Vesicomyid Clams Research Articles

Coupling of bacterial endosymbiont and host mitochondrial genomes in the hydrothermal vent clam Calyptogena magnifica.

The hydrothermal vent clam Calyptogena magnifica (Bivalvia: Vesicomyidae) depends for its nutrition on sulfur-oxidizing symbiotic bacteria housed in its gill tissues. This symbiont is transmitted vertically between generations via the clam's eggs; however, it remains uncertain whether occasionally symbionts are horizontally transmitted or acquired from the environment. If symbionts are transmitted strictly vertically through the egg cytoplasm, inheritance of symbiont lineages should behave as if coupled to the host's maternally inherited mitochondrial DNA. This coupling would be obscured, however, with low rates of horizontal or environmental transfers, the equivalent of recombination between host lineages. Population genetic analyses of C. magnifica clams and associated symbionts from eastern Pacific hydrothermal vents clearly supported the hypothesis of strictly maternal cotransmission. Host mitochondrial and symbiont DNA sequences were coupled in a clam population that was polymorphic for both genetic markers. These markers were not similarly coupled with sequence variation at a nuclear gene locus, as expected for a randomly mating sexual population. Phylogenetic analysis of the two cytoplasmic genes also revealed no evidence for recombination. The tight association between vesicomyid clams and their vertically transmitted bacterial endosymbionts is phylogenetically very young (<50 million years) and may serve as a model for the origin and evolution of eukaryotic organelles.

Evolutionary relationships of deep-sea vent and cold seep clams (Mollusca: Vesicomyidae) of the "pacifica/lepta" species complex

Vesicomyid clams are among the dominant invertebrates of chemosynthesis-based communities found at deep-sea cold seeps and hydrothermal vents, yet there is considerable taxonomic confusion within the family Vesicomyidae. The present study examined phylogenetic relationships among vesicomyid clams be- longing to a cryptic-species complex that includes Ve- sicomya (previously Calyptogena) pacifica (Dall, 1891) and Vesicomya lepta (Dall, 1896). Mitochondrial (mt) COI sequences from clams collected between 1991 and 2001 along the western margin of North America from the San Clemente Basin (32� N latitude) to the Juan de Fuca Ridge (48� N latitude) revealed five discrete evolutionary lineages: V. pacifica (mt-type I), V. lepta (mt-type IV), and three undescribed species defined by mttypes II, III, and V. Separation of the three lineages defined by mt-types I, II, and III was also revealed by an independent nuclear gene (ribosomal ITS-1), and by DNA sequences from vertically transmitted bacterial endosymbionts found in the gills of these clams. Lin- eages I, II, and III occur in Monterey Bay, California, and the data suggest that bathymetric segregation may have played a role in their isolation and divergence.

Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem

Observations from the first submersible reconnaissance of the Blake Ridge Diapir provide the geological and ecological contexts for chemosynthetic communities established in close association with methane seeps. The seeps mark the loci of focused venting of methane from the gas hydrate reservoir, and, in one location (Hole 996D of the Ocean Drilling Program), methane emitted at the seafloor was observed forming gas hydrate on the underside of a carbonate overhang. Megafaunal elements of a chemosynthetically based community mapped onto dive tracks provide a preliminary overview of faunal distributions and habitat heterogeneity. Dense mussel beds were prominent and covered 20×20 m areas. The nearly non-overlapping distributions of mussels and clams indicate that there may be local (meter-scale) variations in fluid flux and chemistry within the seep site. Preliminary evidence suggests that the mussels are host to two symbiont types (sulfide-oxidizing thiotrophs and methanotrophs), while the clams derive their nutrition only from thiotrophic bacteria. Invertebrate biomass is dominated by mussels ( Bathymodiolus heckerae) that reach lengths of up to 364 mm and, to a lesser extent, by small (22 mm length) vesicomyid clams ( Vesicomya cf. venusta). Taking into account biomass distributions among taxa, symbiont characteristics of the bivalves, and stable-isotope analyses, the relative importance of methanotrophic vs thiotrophic bacteria in the overall nutrition of the invertebrate assemblage is on the order of 60% vs 40% (3:2).

