Structural and ultrastructural analysis of the gills in the bacterial-bearing species <Emphasis Type="Italic">Thyasira falklandica</Emphasis> (Bivalvia,Mollusca)

In this study, the cellular organization of the gill that harbors symbiotic bacteria is described in the thyasirid Thyasira falklandica collected from South Shetlands in Antarctic. Sections of the gills revealed that T. falklandica belongs to the gill type 3, as described by Dufour (Biol Bull, 208:200–212, 2005), with an elongated lateral zone along the frontal-abfrontal axis of the gill filaments. The ciliated and intermediary zones looked similar to those described in symbionts-bearing bivalves. The lateral zone is more complex in T. falklandica than in other Thyasiridae already described. Such a zone is composed of four different cell types. Bacteriocytes are abundant in the frontal and abfrontal positions, while the middle part of the lateral zone is occupied mostly by numerous granule cells devoid of bacteria. All along the lateral zone, TEM and SEM observations show some ciliated cells, which are regularly interspersed between bacteriocytes and/or granule cells. Such cells, according to the long double ciliary roots of their cilia, should have a sensory function. Intercalary cells, which have never been observed between bacteriocytes, are restricted to the middle part of the lateral zone where their expansions overlap the adjacent granule cells. Bacterial symbionts occur only extracellularly among long microvilli differentiated by the bacteriocytes. They are abundant, usually spherical in shape (around 0.7 μm length), and covered by the glycocalix from bacteriocyte microvilli. According to TEM views, the empty vesicles located in the periplasmic space should be sulfur storage, as known for other sulfur-oxidizing symbionts.     

Exceptional preservation of a novel gill grade in large Cretaceous inoceramids: systematic and palaeobiological implications

Organised mineralised structures observed in large inoceramids (valves on a metre scale) from the Late Albian, Toolebuc Formation, Australia (Inoceramus sutherlandi McCoy, 1865), and the Santonian, Niobrara Formation, USA (Platyceramus sp.), were investigated using variable pressure scanning electron microscope (SEM) with energy‐dispersive X‐ray spectroscopy (EDX), X‐ray microcomputed tomography (micro‐CT) and X‐ray diffraction (XRD) analyses. These indicate that the structures comprised a phosphate framework of aligned tubes and shallow troughs overlain perpendicularly by evenly spaced structures. In the Toolebuc Inoceramus, these are U‐shaped cross‐structures, whilst in the Niobrara Platyceramus, they comprise bundled fibre elements. Comparison with modern bivalves indicates that the observed phosphatised structures represent soft‐tissue preservation of the gills, as suggested in earlier publications. The tubes and troughs are remnants of a filamental support framework comprising ordinary and primary filaments, whilst the U‐shaped cross‐structures (I. sutherlandi) and fibrous bands (Platyceramus) represent preserved longitudinal gill musculature. Internal perforate and strand‐like fabric observed on the internal surface of some Platyceramus tubular structures suggests that the framework comprised collagen. The presence of primary and ordinary filaments in numerous unusually large plicae, in at least two lamellae, indicates that the gill structures were heterorhabdic. Each plica has at least 40 ordinary filaments, an exceptional number when compared with the maximum of 20 present in modern heterorhabdic gills. The absence of incontrovertible interfilament junctions makes it difficult to say whether inoceramids were filibranch, pseudolamellibranch or eulamellibranch. However, structures that are best attributed to intraplical junctions between filaments suggest the Inoceramidae had gills akin to those of pseudolamellibranch bivalves, although their unusually large number of filaments per plica is more reminiscent of homorhabdic eulamellibranch gills. The general form of the gill is similar to that described in some other pteriomorphs, most specifically Pteria. However, it has more complex junctions and interconnections, although these are not as intricate or pervasive as those observed in the pseudolamellibranch Ostreidae. The connections and well‐developed filament framework allowed the gill to reach its unusually large size, supporting the large size of these inoceramid species. The unusually large size of the gill and its components indicate that the organism fed on the larger suspended organic particles in the water column. It would also have been capable of processing large volumes of water quickly, leading to greater potential for food accumulation and with likely implications for respiratory efficiency. This may help explain the common association of inoceramids with oxygen‐deficient palaeoenvironments, particularly as the general structure of the inoceramid gill is very different to that observed in the commonest extant chemosymbiotic bivalves.     

