Larvae of benthic invertebrates in hydrothermal vent plumes over Juan de Fuca Ridge

Larvae of benthic invertebrates collected in the water column above Juan de Fuca Ridge show distinct variations in abundance and composition in, and away from, the neutrally-buoyant hydrothermal plume emanating from underlying vents. Larvae of vent gastropods (Lepetodrilus sp. and two peltospirid species) occur in significantly higher abundances in the plume than away from it (mean abundance=21.0 individuals 1000 m^?3 vs 1.4 individuals 1000 m^?3), and larvae of vent bivalves (Calyptogena? sp.) occur exclusively in the plume (mean abundance=0.5 individuals 1000 m^?3). Larvae from other benthic taxa known not to be endemic to Juan de Fuca vent communities, such as anthozoans, pholad clams, bryozoans and echinoderms, are less abundant in the plume than away (mean abundance=47.5 vs 16.9 individuals 1000 m^?3) at comparable depths and heights above the bottom. These results support the hypothesis that larvae of vent species are entrained into buoyant hydrothermal plumes and transported at the level of lateral spreading several hundred meters above the seafloor. The discovery of vent-associated larvae in the plume suggests that models used to predict hydrodynamic processes in the plume will also be useful for modeling larval dispersal. Advanced imaging and new molecular-based approaches will be required to resolve taxonomic uncertainties in some larval groups (e.g. certain polychaete families) in order to distinguish vent species and make comprehensive flux estimates of all vent larvae in the neutrally-buoyant plume.     

POM in macro-/meiofaunal food webs associated with three flow regimes at deep-sea hydrothermal vents on Axial Volcano,Juan de Fuca Ridge

Deep-sea hydrothermal vent ecosystems host both symbiotic and non-symbiotic invertebrates. The non-symbiotic vent fauna is generally assumed to rely on free-living chemoautotrophic bacteria as their main food source but other sources such as detritus have recently been suggested to be a part of the invertebrate diets. Little is known about how food availability influences the distribution of vent organisms on a small scale. In addition, the feeding ecology and role of small, often numerically dominant invertebrates, the meiofauna is poorly understood at vents. In this study, we used stable carbon and nitrogen isotopic analysis to investigate the role of particulate detritus in the diets of macro- and meiobenthic invertebrates within three vent assemblages at Axial Volcano, Juan de Fuca Ridge, and Northeast Pacific. Particulate organic matter of a detrital origin became more important in the diet of invertebrates in assemblages typically associated with low-hydrothermal flow intensities. Meiobenthic species occupied several different feeding guilds and trophic levels in the assemblages investigated. We conclude that small-scale spatial variability in food sources is an important feature of vent food webs and that spatial patterns observed here and elsewhere are shaped by variations in hydrothermal discharge.     

A new bathymodioline mussel symbiosis at the Juan de Fuca hydrothermal vents

Until recently, the only major hydrothermal vent biogeographic province not known to include bathymodioline mussels was the spreading centers of the northeast Pacific, but deep-sea dives using DSV Alvin on the Endeavor segment of the Juan de Fuca Ridge (47°56N 129°06W; ∼2,200 m depth) in August 1999 yielded the only recorded bathymodioline mytilids from these northeastern Pacific vents. One specimen in good condition was evaluated for its relatedness to other deep-sea bathymodioline mussels and for the occurrence of chemoautotrophic and/or methanotrophic symbionts in the gills. Phylogenetic analyses of the host cytochrome oxidase I gene show this mussel shares evolutionary alliances with hydrothermal vent and cold seep mussels from the genus Bathymodiolus, and is distinct from other known species of deep-sea bathymodiolines, suggesting this mussel is a newly discovered species. Ultrastructural analyses of gill tissue revealed the presence of coccoid bacteria that lacked the intracellular membranes observed in methanotrophic symbionts. The bacteria may be extracellular but poor condition of the fixed tissue complicated conclusions regarding symbiont location. A single gamma-proteobacterial 16S rRNA sequence was amplified from gill tissue and directly sequenced from gill tissue. This sequence clusters with other mussel chemoautotrophic symbiont 16S rRNA sequences, which suggests a chemoautotrophic, rather than methanotrophic, symbiosis in this mussel. Stable carbon (δ¹³C = −26.6%) and nitrogen (δ¹⁵N = +5.19%) isotope ratios were also consistent with those reported for other chemoautotroph-mussel symbioses. Despite the apparent rarity of these mussels at the Juan de Fuca vent sites, this finding extends the range of the bathymodioline mussels to all hydrothermal vent biogeographic provinces studied to date.     

