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A new species of Lucinoma (Bivalvia: Lucinoidea) from the Oxygen Minimum Zone of the Oman Margin, Arabian Sea.
Abstract
A new species of Lucinoma is described from the upper bathyal zone of the Oman margin, Arabian Sea, living in a low oxygen environment. Shell morphology is conservative within the genus and similar species are found in many oceans. Comparisons are made with all known species but especially with L. bengalensis and L. yoshidai. The anatomy is described and as with other members of the genus the Oman species hosts chemosymbiotic bacteria. No specific adaptations to the low oxygen environment were found, species from cold seep and normal marine sediments being morphologically similar.
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- ... On these grounds and because of the almost uniform very large shell size, we separate the Taiwanese material and describe it as new. Lucinoma gagei Oliver & Holmes, 2006 has a similar outline but does not attain the size of L. taiwanensis and has a much smaller anterior adductor scar in relation to shell size. Another similar species is Lucinoma annulata (Reeve, 1850), as figured by Reeve (1850), Okutani et al. (1988) and Okutani (2000), which was synonymized with Lucina acutilineata Conrad, 1849 by Okutani & Hashimoto (1997). ...... The antero-dorsal margin is divided into two parts with a well marked corner between them. Lucinoma gagei Oliver & Holmes, 2006 from the Oman margin, Arabian Sea, is higher and has a smaller anterior adductor scar. ...... Nevertheless, the other features such as well developed, bifid cardinals and small laterals and a very long and narrow diverging part of the anterior adductor scar place it unambiguously in Lucinoma. The only other species with a similar outline is the Gulf of Cali- fornia species L. heroica (Dall, 1901), figured by Oliver & Holmes (2006). Lucinoma heroica is larger and the posterior area is more pronounced. ...
- ... Both studies sampled from the outer shelf to abyssal depths of 3000m, the latter well below the influence of the OMZ and the stations at this depth may well reflect the overall abyssal fauna of the Indian Ocean. The bivalves collected during the 1994 Arabian Sea survey included some novel species such as Amygdalum anoxicolum Oliver and Lucinoma gagei Oliver & Holmes, both associated with the OMZ (Oliver, 2000;Oliver & Holmes, 2006). The bivalves showed a strong bathymetric zonation in relation to dissolved oxygen concentrations. ...... Distribution of newly described species from Discovery cruise 211 over the OMZ of the Oman Margin in relation to depth and dissolved oxygen concentrations. Compiled fromOliver, 2000Oliver, , 2015Oliver & Holmes, 2006 andOliver & Taylor, 2012 ...The perceived taxonomic uniformity of the Indo-Pacific province is discussed with special reference to the marine faunas of the Indian subcontinent and western Indian Ocean. The deep-sea fauna remains largely unexplored but the Arabian Sea upwelling and related oxygen minimum zones create a strong bathyal zonation and high degree of endemism in the Arabian Sea. The extent of this along the west coast of India remains to be evaluated. The bathyal fauna of the Bay of Bengal exhibits similar zonation and endemicity but the comparisons with the Arabian Sea are few. The abyssal fauna of the Indian Ocean is poorly known but the composition is similar to that of the better known deep Atlantic. Some species appear to be cosmopolitan. The shelf faunas are shown not to be uniform with a mixture of wide-ranging Indo-Pacific species and narrow ranging species to specific subregions. Transition zones are postulated for the coasts of the western Indian Ocean. Oceanographic conditions are discussed as possible mechanisms for isolation and development of regional faunas. Taxonomic congruence and consistency are discussed as factors affecting the interpretation of morphological differences between species and their subsequent role in identifying endemicity. At a finer scale the bivalve faunas of the brackish waters around and adjacent to the Indian subcontinent are reviewed. The endemicity of geographically isolated estuaries, lagoons and backwaters is illustrated and further research is indicated. For each section future directions for malacological research are suggested with that of the molecular identity of morpho-species, reinterpretation of endemicity and exploration of the deep-sea as priorities.