Symbiotic Adaptation for Deeper Habitats in Chemosynthetic Environments

The bacterial endosymbionts of the solemyid clam Acharax johnsoni and the thyasirid clam Parathyasira kaireiae, collected from the Japan Trench, were characterized. Transmission electron microscopic observations showed numerous bacteria in the epithelial cells of the gill tissues of A. johnsoni. Numerous bacteria were also visible in the gill tissues of P. kaireiae, but were not located within the epithelial cells. Phylogenetic analysis based on the 16S ribosomal RNA gene sequences from the gill tissues of both clams indicated that the bacteria were related to sulfur-oxidizing endosymbionts from deep-sea chemosynthetic environments. The symbiont of A. johnsoni formed a monophyletic group with the thioautotrophic symbiont of Solemya reidi, which lives relatively deeper than other solemyid clams. The symbiont of P. kaireiae formed a monophyletic group with symbiont II in Maorithyas hadalis, which lives in the hadal zone of the Japan Trench. In addition, four vesicomyid species living relatively deeper than other vesicomyid clams also have a specific clade of thioautotrophic symbionts. Bacterial chemotrophic endosymbionts as well as the mode of symbiosis might influence host distributions in deep-sea chemosynthetic ecosystems.

16S rDNA-based phylogeny of sulphur-oxidising bacterial endosymbionts in marine bivalves from cold-seep habitats

The phylogenetic relationship of sulphur-oxidising endosymbiotic bacteria from bivalves of the families Vesicomyidae (Calyptogena sp. C1, Calyptogena sp. C3), Solemyidae (Acharax sp.) and Thyasiridae (Conchocele sp.) from cold-seep habitats were determined by 16S rDNA nucleotide sequence analyses. The endosymbiotic bacteria form distinct groups within the gamma-Proteobacteria and are well separated from each other and from free-living sulphur- oxidising bacteria of the genera Beggiatoa, Halothiobacillus and Thiomicrospira. The endosymbiotic bacteria of Acharax sp. from cold seeps off Oregon, Indonesia and Pakistan have sequences highly similar to each other but quite distinct from other thiotrophic endosymbionts. This includes endosym- bionts from Solemya spp., to which they are distantly related. Symbiotic bacteria of Conchocele sp. from a cold seep in the Sea of Okhotsk are similar to those of Bathymodiolus thermophilus and related species, as shown by their overall sequence similarity and by signature sequences. The endosymbiotic bacteria of Calyptogena spp. from cold seeps off Oregon and Pakistan are closely related to those of other vesicomyids. Endosymbiont species found off Oregon corresponded to 2 different clusters of Calyptogena spp. symbionts in the same samples. The results corroborate the hypothesis of a monophyletic origin of the symbionts in vesicomyid clams, and support the existence of deeply branching groups in solemyid symbionts and of divergent lines and distribution for thyasirid symbionts. The results also indicate that certain symbiont species cluster according to the depth distribution of their hosts, and that in consequence host species together with their symbionts may have undergone depth-specific adaptation and evolution.

化学合成がうみだすもうひとつの深海生態系 化学合成共生システム‐ベントスと共生細菌との密接な関係

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.

Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I: Hydrological provinces

Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. The ridge has a northern peak at a depth of about 600 m, which is covered by extensive carbonate deposits, and an 800 m deep southern peak that is predominantly sediment covered. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes from the seafloor vary by several orders of magnitude at sites separated by distances of only a few meters. We identified three distinct active fluid regimes at Hydrate Ridge. The first province is represented by discrete sites of methane gas ebullition, where the bulk of the flow occurs through channels in which gas velocities reach 1 m s −1. At the northern summit of the ridge the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ∼6×10 4 mol day −1 following tidal periodicity. Qualitative observations at the southern peak suggest that the gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor. The second province is characterized by the presence of extensive bacterial mats that overlay sediments capped with methane hydrate crusts, both at the northern and southern summits. Here fluid typically flows out of the sediments at rates ranging from 30 to 100 cm yr −1. The third province is represented by sites colonized by vesicomyid clams, where bottom seawater flows into the sediments for at least some fraction of the time. Away from the active gas release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations. Methane fluxes out of mat-covered sites range from 30 to 90 mmol m −2 day −1, whereas at clam sites the methane flux is less than 1 mmol m −2 day −1.