Vacuolate-attached filaments: highly productive Ridgeia piscesae epibionts at the Juan de Fuca hydrothermal vents

Vacuolate sulfur bacteria with high morphological similarity to vacuolate-attached filaments previously described from shallow hydrothermal vents (White Point, CA) were found at deep-sea hydrothermal vents. These filamentous bacteria grow in dense mats that cover surfaces and potentially provide a significant source of organic carbon where they occur. Vacuolate-attached filaments were collected near vents at the Clam Bed site of the Endeavour Segment of the Juan de Fuca Ridge and from the sediment surface at Escanaba Trough on the Gorda Ridge. A phylogenetic analysis comparing their 16S rRNA gene sequences to those collected from the shallow White Point site showed that all vacuolate-attached filament sequences form a monophyletic group within the vacuolate sulfur-oxidizing bacteria clade in the gamma proteobacteria. Abundance of the attached filaments was quantified over the length of the exterior surface of the tubes of Ridgeia piscesae worms collected from the Clam Bed site at Juan de Fuca yielding a per worm average of 0.070 ± 0.018 cm³ (n = 4). In agreement with previous results for White Point filaments, anion measurements by ion chromatography showed no detectable internal nitrate concentrations above ambient seawater (n = 9). For one R. piscesae tube worm “bush” at the Easter Island vent site, potential gross epibiont productivity is estimated to be 15 to 45× the net productivity of the worms.     

Population structure and reproductive biology of two sympatric hydrothermal vent polychaetes,Paralvinella pandorae and P. palmiformis

The alvinellid polychaetes Paralvinella pandorae Desbruyères and Laubier and P. palmiformis Desbruyères and Laubier occur at deep-sea hydrothermal vents along the Juan de Fuca and Explorer Ridges in the northeast Pacific Ocean. The population structure and reproductive biology of both species were studied in samples taken from three vent sites during six cruises in 1983 and 1984. Size-frequency analyses of two P. pandorae populations produced unimodal histograms, suggesting continuous or semi-continuous juvenile recruitment; in a third population two possible size classes were evident. Histograms of P. palmiformis displayed size-class peaks, which most likely reflected periodic recruitment of juveniles. Both species are gonochoric and gametes develop free in the coelom. Due to the simultaneous presence of a full range of gametogenic stages in P. pandorae populations, including spermatozoa in males, and to the continuous or semi-continuous recruitment pattern suggested by the size-frequency histograms, continuous reproduction is proposed for this species. In P. palmiformis a discrete, possibly synchronized, breeding cycle is thought to occur. Although maximum fecundity of P. pandorae is very low, continual reproduction over a long period of time could enhance its reproductive potential. The estimate of maximum fecundity for P. palmiformis is comparable to estimates for other polychaetes that undergo non-planktotrophic larval development. Maximum observed oocyte size was 215 and 260 m in P. pandorae and P. palmiformis, respectively. It is proposed that P. pandorae broods its young, while P. palmiformis probably undergoes demersal lecithotrophic larval development. The continual production of brooded young by P. pandorae could maintain a vent population, but severely limit dispersal to other vents. Demersal lecithotrophic larvae of P. palmiformis could repopulate vents, and potentially be carried by bottom currents to other vent sites.     