- ... The genus has a latitudinal range from 70° N to 55° S and from the intertidal zone to deeper than 2500 m (summary figure in Taylor and Glover, 2010, fig 5.9). The majority of Lucinoma species are found from >200 m to mid-bathyal depths and are often abundant at hydrocarbon seeps, mud volcanoes and oxygen minimum zones (Cary et al., 1989;Okutani and Hashimoto, 1997;Powell, 1997, 2000;Salas and Woodside, 2002; Olu-Le Roy et al., 2004;Holmes et al., 2005;Cosel, 2006;Cosel and Bouchet, 2008;Oliver and Holmes, 2006;Duperron et al., 2007;Oliver et al., 2012;Zamorano and Hendrickx, 2012). Around 40% of Lucinoma species have been described within the last 15 years suggesting that diversity has not yet been fully sampled. ...... From the eastern Mediterranean, Lucinoma kazani was described from 1700 m deep mud volcanoes (Salas and Woodside, 2002;Olu-Roy et al., 2004) and L. asaphaeus from mud volcanoes in the Strait of Cadiz (Oliver et al., 2012). Additionally two species of Lucinoma are known from sediments in oxygen minimum zones; L. aequizonata from 400 -650 m off southern California (Cary et al., 1989;Zamorano and Hendrickx, 2012;Hendrickx et al., 2016) and Lucinoma gagei from southern Oman at 675-967 m (Oliver and Holmes, 2006). Lucinoma aequizonata (Dall, 1901) has a remarkable tolerance of anoxia being able to survive 262 days without oxygen (Arndt-Sullivan et al., 2008). ...A new species of the lucinid bivalve genus Lucinoma is described from shells dredged at depths of 240-500 m from the edge of the continental shelf off southern Newfoundland. It differs from the other northern species', Lucinoma filosa, in shape, ligament, and characters of the anterior adductor muscle scar. It also differs from the poorly known Lucinoma atlantiv from the outer shelf off Maryland that is longer than high and has both anterior and posterior sulci, and from L. blakeana from deep water off North Carolina, a smaller species with a truncate posterior margin. Other Lucinoma species are recorded further south in the northern Gulf of México, particularly from hydrocarbon seeps, although the taxonomv is confused for those taxa.
- ... Among the two, L. borealis is more similar to Lucinoma persolida because of its similar outline and its straight anterior adductor muscle scar that stays close to the pallial line. The extant L. gagei Oliver and Holmes, 2006 from the Oman margin differs from L. persolida by its narrower anterior adductor muscle scar and its much more distinct posterodorsal groove. ...M. 2018. Chemosymbiotic bivalves from the late Pliocene Stirone River hydrocarbon seep complex in northern Italy. Acta Palaeontologica Polonica 63: xxx-xxx. Seven species of chemosymbiotic bivalves are described from the late Pliocene Stirone River hydrocarbon seep complex in northern Italy, including one new species and two in open nomenclature. The known species are the solemyid Acharax doderleini, the lucinids Lucinoma persolida and Megaxinus ellipticus, and the vesicomyid Isorropodon aff. perplexum; in open nomenclature we report two lucinids, including the largest species of Lucinoma known from the Italian Pliocene to date, and a strongly inflated, large Anodontia sp. The most abundant species at the Stirone seep complex is the lucinid Megaxinus stironensis sp. nov. This Pliocene seep fauna differs from that of the well-known Miocene "Calcari a Lucina" seep deposits by lacking large bathymodiolin mussels and vesicomyid clams; instead, the dominance of the lucinid Megaxinus stironensis gives this fauna a unique character. We speculate that at the Stirone seep complex, Megaxinus had occupied the ecological niche that Meganodontia occupied at the Miocene "Calcari a Lucina" seep sites in the Mediterranean basin, and that the dominance of Megaxinus could be a widespread feature of Pliocene chemosyn-thesis-based ecosystems in Mediterranean Pliocene.