Sulphur signature in the hydrothermal vent mussel Bathymodiolus azoricus from the Mid-Atlantic Ridge

The sulphur compound composition of the hydrothermal vent mussel Bathymodiolus azoricus (Bivalvia: Mytilidae) was analysed in order to determine the specific biochemical characteristics of a thiotrophic mode of nutrition. In specimens collected from two contrasting vent fields on the Mid-Atlantic Ridge (Menez Gwen and Lucky Strike), substantial differences in the sulphur composition were observed between tissues and sampling sites. Total sulphur content was higher for samples from Lucky Strike than for those from Menez Gwen. The amount of elemental sulphur in the gill of B. azoricus was in the same range as values previously reported for the vesicomyid clam Calyptogena phaseoliformis and the lucinid clam Lucinoma annulata. Rings of orthorhombic sulphur or compounds such as polythionates or organopolysulphides excreted in the cytoplasm by the symbionts may account for the large amounts of elemental sulphur evidenced for the first time in an hydrothermal vent mussel. A large proportion of the tissue-sulphur was incorporated into free amino compounds such as taurine, hypotaurine, thiotaurine and cysteine, and in a lesser extent to the tripeptide glutathione. In mantles, sulphur seems to be contained mainly in organic compounds such as proteins, mucopolysaccharides and lipids. However, the occurrence of specific compounds such as thiotaurine and elemental sulphur in gills indicate that in this tissue a large proportion of the sulphur might be involved in specific pathways related to transport, storage and detoxification of sulphide. Moreover, our results suggest a greater reliance on thiotrophy of mussels from Lucky Strike as compared to specimens from Menez Gwen.

Vestimentiferans (Pogonophora) in the Pacific and Indian Oceans: a new genus from Lihir Island (Papua New Guinea) and the Java Trench, with the first report of Arcovestia ivanovi from the North Fiji Basin

Explorations by the German Research Vessel Sonne in the fore-arc and back-arc basins of the western Pacific Ocean have collected vestimentiferan tube worms from both warm vent and cold seep sites. Edison Seamount is a small volcanic cone on the southern flank of Lihir Island, in the Tabar-Feni island chain. Beds of vesicomyid clams on the summit (1450 m depth) are associated with hydrothermal effluent, whereas an uplifted scarp (1600 m depth) nearby is covered by low temperature gas-rich sediments. A methane anomaly has been detected in the water column above the scarp. The benthic fauna includes vestimentiferan tubeworms and bathymodiolid mussels. Bottom photographs show that the vestimentiferans occur singly or in small groups. Four specimens were collected by TV-guided grab. They are described as a new species of a new genus, related to the cold-seep genus Escarpia, of the family Escarpiidae. A single specimen obtained from the landward slope of the Java Trench (1500m) is identified as the same species, extending its area of distribution by some 6000 km westward. Arcovestia ivanovi, already known from hydrothermal vents in the Manus Basin, has now been obtained from a hydrothermal site in the North Fiji Basin.