深海热液喷口周围微氧耐压细菌的培养研究

研究采用析因实验设计,探讨了培养基的水源、培养温度和生长pH等因素对喷口细菌生长繁殖的影响,初步探索了常压下东太平洋某海底喷口周围微氧耐压细菌的生长条件。样品培养10d后,对培养液中细菌量进行显微计数,实验数据用SPSS11.0统计软件中的方差分析程序进行处理。统计结果显示:在标准大气压下,从研究样品中获得最多细菌数的较优培养条件为:培养基水源为单蒸水;生长pH为7.60;培养温度为50℃。在该条件的培养液中细菌平均浓度达2.596×10个/mL。研究结果为进一步认识、鉴定与开发利用深海喷口周围细菌资源打下了基础。     

Larval dispersal of a deep-sea hydrothermal vent bivalve from the Galápagos Rift

The prodissoconch morphology of an unclassified mytilid from a hydrothermal vent on the Galápagos Rift indicates the presence of a planktotrophic larval stage with longrange dispersal capabilities. Recorded abyssal currents are probably sufficient to transport such larvae hundreds of kilometers. It is suggested that one or more aspects of the unusual biological or physico-chemical conditions (such as high microbial densities, elevated water temperatures, and hydrogen sulfide concentrations) encountered at vents, provide a stimulus to larval settlement. Such a behavioral response, perhaps coupled with a gregarious settling response, would provide a means of concentrating relatively sedentary organisms in and around these restricted, geographically isolated regions.     

Genetic comparison of two populations of the deep-sea vent shrimp Rimicaris exoculata (Decapoda: Bresiliidae) from the Mid-Atlantic Ridge

Allozyme data are presented for populations of the bresiliid shrimp Rimicaris exoculata from two hydrothermal vent fields, Trans-Atlantic Geotraverse (TAG) and Broken Spur, located along the Mid-Atlantic Ridge. These indicate that all morphotypes of R. exoculata examined, including those previously interpreted as representing separate species, are conspecific. Conversely, genetic identity between a single specimen of Chorocaris sp. and R. exoculata was high for intergeneric comparisons. Genetic variation in the populations of R. exoculata (H _o =0.034 to 0.056) was in the lower range of that estimated for other vent organisms, but similar to values obtained for other species of caridean shrimps in previous genetic studies. F-statistics were used to examine the population structure of R. exoculata. Estimates of variance of allele frequencies among populations (F _ST ) between TAG and Broken Spur were very low (mean F _ST =0.001), indicating no significant genetic differentiation between these populations although they were separated by 370 km. The number of migrants per generation was estimated from F _ST and by a private-alleles method, and indicates that migration between the two fields exceeds 100 individuals per generation. This may be because of efficient larval or adult migration or a combination of both. Estimates of the correlation between homologous alleles between individuals within local populations (F _IS ) of R. exoculata were high at two enzyme loci and indicate a heterozygote deficiency which caused a significant deviation from genotype frequencies expected under Hardy-Weinberg conditions. This deficiency was caused by the occurrence of rare homozygous genotypes in small individuals. In large individuals, rare alleles decreased in frequency or disappeared completely. This is discussed in relation to previous genetic investigations on other vent and nonvent organisms.     

Elemental composition of the hydrothermal vent clam Calyptogena magnifica from the East Pacific Rise

Specimens of the hydrothermal vent clam Calyptogena magnifica were collected at a depth of 2 600 m from 21° N on the East Pacific Rise in spring, 1982. The elemental composition was determined for the total soft tissues, individual organs, and the shell. The soft parts contained high concentrations of a number of trace metals. Iron, copper, and zinc, at 760, 148 and 2 152 g g^-1 dry weight, respectively, were present at the highest concentrations. Silver and antimony exhibited the greatest enrichments when their concentrations in C. magnifica were compared with those measured in a shallow-water marine mussel, Mytilus edulis. In contrast to the soft parts, the shell was not a site of metal accumulation or deposition. Within the soft parts; gills, kidney, pericardium, and mantle were involved in the concentration of various metals; the degree of concentration varied depending on the particular metal under consideration. C. magnifica possesses metal body burdens which are ordinarily considered to be very high and potentially toxic in other species.     