Coexistence of multiple proteobacterial endosymbionts in the gills of the wood-boring Bivalve Lyrodus pedicellatus (Bivalvia: Teredinidae)

Wood-boring bivalves of the family Teredinidae (commonly called shipworms) are known to harbor dense populations of gram-negative bacteria within specialized cells (bacteriocytes) in their gills. These symbionts are thought to provide enzymes, e.g., cellulase and dinitrogenase, which assist the host in utilizing wood as a primary food source. A cellulolytic, dinitrogen-fixing bacterium, Teredinibacter turnerae, has been isolated from the gill tissues of numerous teredinid bivalves and has been proposed to constitute the sole or predominant symbiont of this bivalve family. Here we demonstrate that one teredinid species, Lyrodus pedicellatus, contains at least four distinct bacterial 16S rRNA types within its gill bacteriocytes, one of which is identical to that of T. turnerae. Phylogenetic analyses indicate that the three newly detected ribotypes are derived from gamma proteobacteria that are related to but distinct (>6.5% sequence divergence) from T. turnerae. In situ hybridizations with 16S rRNA-directed probes demonstrated that the pattern of occurrence of symbiont ribotypes within bacteriocytes was predictable and specific, with some bacteriocytes containing two symbiont ribotypes. However, only two of the six possible pairwise combinations of the four ribotypes were observed to cooccur within the same host cells. The results presented here are consistent with the existence of a complex multiple symbiosis in this shipworm species.     

Magnetosome-containing bacteria living as symbionts of bivalves

Bacteria containing magnetosomes (protein-bound nanoparticles of magnetite or greigite) are common to many sedimentary habitats, but have never been found before to live within another organism. Here, we show that octahedral inclusions in the extracellular symbionts of the marine bivalve Thyasira cf. gouldi contain iron, can exhibit magnetic contrast and are most likely magnetosomes. Based on 16S rRNA sequence analysis, T. cf. gouldi symbionts group with symbiotic and free-living sulfur-oxidizing, chemolithoautotrophic gammaproteobacteria, including the symbionts of other thyasirids. T. cf. gouldi symbionts occur both among the microvilli of gill epithelial cells and in sediments surrounding the bivalves, and are therefore facultative. We propose that free-living T. cf. gouldi symbionts use magnetotaxis as a means of locating the oxic–anoxic interface, an optimal microhabitat for chemolithoautotrophy. T. cf. gouldi could acquire their symbionts from near-burrow sediments (where oxic–anoxic interfaces likely develop due to the host''s bioirrigating behavior) using their superextensile feet, which could transfer symbionts to gill surfaces upon retraction into the mantle cavity. Once associated with their host, however, symbionts need not maintain structures for magnetotaxis as the host makes oxygen and reduced sulfur available via bioirrigation and sulfur-mining behaviors. Indeed, we show that within the host, symbionts lose the integrity of their magnetosome chain (and possibly their flagellum). Symbionts are eventually endocytosed and digested in host epithelial cells, and magnetosomes accumulate in host cytoplasm. Both host and symbiont behaviors appear important to symbiosis establishment in thyasirids.     

Extensive variation in intracellular symbiont community composition among members of a single population of the wood-boring bivalve Lyrodus pedicellatus (Bivalvia: Teredinidae)

