging to the families Vesicomyidae and Mytilidae, which thrive in deep-sea hydrothermal vents and cold seeps, live in symbiosis with chemoautotrophic bacteria. The present paper discusses the significance of differences in the life-styles of deep-sea bivalves and shallow-water ones, considering both those buried in the sediment and those attached to objects. The life-styles of bivalves that live buried in the sediment in shallow water are shaped by adaptations related to feeding on plankton and detritus, orientation, burrowing, and so on. Mussels living in shallow water use byssus to attach themselves to solid objects for feeding and orientation. The byssus also serves to protect them from strong wave action, preventing them from being swept off the rock surface. Deep-sea clams of the genus Calyptogena absorb hydrogen sulfide from the bottom at deep-sea hydrothermal vents and cold seeps. They do not feed on particulate matter. Deep-sea mussels of the genus Bathymodiolus attach themselves to chimneys and rocks, and to living and dead shells of the same species. Because of high hydrothermal vent activity, some of the chimneys grow 40 to 50 cm taller within a day. The surface structure of the chimneys and the temperature of their environment vary from day to day. In the case of the wave-free hydrothermal vent environment, tight attachment by byssus provides no advantage. The functional and/or ecological significance of the attachment is most likely different between shallowwater mussels and deep-sea ones.

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The spatial distribution around hydrothermal vents, population structure, and relative growth parameters of the galatheid crab Shinkaia crosnieri were examined. Surveys were done by the Shinkai 2000 on the Hatoma and Dai-yon Yonaguni Knolls in the southern Okinawa Trough. On the Hatoma Knoll, S. crosnieri inhabited areas (temp. 4.0-6.2°C) about 0.2-2 m away from the active vent (temp. 301°C). In the outer area of the habitat of S. crosnieri (temp. 3.0-3.7°C), dense beds of Bathymodiolus mussels occurred and aggregations of Alvinocaris shrimp were observed. In this survey, 248 specimens of S. crosnieri were collected. Small, probably just post-metamorphic juveniles and large, mature adults co-occurred. Chelipeds of males were proportionally larger than those of females, while abdomens of females were proportionally larger than those of males. Larger chelipeds in males are thought to have evolved through male-male competition for females, and wider abdomens in females are thought to be related to the attachment of fertilized eggs to the abdominal appendages.

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