Mineralization of major lateral teeth in the radula of a deep-sea hydrothermal vent limpet (Gastropoda:Neolepetopsidae)

Radular teeth of a neolepetopsid patellogastropod limpet, Paralepetopsis ferrugivora, from a deep-sea hydrothermal vent site have similar elemental composition (O, Si, P, S, Cl, K and Fe) to radular teeth of the shallow-waters patellogastropod limpets. However, in contrast to shallow waters limpets, the fully mature teeth of P. ferrugivora do not show any crystalline phases. Amorphous silica was found in the cusp of the teeth and amorphous iron oxide in the junction zone and base of the teeth. Ferritin-rich vesicles were observed in cells adjacent to the junction zone of the early mature teeth, suggesting that these vesicles can mediate the delivery of iron to the tooth matrix. The similarity in the elemental composition between the hydrothermal limpet P. ferrugivora and shallow-waters patellogastropod limpets shows that the extreme hydrothermal environment did not alter the elemental composition of the radular teeth in the deep-sea species.Communicated by P. W. Sammarco, Chauvin     

Evolutionary relationships of deep-sea hydrothermal vent and cold-water seep clams (Bivalvia: Vesicomyidae): results from the mitochondrial cytochrome oxidase subunit I

Phylogenetic relationships among vesicomyid clams (Bivalvia: Vesicomyidae) and their placement within the order Heterodonta were examined using mitochondrial encoded cytochrome oxidase subunit I (COI) DNA sequences. The presently analyzed vesicomyids represent a recent monophyletic radiation that probably occurred within the Cenozoic. Nucleotide phylogenetic analyses resolved discrete clades that were consistent with currently recognized species: Calyptogena magnifica, C. ponderosa, Ectenagena extenta, C. phaseoliformis, Vesicomya cordata, Calyptogena n. sp. (Gulf of Mexico), C. kaikoi, C. nautilei, C. solidissima and C. soyoae (Type-A). However, specimens variously identified as: V. gigas, C. kilmeri, C. pacifica, and V. lepta comprised two “species complexes”, each composed of multiple evolutionary lineages. Most taxa are limited to hydrothermal-vent or cold-seep habitats, but the “vent” versus “seep” clams do not constitute separate monophyletic groups. Current applications of the generic names Calyptogena, Ectenagena, and Vesicomya are not consistent with phylogenetic inferences. Received: 24 July 1997 / Accepted: 22 August 1997     

Elemental concentrations of hydrothermal vent organisms from the Galápagos Rift

Concentrations of 40 elements were determined in tissues and whole bodies of mussels (Bathymodiolus thermophilus) and/or whole bodies of limpets (Neomphalus fretterae) collected from the submarine thermal springs on the Galápagos Rift in 1977. Concentrations of Ag, As, Fe, Mn, Mo, Se, Sr, and Zn were comparable or elevated relative to those of intertidal mollusks, which have been utilized as indicators of anthropogenic trace-metal contamination. Both species contained concentrations of some elements (e.g. Ag, As, Fe, Mn, Mo, Se, Zn) which corresponded with the elevated levels of those elements discharged from hydrothermal vents. Many elements were most enriched in the gills and digestive glands of the mussels. Iron:manganese ratios of those tissues were similar to ratios of pelagic and transitional sediments, indicating that metal-rich suspended particulates had been absorbed onto mucous membranes. Comparisons with other benthopelagic organisms were precluded, since the present data represent essentially the only trace-element analyses of deep-sea macroinvertebrates.     