Shipworms (wood-boring bivalves of the family Teredinidae) harbor in their gills intracellular bacterial symbionts thought to produce enzymes that enable the host to consume cellulose as its primary carbon source. Recently, it was demonstrated that multiple genetically distinct symbiont populations coexist within one shipworm species, Lyrodus pedicellatus. Here we explore the extent to which symbiont communities vary among individuals of this species by quantitatively examining the diversity, abundance, and pattern of occurrence of symbiont ribotypes (unique 16S rRNA sequence types) among specimens drawn from a single laboratory-reared population. A total of 18 ribotypes were identified in two clone libraries generated from gill tissue of (i) a single specimen and (ii) four pooled specimens. Phylogenetic analysis assigned all of the ribotypes to a unique clade within the gamma subgroup of proteobacteria which contained at least five well-supported internal clades (phylotypes). By competitive quantitative PCR and constant denaturant capillary electrophoresis, we estimated the number and abundance of symbiont phylotypes in gill samples of 13 individual shipworm specimens. Phylotype composition varied greatly; however, in all specimens the numerically dominant symbiont belonged to one of two nearly mutually exclusive phylotypes, each of which was detected with similar frequencies among specimens. A third phylotype, containing the culturable symbiont Teredinibacter turnerae, was identified in nearly all specimens, and two additional phylotypes were observed more sporadically. Such extensive variation in ribotype and phylotype composition among host specimens adds to a growing body of evidence that microbial endosymbiont populations may be both complex and dynamic and suggests that such genetic variation should be evaluated with regard to physiological and ecological differentiation.     

Bacterial symbiont transmission in the wood-boring shipworm Bankia setacea (Bivalvia: Teredinidae)

The Teredinidae (shipworms) are a morphologically diverse group of marine wood-boring bivalves that are responsible each year for millions of dollars of damage to wooden structures in estuarine and marine habitats worldwide. They exist in a symbiosis with cellulolytic nitrogen-fixing bacteria that provide the host with the necessary enzymes for survival on a diet of wood cellulose. These symbiotic bacteria reside in distinct structures lining the interlamellar junctions of the gill. This study investigated the mode by which these nutritionally essential bacterial symbionts are acquired in the teredinid Bankia setacea. Through 16S ribosomal DNA (rDNA) sequencing, the symbiont residing within the B. setacea gill was phylogenetically characterized and shown to be distinct from previously described shipworm symbionts. In situ hybridization using symbiont-specific 16S rRNA-directed probes bound to bacterial ribosome targets located within the host gill coincident with the known location of the gill symbionts. These specific probes were then used as primers in a PCR-based assay which consistently detected bacterial rDNA in host gill (symbiont containing), gonad tissue, and recently spawned eggs, demonstrating the presence of symbiont cells in host ovary and offspring. These results suggest that B. setacea ensures successful inoculation of offspring through a vertical mode of symbiont transmission and thereby enables a broad distribution of larval settlement.     

Evidence for Methylotrophic Symbionts in a Hydrothermal Vent Mussel (Bivalvia: Mytilidae) from the Mid-Atlantic Ridge

Symbioses between chemolithoautotrophic bacteria and the major macrofaunal species found at hydrothermal vents have been reported for numerous sites in the Pacific Ocean. We present microscopical and enzymatic evidence that methylotrophic bacteria occur as intracellular symbionts in a new species of mytilid mussel discovered at the Mid-Atlantic Ridge hydrothermal vents. Two distinct ultrastructural types of gram-negative procaryotic symbionts were observed within gill epithelial cells by transmission electron microscopy: small coccoid or rod-shaped cells and larger coccoid cells with stacked intracytoplasmic membranes typical of methane-utilizing bacteria. Methanol dehydrogenase, an enzyme diagnostic of methylotrophs, was detected in the mytilid gills, while tests for ribulose-1,5-bisphosphate carboxylase, the enzyme diagnostic of autotrophy via the Calvin cycle, were negative. Stable carbon isotope values (δ¹³C) of mytilid tissue (−32.7 and −32.5% for gill and foot tissues, respectively) fall within the range of values reported for Pacific vent symbioses but do not preclude the use of vent-derived methane reported to be isotopically heavy relative to biogenically produced methane.     

Bacterial Symbiont Transmission in the Wood-Boring Shipworm Bankia setacea (Bivalvia: Teredinidae)