- ... This may reflect a sampling deficiency, for lucinids are often deeply buried in the sediment and not frequently captured with gear from ROVs. It seems likely that significant populations of deeper water lucinids are present only at locations with some organic enrichment, including sites of accumulation of sunken vegetation, for example around Indonesia and Philippines (see vonCosel and Bouchet 2008), oxygen minimum zones (Cary et al. 1989;Oliver and Holmes 2006b), hydrocarbon seeps and mud volcanoes. ...
- ... Lucinidae occupy a great range of habitats, from intertidal mangroves, seagrass beds, shallow suboxic sediments to bathyal depths including hydrocarbon seeps, but they are particularly rich in coral reef environments of the Indo-West Pacific and western Atlantic (Glover & Taylor, 2007, in press; Mikkelsen & Bieler, 2008). The discovery of the chemosymbiotic life style of lucinids triggered a surge in taxonomic activity in the previously neglected family and many new species and genera from the Indo-West Pacific have been described over the last 20 years (Glover & Taylor, 2001; Bouchet & Cosel, 2004; Cosel, 2006; Oliver & Holmes, 2006; Glover & Taylor, 2007; Cosel & Bouchet 2008; Okutani, 2011; Glover & Taylor, 2016). Molecular analyses have concurrently developed a radical new classification and phylogeny of the family and enhanced taxonomic discrimination based on traditional shell characters (Taylor et al., 2011Taylor et al., , 2014). ...A survey of the lucinid bivalves of Singapore recorded 18 species, 12 of these located during the Singapore Strait Biodiversity Workshop, two others previously collected from the Straits of Johor and a further four species identified from museum specimens. These are illustrated and briefly described. In 2013 survey, lucinids were uncommon at most locations but a seagrass bed at the southern end of the artificially constructed beach joining Seringat and Lazarus islands, yielded 9 species of lucinids and numerous other infaunal bivalves. By far the most abundant species was the small Pillucina profusa, with fewer numbers of Euanodontia ovum, Cardiolucina macassari, Cavatidens bullula, Leucosphaera philippinensis and Liralucina lyngei. The associated infaunal bivalves included 36 species from 10 families; the most diverse were Tellinidae with 14 species and these are illustrated to assist local identification. Three lucinids, Austriella corrugata, Indoaustriella dalli, and Pegophysema philippiana occurred amongst mangroves in the Strait of Johor. Species for which only museum records are available, such as Codakia paytenorum and Lepidolucina venusta, may be locally extinct. Five Singapore lucinid species were included in a new molecular analysis (18S, 28S and cytochrome b genes) to establish their phylogenetic relationships and anatomical details including, in particular, ctenidial structure and morphology of bacterial symbionts for Eunanodontia ovum and Pillucina profusa.
- ... , 2004 ; Holmes et al . , 2005 ; Cosel , 2006 ; Oliver & Holmes , 2006 ; Duperron et al . , 2007 ; Cosel & Bouchet , 2008 ; Oliver et al . ...Although species of the chemosymbiotic bivalve family Lucinidae are often diverse and abundant in shallow water habitats such as seagrass beds, new discoveries show that the family is equally speciose at slope and bathyal depths, particularly in the tropics, with records down to 2500 m. New molecular analyses including species from habitats down to 2000 m indicate that these cluster in four of seven recognized subfamilies: Leucosphaerinae, Myrteinae, Codakiinae, and Lucininae, with none of these comprising exclusively deep-water species. Amongst the Leucosphaerinae, Alucinoma, Epidulcina, Dulcina, and Myrtina live mainly at depths greater than 200 m. Most Myrteinae inhabit water depths below 100 m, including Myrtea, Notomyrtea, Gloverina, and Elliptiolucina species. In the Codakinae, only the Lucinoma clade live in deep water; Codakia and Ctena clades are largely restricted to shallow water. Lucininae are the most speciose of the subfamilies but only four species analyzed, Troendleina sp., ‘Epicodakia’ falkandica, Bathyaustriella thionipta, and Cardiolucina quadrata, occur at depths greater than 200 m. Our results indicate that slope and bathyal lucinids have several and independent originations from different clades with a notable increased diversity in Leucosphaerinae and Myrteinae. Some of the deep-water lucinids (e.g. Elliptiolucina, Dulcina, and Gloverina) have morphologies not seen in shallow water species, strongly suggesting speciation and radiation in these environments. By contrast, C. quadrata clusters with a group of shallow water congenors. Although not well investigated, offshore lucinids are usually found at sites of organic enrichment, including sunken vegetation, oxygen minimum zones, hydrocarbon seeps, and sedimented hydrothermal vents. The association of lucinids with hydrocarbon seeps is better understood and has been traced in the fossil record to the late Jurassic with successions of genera recognized; Lucinoma species are particularly prominent from the Oligocene to present day. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, ●●, ●●–●●.