Influence of habitat on the reproductive biology of the deep-sea hydrothermal vent limpet <Emphasis Type="Italic">Lepetodrilus fucensis</Emphasis> (Vetigastropoda: Mollusca) from the Northeast Pacific

Habitat selection by the hydrothermal vent limpet, Lepetodrilus fucensis, in Northeast Pacific hydrothermal vent ecosystems, may influence its reproductive output, as it occupies habitats with varying physico-chemical conditions that reflect the availability of nutritional resources. Histological techniques were used to determine size at first reproduction, gametogenesis, reproductive output, and fecundity in relation to shell length (SL), through examination of the gonads of male and female L. fucensis, collected from five different hydrothermal vent habitat types with different temperature anomalies and hydrothermal fluid flow vigour: vigorous (VIG), diffuse (DIF), tubeworm bushes (TWB), peripheral (PER), and senescent areas (SEN). Both male and female L. fucensis exhibited early maturity, with the first reproductive event occurring at 3.8 and 3.9 mm shell length, respectively. All stages of gamete development were present in the gonads of males and females, suggesting continuous gametogenesis and asynchronous reproduction in this species. Gametogenic maturity of limpets did not vary among actively venting habitats (VIG, DIF, TWB, and PER), but was significantly lower in males and females from SEN habitats. Mean oocyte diameter was largest in females from VIG habitats, and smallest in females from SEN habitats, than in those from the other habitats (DIF, TWB, and PER). Females from actively venting habitats also had greater actual fecundity than those from senescent habitats. While the gametogenic pattern of L. fucensis appears phylogenetically constrained, selection of actively venting habitats by L. fucensis maximizes its reproductive output. The multiple feeding strategies of L. fucensis may allow for a constant supply of energy to be allocated to reproduction in any habitat except senescent vents. Early maturity, high fecundity, and continuous production of gametes suggest a reproductive strategy characteristic of an opportunistic species, and may be contributing to the extremely abundant populations of L. fucensis observed in the Northeast Pacific vent ecosystem.     

Carbon and nitrogen isotopic composition of the fauna from the Broken Spur hydrothermal vent field

 The carbon and nitrogen isotopic composition of 18 faunistic groups collected during the 39th cruise of the R.V. “Akademik Mstislav Keldysh” in September 1996 at the Broken Spur vent field, MAR, was studied. The trophic structure of the Broken Spur vent community is considered. All age stages of the shrimp Rimicaris exoculata living 5 m below the main aggregations at black smokers show higher δ¹⁵N and more depleted δ¹³C values than the same stages inhabiting the black smokers themselves. The shrimps R. exoculata and Chorocaris chacei demonstrate ontogenetic changes in δ¹³C (the former also in δ¹⁵N), with smaller individuals showing higher δ¹⁵N and more depleted δ¹³C values than larger shrimps. Benthopelagic and benthic components of the vent community differ significantly in δ¹³C and δ¹⁵N, the benthic fauna being less dependent upon chemosynthetic production. Received: 30 March 1999 / Accepted: 28 September 1999     

Population structure of yellowfin tuna (Thunnus albacares) in the western Pacific Ocean, inferred from microsatellite loci

Five polymorphic microsatellite loci were examined in 1391 yellowfin tuna (Thunnus albacares) from eight regions of the western (Coral Sea, eastern Australia, Fiji, Indonesia, Philippines and Solomon Islands) and eastern (California and Mexico ) Pacific Ocean. Across all samples, numbers of alleles per locus ranged from 7 to 30 (mean: 17.0), and observed heterozygosities per locus ranged from 0.223 to 0.955 (mean: 0.593). Temporal collections were available for three areas: no significant temporal heterogeneity was observed for the Coral Sea (1991/1992 and 1995/1996 collections) or eastern Australia (1994/1995, 1995/1996, 1996/1997 and 1997/1998), but there was slight but significant heterogeneity at one locus (cmrTa-161) between the two Philippines collections (1994/1995 and 1996/1997). Genotypes generally showed a good fit to Hardy-Weinberg expectations within populations; only cmrTa-208 in the pooled Coral Sea population gave a significant deviation after Bonferroni correction for 40 tests, with a small but significant excess of homozygotes. Four loci showed no evidence of population differentiation following contingency Chi-squared and F_ST analyses. The fifth locus, cmrTa-161, showed small but significant differentiation (F_ST=0.002, P<0.001). This heterogeneity was largely a result of the Philippines 1994/1995 and Fiji collections; there was no correlation with geographic distance. The average F_ST across all five loci was very low (F_ST=0.002), but it was significant (P<0.001). It is unclear whether this low but significant differentiation reflects noise in the dataset, perhaps arising from experimental error, or real population differentiation. The finding of very limited population heterogeneity accords with most of the earlier allozyme and mitochondrial DNA studies of yellowfin tuna in the Pacific Ocean.     