Macrofaunal community structure and sulfide flux at gas hydrate deposits from the Cascadia convergent margin, NE Pacific

Gas hydrates occur at the sediment surface on the southern summit of Hydrate Ridge, Cascadia convergent margin. The hydrates are found in mounds several meters in diameter and up to 2 m high, and are covered by sediment and mats of the filamentous sulfur-oxidizing bacteria Beggiatoa. The mounds are surrounded by vesicomyid clams (Calyptogena pacifica, C. kilmeri), which in turn are encircled by solemyid bivalves (Acharax sp.). The zonation pattern of 3 species (Calyptogena spp. and Acharax sp., which harbor chemoautotrophic bacteria in their gills, and the chemoautotrophic Beggiatoa), is also reflected in a change in the entire community structure. Beggiatoa, Calyptogena spp. and Acharax sp. are shown to be characteristic species for the different communities. The Beggiatoa community directly overlaying the gas hydrates consists of seep endemic species in high densities: gastropods (Provanna laevis, P. lomana, Pyropelta corymba, Hyalogyrina sp. nov.), bivalves (Nuculana sp. nov.) and polychaetes (Ampharetidae, Polynoidae, Dorvilleidae). Based on pooled samples, the rarefaction curves show a decrease in species diversity in the Beggiatoa and Calyptogena communities. The hydrogen sulfide gradients in the porewater of sediments below the different communities dominated by either Beggiatoa, Calyptogena spp. or Acharax sp. vary by 3 orders of magnitude. The diffusive sulfide flux based on the measured sulfide concentration gradients is highest in Beggiatoa sp. communities (23 ± 13 mol m-2 yr-1), slightly less in Calyptogena communities (6.6 ± 2.4 mol m-2 yr-1), and low in Acharax communities (0.05 ± 0.05 mol m-2 yr-1). The difference in the sulfide environment is a factor influencing the distribution patterns of the chemoautotrophy-dependant and heterotrophic species at the deep-sea sediments containing gas hydrate.

Species-specific variation in sulfide physiology between closely related Vesicomyid clams

Symbioses involving sulfide-oxidizing bacteria and metazoan phyla dominate inverte- brate assemblages at cold seeps and hydrothermal vents worldwide. The predominant species inhab- iting cold seeps in Monterey Bay are the vesicomyid clams Calyptogena kilmeri and C. pacifica. The growth and survival of these clams depend directly upon the productivity of their chemoautotrophic endosymbionts, which is fueled by the oxidation of sulfide. For this reason, sulfide availability and sulfide-related physiology are thought to be the most influential factors governing the productivity of these associations. Both species inhabit sulfide-rich sediments and depend nutritionally on their sym- bionts, yet many aspects of their life histories differ considerably. Our results indicate that C. pacifica, which inhabits areas with lower environmental sulfide levels, is physiologically poised for the uptake and transport of sulfide, indicated by increased sulfide consumption rates, sulfide-binding ability, and internal sulfide levels. C. pacifica also has a greater potential for symbiont energy turnover, sup- ported by increased sulfide oxidation potential, enzymes involved in sulfur metabolism, and bacter- ial densities. Conversely, C. kilmeri demonstrates a less effective sulfide uptake mechanism and, therefore, a specific need for higher environmental sulfide levels. It appears that the abilities of these 2 species to process sulfide differ greatly and reflect not only the environments in which they are found but also the capabilities of their symbionts. This research represents the first comparative investigation of the physiological functioning of closely related species in chemosynthetic symbioses and elucidates the constraints and advantages posed by different modes of sulfide (energy) uptake and assimilation in these, and perhaps other, symbiotic organisms.

Mass wasting, methane venting, and biological communities on the Mendocino transform fault