The Teredinidae (shipworms) are a morphologically diverse group of marine wood-boring bivalves that are responsible each year for millions of dollars of damage to wooden structures in estuarine and marine habitats worldwide. They exist in a symbiosis with cellulolytic nitrogen-fixing bacteria that provide the host with the necessary enzymes for survival on a diet of wood cellulose. These symbiotic bacteria reside in distinct structures lining the interlamellar junctions of the gill. This study investigated the mode by which these nutritionally essential bacterial symbionts are acquired in the teredinid Bankia setacea. Through 16S ribosomal DNA (rDNA) sequencing, the symbiont residing within the B. setacea gill was phylogenetically characterized and shown to be distinct from previously described shipworm symbionts. In situ hybridization using symbiont-specific 16S rRNA-directed probes bound to bacterial ribosome targets located within the host gill coincident with the known location of the gill symbionts. These specific probes were then used as primers in a PCR-based assay which consistently detected bacterial rDNA in host gill (symbiont containing), gonad tissue, and recently spawned eggs, demonstrating the presence of symbiont cells in host ovary and offspring. These results suggest that B. setacea ensures successful inoculation of offspring through a vertical mode of symbiont transmission and thereby enables a broad distribution of larval settlement.     

Genus age,provincial area and the taxonomic structure of marine faunas

Species are unevenly distributed among genera within clades and regions, with most genera species-poor and few species-rich. At regional scales, this structure to taxonomic diversity is generated via speciation, extinction and geographical range dynamics. Here, we use a global database of extant marine bivalves to characterize the taxonomic structure of climate zones and provinces. Our analyses reveal a general, Zipf–Mandelbrot form to the distribution of species among genera, with faunas from similar climate zones exhibiting similar taxonomic structure. Provinces that contain older taxa and/or encompass larger areas are expected to be more species-rich. Although both median genus age and provincial area correlate with measures of taxonomic structure, these relationships are interdependent, nonlinear and driven primarily by contrasts between tropical and extra-tropical faunas. Provincial area and taxonomic structure are largely decoupled within climate zones. Counter to the expectation that genus age and species richness should positively covary, diverse and highly structured provincial faunas are dominated by young genera. The marked differences between tropical and temperate faunas suggest strong spatial variation in evolutionary rates and invasion frequencies. Such variation contradicts biogeographic models that scale taxonomic diversity to geographical area.     

Worth a second look: gill structure in Hemipecten forbesianus (Adams & Reeve, 1849) and taxonomic implications for the Pectinidae

The gill of the reef-dwelling Hemipecten forbesianus was examinedusing histology and scanning electron microscopy, in order toverify its reported homorhabdic state, and to provide the firstdetailed information on tropical pectinid gill structure. Dueto the difficulty of obtaining live individuals, museum referencespecimens were used for this study. The gill is heterorhabdic,and presents several notable differences compared to the larger,temperate species studied to date: (1) the small number of ordinaryfilaments per plica (probably associated with the very smallsize of these specimens), (2) the outer demibranch is approximately15% shorter than the inner demibranch, (3) several characteristicswhich confer gill cohesion in other pectinid gills are absentand (4) the principal filaments are devoid of mucocytes, suchthat particle transport of positively-selected particles mustbe entirely effected in water. The apparent universality ofthe heterorhabdic condition within the Pectinidae leads us torecommend its use as a reliable soft-body taxonomic characterat the family level. The anatomical differences between thegill of H. forbesianus and that of the larger, temperate speciescall for further comparison with both large and small, temperateand tropical species, in order to elucidate the effects of sizeand habitat on gill structure in this family. (Received 26 June 2007; accepted 14 January 2008)     

Extensive Variation in Intracellular Symbiont Community Composition among Members of a Single Population of the Wood-Boring Bivalve Lyrodus pedicellatus (Bivalvia: Teredinidae)

Shipworms (wood-boring bivalves of the family Teredinidae) harbor in their gills intracellular bacterial symbionts thought to produce enzymes that enable the host to consume cellulose as its primary carbon source. Recently, it was demonstrated that multiple genetically distinct symbiont populations coexist within one shipworm species, Lyrodus pedicellatus. Here we explore the extent to which symbiont communities vary among individuals of this species by quantitatively examining the diversity, abundance, and pattern of occurrence of symbiont ribotypes (unique 16S rRNA sequence types) among specimens drawn from a single laboratory-reared population. A total of 18 ribotypes were identified in two clone libraries generated from gill tissue of (i) a single specimen and (ii) four pooled specimens. Phylogenetic analysis assigned all of the ribotypes to a unique clade within the γ subgroup of proteobacteria which contained at least five well-supported internal clades (phylotypes). By competitive quantitative PCR and constant denaturant capillary electrophoresis, we estimated the number and abundance of symbiont phylotypes in gill samples of 13 individual shipworm specimens. Phylotype composition varied greatly; however, in all specimens the numerically dominant symbiont belonged to one of two nearly mutually exclusive phylotypes, each of which was detected with similar frequencies among specimens. A third phylotype, containing the culturable symbiont Teredinibacter turnerae, was identified in nearly all specimens, and two additional phylotypes were observed more sporadically. Such extensive variation in ribotype and phylotype composition among host specimens adds to a growing body of evidence that microbial endosymbiont populations may be both complex and dynamic and suggests that such genetic variation should be evaluated with regard to physiological and ecological differentiation.     