Nematode community composition in hydrothermal vent and adjacent non-vent fields around Myojin Knoll,a seamount on the Izu-Ogasawara Arc in the western North Pacific Ocean

In contrast to specific large benthic invertebrates in chemosynthetic ecosystems such as hydrothermal vents, meiofaunal communities in such habitats have been reported to have strong taxonomic overlap with meiofauna in the adjacent “normal” environments. However, meiofauna have only recently been included in studies of those environments and detailed information on these communities is still rare. This is especially true in the Northwest Pacific Ocean, even though there are many seamounts with active vents in the calderas of the region. Nematode community composition at the genus level in sediments from a hydrothermal vent field in the caldera of Myojin Knoll (32°06′N, 139°52′E, depth 1,300 m), a seamount on the Izu-Ogasawara Arc, Japan, was investigated for the first time and was compared with adjacent non-vent areas inside and outside the caldera. Multivariate analyses showed that the composition of nematodes in the hydrothermal field was significantly different from that in the non-hydrothermal fields around the caldera. However, the common genera, such as Oxystomina, Pareudesmoscolex, Desmoscolex, and Microlaimus were found in two, or all three vent fields while their rank contributions differed among the three fields. When the data from Myojin Knoll were compared with those from other deep-sea vent environments in different regions (e.g., North Fiji Basin, East Pacific Rise, Mid-Atlantic Ridge), the nematode composition in the vent field of the Myojin caldera was more similar to that of the non-vent fields around the caldera than the composition in vent fields of other regions. These data from the Northwest Pacific Ocean also suggest the absence of long-range transport systems and local adaptations for meiofauna in hydrothermal vent fields.     

Bacterial colonization and endocytosis on the gill of a new limpet species from a hydrothermal vent

Calcification rates in different fragments along branches of the hermatypic coral Stylophora pistillata were tested in the laboratory using a new technique, the optic glassfiber method. By this method, the tested colony remains constantly in dark conditions while a narrow beam of light, transferred through the optic fiber, illuminates a small distinct point of coral tissue (on a branch tip or base). The selected illuminated portion of the branch serves as the experimental fragment, while all the other parts of the same colony serve as the dark controls. The results indicate that significantly more calcium is incorporated in the tip fragments than in the bases, both in light and in dark conditions (4.1 to 13.2 times more). Illumination of the tips or the bases did not stimulate or enhance the calcification rates of these fragments. Thus, in all colonies tested, the calcification rates of the illuminated fragments were not significantly different from the average rates of other similar, non-illuminated fragments of the same colony. It is suggested that light does not directly enhance calcification in hermatypic corals, but rather, that light enhances O₂ production, which consequently stimulates coral metabolism. Our preliminary results indicate that calcification rates recorded in aerated dark experiments are significantly higher than calcification rates of non-aerated dark controls.     