Chemosynthetic cold-seep vestimentiferan tubeworms and vesicomyid clams inhabiting oceanic basaltic rock have been discovered on the Gorda Escarpment sector of the Mendocino transform fault 73 km west of Cape Mendocino, northern California. The sparse cold-seep animals are biomarkers that identify zones of focused fluid venting from a methane gas horizon seismically imaged as a bottom-simulating reflector (BSR) within sediments on the southern flank of the escarpment. This is the first example of a methane-based cold-seep community on exposed oceanic basement rock within an area dominated by transform tectonics. This discovery extends the range of known environments in which the subsurface flow and venting of methane-rich fluids are linked both with geological activity and chemosynthetic communities. Observations by remotely operated vehicle of the distribution of the animals, sediment, basaltic talus, and basement outcrop delineate a large slump headscarp that channels subsurface fluid. Seismic surveys of the southern flank of the Mendocino transform fault (the Vizcaino block) define a BSR hosted in the thick sedimentary sequence that projects to the wall of the Gorda Escarpment at the same depth as the chemosynthetic community. The well-defined BSR in the marine sediment of the Vizcaino block results from an accumulation of methane gas possibly capped by methane hydrate. The isotopic composition of Mg-calcite found along the headscarp (δ 1 3 C = -65‰; δ 1 8 O = 4.8‰) is consistent with fluids derived from dissociated methane hydrate. We propose that the tectonic uplift along this transform margin has resulted in the lateral, northward movement of methane from the Vizcaino sedimentary sequence to the east-trending wall of the north-facing Gorda Escarpment.

Phylogenetic characterization and biomass estimation of bacterial endosymbionts associated with invertebrates dwelling in chemosynthetic communities of hydrothermal vent and cold seep fields

Molecular phylogenetics for endosymbiotic bacteria recovered from vestimentiferan tubeworm Lamellibrachia satsuma from Kagoshima Bay, vesicomyid clam Calyptogena laubieri from the Nankai Trough and mytilid mussel Bathymodiolus sp. from the Mariana Back-arc Basin, were examined by PCR-aided 16S rDNA cloning and sequencing, and quinone profiling. The 16SrRNA clones of the endosymbionts from the 3 organisms fall within γ-Proteobacteria and showed distinct lines of descent specific to their respective host. The 16S rRNA gene phylogeny confirms the host- endosymbiont specificity in the co-evolutionary process. Ubiquinones with 9 isoprene units (Q-9) or Q-10 were found as the major quinones in all test tissues of the host. Larger amounts of Q-8 were detectable only in those host body parts (gill or trophosome) harboring the endosymbiotic bacteria. These observations suggest that Q-8 is the major quinone of the endosymbionts. Based on the bac- terial quinone concentration, the population densities of the bacteria present were estimated to be 10 10 to 10 11 cells g -1 wet wt of host tissue. The quantitative determination of quinones may provide information about physiological activity of the chemosynthetic communities as well as their biomass production.

Reconstructing the history of fluid flow at cold seep sites from Ba/Ca ratios in vesicomyid clam shells

Hydrogen sulfide discharge at cold seep sites is recorded as enrichment in the barium to calcium (Ba/Ca) ratio in shells of vesicomyid clams collected live from cold seeps in Monterey Canyon and the Cascadia margin. A direct relationship between increased Ba fluxes from cold seeps and Ba incorporation into shells was established for the Cascadia margin site. For the Monterey canyon site, a 2‐yr episode of high fluid flow centered on 1992 was inferred from coherent changes in the Ba/Ca profiles of three Calyptogena kilmeri shells. Comparison with precipitation and δ18O data indicates that this high‐flow period may have been driven by an increase in rainfall after the 1988–1990 California drought. High‐resolution records preserved in clam shells are shown to be useful in elucidating characteristics, history, and possible mechanisms driving fluid discharge at continental margin seeps, thus establishing their potential use as paleotracers of fluid seepage events.

Raman spectroscopic and laser scanning confocal microscopic analysis of sulfur in living sulfur-precipitating marine bacteria