Evolutionary changes in symbiont community structure in ticks

Ecological specialization to restricted diet niches is driven by obligate, and often maternally inherited, symbionts in many arthropod lineages. These heritable symbionts typically form evolutionarily stable associations with arthropods that can last for millions of years. Ticks were recently found to harbour such an obligate symbiont, Coxiella‐LE, that synthesizes B vitamins and cofactors not obtained in sufficient quantities from blood diet. In this study, the examination of 81 tick species shows that some Coxiella‐LE symbioses are evolutionarily stable with an ancient acquisition followed by codiversification as observed in ticks belonging to the Rhipicephalus genus. However, many other Coxiella‐LE symbioses are characterized by low evolutionary stability with frequent host shifts and extinction events. Further examination revealed the presence of nine other genera of maternally inherited bacteria in ticks. Although these nine symbionts were primarily thought to be facultative, their distribution among tick species rather suggests that at least four may have independently replaced Coxiella‐LE and likely represent alternative obligate symbionts. Phylogenetic evidence otherwise indicates that cocladogenesis is globally rare in these symbioses as most originate via horizontal transfer of an existing symbiont between unrelated tick species. As a result, the structure of these symbiont communities is not fixed and stable across the tick phylogeny. Most importantly, the symbiont communities commonly reach high levels of diversity with up to six unrelated maternally inherited bacteria coexisting within host species. We further conjecture that interactions among coexisting symbionts are pivotal drivers of community structure both among and within tick species.     

Carbon isotope signatures reveal that diet is related to the relative sizes of the gills and palps in bivalves

In marine bivalves, the relative sizes of the gills and palps appear to be a useful functional trait that reflect feeding mode, i.e. suspension feeders have relatively larger gills than palps for pumping, whereas deposit feeders have relatively larger palps than gills for sorting. Also, within a species, the relative sizes of the gills and palps are related to changes in local food conditions. However, there is still no firm evidence showing that differences in the relative gill and palp sizes between species are related to diet selection. Based on the knowledge that carbon and nitrogen isotope signatures of an animals tissues reflect past diet, we compared the relative gill and palp sizes of bivalves from Roebuck Bay, northwestern Australia with their carbon and nitrogen isotope signatures. The carbon isotope signatures distinguished clear differences in diet between bivalves along a gradient from suspension to deposit feeding, and strikingly this pattern was closely followed by the relative sizes of the gills and palps of the bivalves. This study confirms that relative gill and palp sizes in bivalves are a functional trait that can be used to compare resource use between species. Furthermore, these data may suggest that morphospace occupation, as determined by relative gill and palp sizes of bivalves, could reflect a gradient of resource use between species.     

Spirotrichonymphea (Parabasalia) symbionts of the termite Paraneotermes simplicicornis

The desert dampwood termite Paraneotermes simplicicornis harbors several species of obligately symbiotic protists that support its nutrition by fermenting lignocellulose. Among them are three morphotypes with the dexiotropic spiraling flagellar bands characteristic of Spirotrichonymphea (Parabasalia). The largest morphotype, characterized by an elongated cell apex with axial columella and internally positioned spiraling flagellar bands, was previously described as Spirotrichonympha polygyra. A smaller morphotype, with similarly internalized flagellar bands but a more rounded posterior without a protruding axostyle, was previously reported but not named. The smallest morphotype has surface flagellar bands and can attach to other protist cells by its apex. In this study, we combine light microscopy of live specimens and 18S rRNA gene sequencing of individually isolated cells to better understand the diversity of symbionts in P. simplicicornis. We found that S. polygyra branches distantly from true Spirotrichonympha, which are associated with Reticulitermes termites. Thus, we propose the new genus Cuppa to accommodate C. polygyra n. comb. (type species) and the similar but smaller morphotype Cuppa taenia n. sp. The undescribed smallest morphotype can be excluded from all previously described Spirotrichonymphea genera by molecular and behavioral evidence, so we propose Fraterculus simplicicornis n. gen., n. sp., to accommodate this organism.     