Population structure and reproductive patterns of the NW Mediterranean deep-sea macrourid Trachyrincus scabrus (Rafinesque, 1810)

Macrourid fish are one of the most abundant marine species on continental margins worldwide. Although they play an important role in the ecosystem, little is known about their overall biology. We report here a large data set of the most abundant macrourid in Mediterranean waters, Trachyrincus scabrus, showing the main population and reproductive characteristics. The study was based on 3,239 specimens collected between 300 and 1,500 m depth on the northwestern Mediterranean in 2003–2004 and 2008–2009. The population showed a depth-related structure with the largest individuals at 1,100 m depth and the smallest (i.e., immature) at shallower depths. Macroscopic and microscopic analyses of the gonads showed that T. scabrus has a highly seasonal reproductive pattern. Spawning females were found during winter when the organic matter fluxes were highest on the continental slope. T. scabrus is a batch spawner with group-synchronous oocyte development and present low average fecundity of 14,191 oocytes.     

Genetic differentiation between spatially-disjunct populations of the deep-sea,hydrothermal vent-endemic amphipod Ventiella sulfuris

Species endemic to deep-sea hydrothermal vent ecosystems have disjunct distributions imposed by the island-like arrangement of their specialized habitats. Using allozyme electrophoresis, we examined genetic population structure of the hydrothermal vent amphipod Ventiella sulfuris Barnard and Ingram, 1990. Samples from five sites along the East Pacific Rise (EPR) and two along the Galapagos Rift were collected in 1990 and 1988, respectively. Variability, based on 12 enzyme loci, was relatively high (proportion of polymorphic loci whose most common allele not greater than 0.95 in frequency) =41.6%; (mean heterozygosity) =0.158] compared with shallow-water marine and freshwater amphipods, and similar to the deep-sea lysianassid Eurythenes gryllus. Genetic divergence among populations spread along a cöntiguous rift axis (i.e., EPR) was low [Nei's genetic distance (D) ranged from <0.001 to 0.018]. Genetic tructure analysis suggests that along a contiguous ridge axis migration occurs in a stepping stone manner and is unconstrained by distances as great as 1200 km (migration rate, , ranged from 1.9 to 67.8 ind. generation^–1). However, genetic divergence between populations on disjunct ridge axes was extremely high (D ranged from 0.438 to 0.476). Most of the variance in gene frequencies was due to the differences between the major subpopulations inhabiting the two distinct ridge axes, EPR and Galapagos Rift. Apparently, very little migration and gene flow occur between these major subpopulations (M1). This level of genetic divergence may be sufficient to justity separation of EPR and Galapagos Rift populations at the species level. Futher analyses of morphological characters is required before taxonomic status can be assigned.     

Ultrastructure of the gill of the hydrothermal vent bivalveCalyptogena magnifica,with a discussion of its nutrition

Calyptogena magnifica Boss and Turner, 1980, a new Vesicomyidae found during the Galápagos expedition in hydrothermal vents of the East Pacific Rise, was collected in the same Rise at 21°N during the Oasis expedition (March 1982), and samples of the gill were fixed for ultrastructural observations. The large size and structure of the gill indicate that this is the organ mainly involved in the nutritional processes ofC. magnifica. Despite the classic structural appearance of the external cilia of its gill, and an obvious production of mucus,C. magnifica is not a filter-feeder, as it does not use filtering processes to provide its major source of nutrition. Negligible particulate transfer is evidenced by reduction of the ciliary groove, of the labial palps and of the digestive tube, as well as by the absence of mucous strings. Histological and ultrastructural observations endorse the hypothesis that endocellular chemoautotrophic bacteria play an important role in the nutrition of the clam. Except for a superficial zone of ciliated cells, most of the gill tissue is comprised of cells which appear to be bacteriocytes, and which are perfectly integrated into the gill tissue and contain abundant and normally reproducing bacteria. The differences observed in the structure of the bacteriocytes suggest a cyclic process of their colonization by bacteria, their possible resorption, and their replacement by new bacteria-infected cells. Energetic substrates (sulfides and organic molecules) are probably directly absorbed by the bacteriocytes through the microvilli of the epithelial cells. Abundant fingerprint-like mitochondria in ciliate cells attest to a particularly high metabolic activity, perhaps related to active biosynthesis.