Laser Raman microprobe spectroscopy and laser scanning confocal microscopy were used to determine the presence and speciation of sulfur in sulfur-oxidizing, marine bacteria from Monterey Bay, CA. The bacteria studied include: large, filamentous Thioploca and Beggiatoa, endosymbionts in the vesicomyid clam Calyptogena kilmeri, and a filamentous bacterium of undetermined species. All of these bacteria were shown spectroscopically to store elemental sulfur in submicrometer to several micrometer diameter vesicles. More detailed Raman spectroscopic study of the vesicles in Thioploca and Beggiatoa provided further chemical and structural characterization of the elemental sulfur. The sulfur is bonded in the common, stable S 8 ring configuration and is of an extremely fine-grained microcrystalline form. No additional (organo) sulfur compounds were detected spectroscopically in the vesicles under the low laser powers required to preserve the molecular structure of the sulfur. The present spectroscopic and optical data stand in contrast to reports and inferences of liquid-like elemental sulfur or homogeneous, complex sulfur compounds in other sulfur-oxidizing bacteria. The findings of this study are compatible with a model of sulfur vesicles as dominated by microcrystalline solid elemental sulfur, perhaps embedded in a matrix and/or confining membrane of organic material. The high reactivity and solubility observed in these vesicles is attributed to the extremely fine grain size of the solid elemental sulfur.

Different energy sources for three symbiont-dependent bivalve molluscs at the Logatchev hydrothermal site (Mid-Atlantic Ridge)

The vent mussel Bathymodiolus puteoserpentis, a large vesicomyid clam and a smaller thyasirid were collected from an area of sediment subject to diffuse hydrothermal flow. The mussels live on the surface, the vesicomyids are partly buried and the thyasirids burrow in the sediment. The fine structure of the gills differs in the three bivalves. Bathymodiolus puteoserpentis hosts two types of bacterial symbiont, one methanotrophic, and another probably thiotrophic. The other two bivalves have single types of symbiont of different shapes. Stable isotope ratios of carbon and nitrogen indicate thiotrophy in the vesicomyid and thyasirid, but a predominance of methanotrophy in the mussel. This is the first time that such an assemblage has been found at a hydrothermal site on the Mid-Atlantic Ridge (MAR), with the different faunistic elements exploiting different energy resources.

Dual symbiosis in the cold-seep thyasirid clam Maorithyas hadalis from the hadal zone in the Japan Trench, western Pacific

The bacterial endosymbionts of the thyasirid clam Maorithyas hadalis collected from the hadal zone in the Japan Trench were characterized. Two distinct phylotypes of endosymbiotic bacte- ria were discovered within the gill tissues by molecular phylogenetic analysis and in situ hybridiza- tion. Symbiont Type I was affiliated with thioautotrophic symbionts of vesicomyid clams and deep-sea mussels from deep-sea hydrothermal vents and cold seeps. Symbiont Type II was not related to pre- viously reported bacterial symbionts, and was distantly related to the free-living chemoautotrophic bacteria (genera Thiomicrospira and Hydrogenovibrio). In situ hybridization experiments indicated spatial partitioning between the 2 M. hadalis symbionts, with Symbiont I occurring mainly in the outer regions of bacteriocyte zones and Symbiont II situated predominantly within inner regions of bacteriocyte zones. This is the deepest chemosynthetic symbiosis ever recorded and the first report to show spatial partitioning between the intracellular endosymbionts in marine invertebrates.

Monterey Bay cold seep biota: Euglenozoa with chemoautotrophic bacterial epibionts

Summary Deep-sea cold seeps in Monterey Bay, California, are prominent sites of chemosynthetic production, mediated through bacterial oxidation of sulfur. Bacterial mats and vesicomyid clams rely upon hydrogen-sulfide oxidation. Euglenozoa comprise a large and diverse component of the protistan assemblage associated with the dense chemoautotrophic bacterial mats found in the top 1 cm of sediment at some of these seeps. A high percentage of these euglenozoans have conspicuous coats of rod-shaped bacteria that cover virtually their entire surface. In at least one of the three morphotypes identified that possess these epibionts, the epibiotic bacteria contain membrane bound translucent sulfur vesicles. Total sulfide concentration in the top 1 cm of the sediment ranged from undetectable at the sediment-water interface to a maximum of 350 uM. The presence of other hydrogen sulfide oxidizing bacteria (bacterial mats and endosymbiotic bacteria of the clams), the measurable levels of hydrogen sulfide and the distinct membrane surrounding the sulfur inclusion suggests that the epibiotic bacteria on at least some of the euglenozoans are sulfur oxidizers. This is the first report of flagellates possessing sulfur oxidizing epibiotic bacteria. No evidence has been found that the euglenozoans ingest their epibiotic bacteria, however, and the degree and mode of any nutritional contribution the bacteria might make to the host is unknown.