Phylogenetic diversity of bacterial symbionts of Solemya hosts based on comparative sequence analysis of 16S rRNA genes.

The bacterial endosymbionts of two species of the bivalve genus Solemya from the Pacific Ocean, Solemya terraeregina and Solemya pusilla, were characterized. Prokaryotic cells resembling gram-negative bacteria were observed in the gills of both host species by transmission electron microscopy. The ultrastructure of the symbiosis in both host species is remarkably similar to that of all previously described Solemya spp. By using sequence data from 16S rRNA, the identity and evolutionary origins of the S. terraeregina and S. pusilla symbionts were also determined. Direct sequencing of PCR-amplified products from host gill DNA with primers specific for Bacteria 16S rRNA genes gave a single, unambiguous sequence for each of the two symbiont species. In situ hybridization with symbiont-specific oligonucleotide probes confirmed that these gene sequences belong to the bacteria residing in the hosts gills. Phylogenetic analyses of the 16S rRNA gene sequences by both distance and parsimony methods identify the S. terraeregina and S. pusilla symbionts as members of the gamma subdivision of the Proteobacteria. In contrast to symbionts of other bivalve families, which appear to be monophyletic, the S. terraeregina and S. pusilla symbionts share a more recent common ancestry with bacteria associating endosymbiotically with bivalves of the superfamily Lucinacea than with other Solemya symbionts (host species S. velum, S. occidentalis, and S. reidi). Overall, the 16S rRNA gene sequence data suggest that the symbionts of Solemya hosts represent at least two distinct bacterial lineages within the gamma-Proteobacteria. While it is increasingly clear that all extant species of Solemya live in symbiosis with specific bacteria, the associations appear to have multiple evolutionary origins.     

On the growth of bivalve gills initiated from a lobule-producing budding zone

The growth of bivalve gills proceeds at the posterior end of the gill from a meristem-like budding zone, that is, an undifferentiated terminal organ, which continuously proliferates new gill elements in growing bivalves. In representatives of protobranch, filibranch, and eulamellibranch gills (13 species from Protobranchia, Pteriomorphia, Palaeoheterodonta, and Heterodonta), the first growth steps demonstrate a uniform basic pattern. The budding zone produces either transverse folds that split after a transition zone into parallel pairs of lobules (which themselves later differentiate into the inner and outer demibranchs), or it produces the lobules directly, without first forming a transition zone. The lobules elongate, differentiate into lobes, and transform into leaflet-like structures (protobranchs) or into filaments (filibranchs and eulamellibranchs). The filaments represent the differentiated outer margins of each lobe, of which the central tissue (interlamellar septum) becomes incised or fenestrated, or transformed by tissue junctions. A distally located main growth zone for each lobe is suggested. With regard to the delayed onset of the differentiation of the outer demibranch in juvenile unionids, an additional temporary growth zone for filaments is suggested to exist at the anterior end of the outer demibranch.     

Phylogenic Diversity and Taxonomic Problems of the Dictyosphaerium Morphotype within the Parachlorella Clade (Chlorellaceae,Trebouxiophyceae)

The Parachlorella clade was put forward as a group within the family Chlorellaceae in 2004. Recent molecular analyses have revealed that Dictyosphaerium morphotype algae form several independent lineages within the Parachlorella clade, and new genera and species have been established. In this study, we focus on the diversity of Dictyosphaerium morphotype algae within the Parachlorella clade, based on 42 strains from China. We used combined analyses of morphology and molecular data based on SSU and internal transcribed spacer region (ITS) rDNA sequences to characterize these algae. In addition, the secondary structure of ITS2 was compared to delineate new lineages. Our results revealed high phylogenic diversity of Dictyosphaerium morphotype algae, and we describe five distinct lineages. We examined the morphological features of these five lineages, and morphological differences are difficult to find compared with other Dictyosphaerium morphotype algae. The five distinct lineages were not described as new genera currently. We lastly discuss the taxonomic problems regarding the Dictyosphaerium morphotype within the Parachlorella clade, and possible solutions are considered.     