- ... Over the last 20 years there has been much taxonomic interest in the chemosymbiotic bivalve family Lucinidae with many new genera and species described from shallow to bathyal depths of the tropical Indo-West Pacific (Taylor and Glover 1997, 2002, 2005; Glover and Taylor 2001, 2007, 2008; Glover et al. 2008; Oliver and Holmes 2006; Bouchet and Cosel 2004, Cosel and Bouchet 2008, Okutani 2011, Glover and Taylor in press). Despite this activity there remain many undescribed or unregarded species whose systematic position is obscure. ...Four new species and a new genus of lucinid bivalves are described from shallow and deeper waters in the Indian and West Pacific Oceans. The new genus Scabrilucina (subfamily Lucininae) includes the little-known Scabrilucina victorialis (Melvill, 1899) from the Arabian Sea and Scabrilucina vitrea (Deshayes, 1844) from the Andaman Sea as well as a new species Scabrilucina melvilli from the Torres Strait off northeastern Australia. Ferrocina brunei new species (Lucininae) was recovered from 60 m near oil drilling activities off Borneo; its anatomy confirmed the presence of symbiotic bacteria. Two unusual deeper water species of Leucosphaerinae are described, both species included in on-going molecular analyses; Gonimyrtea ferruginea from 400-650 m in the southwest Pacific and Myrtina reflexa from 200-825 m off Zanzibar and Madagascar.
- ... Typically lucinids live at the boundary between oxic and suboxic zones in the sediment from where sulphiderich interstitial water is channeled into the mantle cavity by probing and tunneling activities of the foot (Stanley, 1970;Dando et al., 1986Dando et al., , 1994). Lucinids are often associated with organic-rich habitats such as seagrass beds (Jackson, 1972;Barnes & Hickman, 1999;Johnson et al., 2002), mangrove fringes (Lebata & Primavera, 2001;Glover et al., 2008), and hydrocarbon rich sites such as cold seeps (Oliver & Holmes, 2006), but many others inhabit intertidal and subtidal, unvegetated sand and mud. Although the biology of a few lucinid species, notably from the northern Atlantic, is known in some detail (e.g.,Fisher & Hand, 1984;Frenkiel & Mouëza, 1995;Frenkiel et al., 1996;Johnson & Fernandez, 2001;Gros et al., 2003), for the tropical Indo-West Pacifi c, most species remain unstudied and lack even basic information of distributional range, habitat preferences or estimates of abundance.Johnson et al. (2002)highlighted the importance of chemoautotrophic symbioses in estimates of the primary production of seagrass beds, but for most areas, information is lacking. ...A quantitative survey of molluscs inhabiting a disturbed, intertidal muddy-sand fl at near mangroves in a sheltered bay in southeastern Thailand recorded a high abundance (to 1,380 m -2 ) of the small, chemosymbiotic lucinid bivalves Pillucina vietnamica and Indoaustriella dalli. Two other larger lucinids, Anodontia bullula and A. philippiana, were recorded living near the transects, whereas another, Ctena delicatula, inhabited a cobble habitat on a rocky headland at the entrance to the bay. The presence of symbiotic bacteria in the ctenidial fi laments was confi rmed for each species. Most abundant of the associated molluscan fauna were the epifaunal gastropods Cerithideopsilla cingulata, Cerithium coralium and Clithon oualaniensis, and the infaunal bivalves Gafrarium tumidum, Anomalocardia squamosa, Pristis capsoides and Anadara troscheli. Biomass estimates for the lucinids ranged from 0.51 to 3.95 gm -2 . The results highlight the ecological importance of chemoautotrophic bivalves in peri-mangrove communities.