Deep-sea hydrothermal vent communities of the Logatchev area (14°45′N, Mid-Atlantic Ridge): diverse biotopes and high biomass

Results from recent biological studies (including three Alvin dives in July 1997) of the southernmost deep-sea hydrothermal area known on the Mid-Atlantic Ridge (MAR), the Logatchev hydrothermal area, are described. The Logatchev area includes three vent fields, each with a different geological setting corresponding to: (1) a large sulphide mound with ‘smoking craters’; (2) an active chimney complex (known as ‘Irina-2’); and (3) a diffuse flow through soft sediment (‘Anya's Garden’). Hydrothermal communities found at these fields differ considerably: Anya's Garden is dominated by vesicomyid clams, Ectenagena sp. aff. kaikoi, Irina-2 by mytilid mussels, Bathymodiolus sp. aff. puteoserpentis, with swarms of shrimps and other rich associated fauna, whereas on the main mound the fauna as a whole is less abundant and less diverse. This is the first record of a living clam population on the MAR. Other bivalves found on the clam field included thyasirids, Thyasira (Parathyasira) spp., not reported previously from deep-sea hot vent communities. The biomass on the mussel bed at Irina-2 exceeds 70 kg m−2 (wet weight with shells) and is the highest known on the MAR. The large shrimp population at Irina-2 includes swarms of Rimicaris exoculata. A diverse shrimp fauna (including a new species Alvinocaris sp. aff. muricola) on the mussel bed is dominated by Mirocaris keldyshi. Overall the Logatchev area is dominated by mussels which may be attributable to the presence of both methane-oxidizing (dominant type) and sulphur-oxidizing bacterial symbionts in their gills. The diversity of vent organisms and biotopes at Logatchev appears to be the highest on the MAR.

Calcium carbonate dissolution rates in deep-sea bivalve shells on the East Pacific Rise at 21°N: results of an 8-year in-situ experiment

Analysis of shell fragments of two common deep-sea hydrothermal vent bivalves, the vesicomyid clam Calyptogena magnifica and the mytilid Bathymodiolus thermophilus, deployed more than 100 m from any active hydrothermal vent location at 20°50′N, 109°06′W on the East Pacific Rise indicates significant variation of calcium carbonate dissolution in in-situ exposures of more than 8 years. Shell fragments embedded in epoxy blocks, mounted on a buoyed and anchored polypropylene line, and deployed by DSV Alvin (depth = 2615 m) were continuously exposed to ambient seawater (∼2°C) conditions immediately above the seafloor. The mean dissolution rate for the aragonitic shell of C. magnifica amounted to 27.7 μm/yr. Dissolution rates varied significantly among different shell layers of the clam, with the middle fine to irregular complex crossed lamellar shell layer exhibiting the lowest rates (mean = 22.2 μm/yr) and the irregular prismatic shell layer the highest rates (mean = 36.9 μm/yr). In the mytilid B. thermophilus, the dissolution rate of the aragonitic shell layer averaged 41.6 μm/yr, while that of the calcitic shell layer was immeasurable. The rates of calcium carbonate dissolution reported here for a ridge-crest site remote from any active hydrothermal vent are much lower than those previously documented for active vent sites at 21°N, the Guaymas Basin (Southern Trough), and Galapagos Rift (Rose Garden). Assuming a constant rate of dissolution, we estimate that empty adult shell valves of C. magnifica at the experimental site would dissolve completely in a period of ∼300 years, which has important implications for determining the longevity of hydrothermal vent activity along the rise axis.