Diversity and characterization of bacterial communities of five co‐occurring species at a hydrothermal vent on the Tonga Arc

Host–symbiont relationships in hydrothermal vent ecosystems, supported by chemoautotrophic bacteria as primary producers, have been extensively studied. However, the process by which densely populated co‐occurring invertebrate hosts form symbiotic relationships with bacterial symbionts remains unclear. Here, we analyzed gill‐associated symbiotic bacteria (gill symbionts) of five co‐occurring hosts, three mollusks (“Bathymodiolus” manusensis, B. brevior, and Alviniconcha strummeri) and two crustaceans (Rimicaris variabilis and Austinograea alayseae), collected together at a single vent site in the Tonga Arc. We observed both different compositions of gill symbionts and the presence of unshared operational taxonomic units (OTUs). In addition, the total number of OTUs was greater for crustacean hosts than for mollusks. The phylogenetic relationship trees of gill symbionts suggest that γ‐proteobacterial gill symbionts have coevolved with their hosts toward reinforcement of host specificity, while campylobacterial Sulfurovum species found across various hosts and habitats are opportunistic associates. Our results confirm that gill symbiont communities differ among co‐occurring vent invertebrates and indicate that hosts are closely related with their gill symbiont communities. Considering the given resources available at a single site, differentiation of gill symbionts seems to be a useful strategy for obtaining nutrition and energy while avoiding competition among both hosts and gill symbionts.     

Sulfur-oxidizing bacterial endosymbionts: analysis of phylogeny and specificity by 16S rRNA sequences.

The 16S rRNAs from the bacterial endosymbionts of six marine invertebrates from diverse environments were isolated and partially sequenced. These symbionts included the trophosome symbiont of Riftia pachyptila, the gill symbionts of Calyptogena magnifica and Bathymodiolus thermophilus (from deep-sea hydrothermal vents), and the gill symbionts of Lucinoma annulata, Lucinoma aequizonata, and Codakia orbicularis (from relatively shallow coastal environments). Only one type of bacterial 16S rRNA was detected in each symbiosis. Using nucleotide sequence comparisons, we showed that each of the bacterial symbionts is distinct from the others and that all fall within a limited domain of the gamma subdivision of the purple bacteria (one of the major eubacterial divisions previously defined by 16S rRNA analysis [C. R. Woese, Microbiol. Rev. 51: 221-271, 1987]). Two host specimens were analyzed in five of the symbioses; in each case, identical bacterial rRNA sequences were obtained from conspecific host specimens. These data indicate that the symbioses examined are species specific and that the symbiont species are unique to and invariant within their respective host species.     

Giant alatoform bivalves in the Upper Triassic of western North America

ABSTRACT. Large, alatoform bivalves, Wallowaconcha raylenea gen. et sp. nov., from the Upper Triassic of north-eastern Oregon, are described and placed in a new family, Wallowaconchidae, within the Megalodontoidea, which also contains the families Megalodontidae and Dicerocardiidae (herein transferred). Major character innovations of wallowaconchids are the internal partitioning of wings and development of non-articulating thin vanes on the hingeplate. The wallowaconchid hinge, which changed during ontogeny, differs greatly from the hinge of other bivalves. Wallowaconchids probably evolved from a species of the megalodontid Triadomegalodon . Although wallowaconchids are homeomorphs of the alate Permian alatoconchid bivalves, hinge structure shows that these two groups are unrelated.
Large wings on wallowaconchids were used for snowshoe support. They may have utilized elaborate vanes and wing chambers to culture microbial symbionts, either microalgae or bacteria. These bivalves are endemic to displaced island arc terranes in western North America, occurring in Yukon, Canada (Stikine terrane), Oregon, USA (Wallowa terrane), and Sonora, Mexico (Antimonio terrane). They occupied environmental niches similar to those of large megalodontid bivalves of Triassic tropical provinces.