- Seven species of Thyasiridae are reported from the Oman Margin of the Arabian Sea at depths between 688 m and 3356m. Hypoxic conditions exist at depths between 400 and 1200 m and three species are restricted to this zone and to the Arabian Sea. Leptaxinus indusarium has also been recorded from the Indus Fan and Channelaxinus investigators from off Sri Lanka. A new species Thyasira anassa sp. nov. is described from the hypoxic zone. Another four species are recorded from the abyssal zone where oxygen levels are typical for the deep ocean. Here another new species is described, Parathyasira bamberi sp. nov. but the other species could not be conclusively identified because of close affinity with populations from other oceans. Deep water Atlantic species Axinulus croulinensis and Mendicula ferruginosa are apparently present in the abyssal Indian Ocean while another thyasirid shell is very close to Channelaxinus excavatus from the Eastern Pacific and C. perplicata from the Atlantic. Accompanying these abyssal thyasirids were other bivalve species, Deminucula atacellana, Limopsis pelagica and Bentharca asperula that cannot be distinguished by morphology from their Atlantic populations. It is concluded that using morphology alone that the abyssal species may well be cosmopolitan in distribution.
- Chemoautotrophically-based benthic communities on the Louisiana continental slope are currently producing the only significant localized, autochthonous shell accumulations in the northern Gulf of Mexico shelf and slope region. Five distinctive biofacies are associated with petroleum seepage, dominated respectively by vestimentiferan tubeworms, lucinid, thyasirid and vesicomyid clams and mytilid mussels. The taphonomy of petroleum seeps includes dissolution as the most pervasive mode of shell alteration throughout all the biofacies. The degree of fragmentation is high and is likely caused by biological breakage and extreme dissolution. Characteristics common to seep and many ancient autochthonous assemblages include: (1) low species richness; (2) high density; (3) size frequency dominated by large individuals; (4) variable articulation frequency. The community taphonomic attributes of petroleum seep death assemblages are very similar to ancient autochthonous benthic assemblages. -from Authors
- The lucinid bivalve Lucina pensylvanica possesses an unusual flexible commarginal shell sculpture formed from calcified periostracal lamellae. The lamellae comprise thick, recurved, periostracal extensions with distal calcified scales. The periostracum is also densely embedded with calcareous granules around 2.0-2.5 μm in diameter and a thin (10 μm) layer of prismatic aragonite covers the ventral face of each lamella. Other species of Lucina in the western Atlantic possess calcified scales but with different morphologies and the continuous commarginal ridges of the eastern Atlantic Lucina adansoni and other African species are similarly constructed and homologous. The periostracal lamellae are a probable apomorphy of the genus Lucina and morphology of the calcified structures provides a set of systematic characters of importance in the discrimination of species.
- The Lucinacea (Eulamellibranchia) have been studied and particular attention has been paid to the feeding, respiratory and cleansing currents and their relation to the morphology and habits of the group. The Lucinacea comprise three families, the Ungulinidae, Thyasiridae and Lucinidae, and living specimens of thirteen species were studied; namely, Diplodonta rotundata, D. punctata, D. semi-aspera (Ungulinidae); Thyasira flexuosa (Thyasiridae); Lucina pennsylvanica, L. chrysostoma. Codakia orbicularis, C. orbiculata, C. costata, Loripes lucinalis, Phacoides borealis, Myrtea spinifera, Divaricella quadrisulcata (Lucinidae). These species occur on a variety of substrata ranging from coarse sand to fine mud particularly where the associated fauna is sparse. Shallow-water species are often found among the roots of marine grasses such as Zostera and Thalassia. The mantle is described, notably the extensive glandular tissue of the mantle margin, also the 'mantle gills' and the unique form of the exhalent siphon both of which are confined to the Lucinidae. The great development of the glandular tissue at the mantle margin is probably related to large quantities of sediment entering the mantle cavity. While many of the gland cells close to the epithelium show typical basiphilic reactions of mucus-secreting cells, those further from the epithelium show marked eosinophilic-staining reactions. The only visible difference between the cells appears to be a difference in cytoplasmic granule size, those to the inside possessing the larger granules. 'Mantle gills' consist of convoluted folds of the inner mantle epithelium close to the anterior adductor muscle. They occur in Codakia orbicularis and Lucina pennsylvanica. The folds are parallel to the direction of the ciliary currents. There are no gland cells, but there are many blood vessels below the folds and the latter are probably concerned with respiration. The exhalent siphon in the Lucinidae does not contract into a pallial sinus, but turns inside out and lies in the suprabranchial cavity, a method that appears to be unique. The anterior adductor muscle is elongate and the greater part of it lies ventral and posterior to the mouth. Its surface epithelium is ciliated and acts as a sorting area for particles entering by the anterior inhalent tube. The Ungulinidae are least specialized in this respect and the Lucinidae the most specialized. The variations of the ciliary currents on the body are described. The foot is highly specialized, it is used in burrowing and also forms the anterior inhalent tube. It is typically long and vermiform often with a distinct tip. In the Lucinidae the heel is well developed and may be distinct from the vermiform portion. The latter is chiefly concerned with tube formation and the heel with digging. In some species the foot can extend to more than ten times the length of the shell. The tip of the foot is profusely supplied with glands which supply material for tube formation. An account of this and of burrowing is given. The morphology and ciliary mechanisms of the gills are described. In the Ungulinidae and Thyasiridae the outer demibranch is reduced and in the Lucinidae this demibranch is absent. All the species studied have homorhabdic gills. In the Thyasiridae and Lucinidae development of the subfilamentar tissue gives a fleshy appearance to the gills. The palps show progressive reduction in size in the three families. The Ungulinidae and the Thyasiridae retain the typical ciliated folds. The palps of the Lucinidae are slight enlargements of the walls of the oral groove. Vestiges of not more than three folds remain which are still capable of a limited amount of sorting. The stomach and digestive diverticula are described; the sorting mechanisms are progressively reduced. They are least reduced in the Ungulinidae being essentially the same as those of other Eulamellibranchia. In the Thyasiridae and the Lucinidae there is reduction in the sorting areas of the stomach and the number of ducts of the digestive diverticula. The resulting simplification of the stomach emphasizes the importance of (1) the acceptance tract, (2) the typhlosoles, (3) the left caecum, (4) the dorsal groove, and (5) the tooth on the gastric shield. The digestive diverticula of the Ungulinidae are of the typical eulamellibranch pattern, those of the Lucinidae and Thyasiridae have much larger ducts and tubules, both in the size of the lumen and of the cells. Tubules only are present in the Thyasiridae. The excretory, circulatory and nervous systems do not differ significantly from those of other Eulamellibranchia. The evolution of the group is discussed particularly in relation to the anterior inhalent current and the correlated reduction in sorting mechanisms of the gills, palps and stomach, together with the development of the anterior adductor muscle, foot and exhalent siphon.
- Chemoautotrophic bacteria live symbiotically in gills of Lucinoma aequizonata, an infaunal clam inhabiting an oxygen-poor environment. These intracellular symbionts respire nitrate, i.e. they use nitrate instead of oxygen as a terminal electron acceptor in the respiratory chain. Nitrate is only reduced to nitrite and not further to nitrogen gas. Nitrate is respired by the symbionts under fully aerobic conditions at the same rate as under anaerobic conditions. The bacterial symbionts contain a nitrate reductase that is associated with the membrane-containing fraction of the symbiont cell and that is sensitive to respiratory inhibitors; both features are consistent with the respiratory role of this enzyme. A review of nitrate reductase in chemoautotrophic symbionts suggests that nitrate respiration may be common among these symbioses. Symbiont nitrate reductase may be an ecologically important factor permitting the survival of animal hosts in oxygen-poor environments.
- A population of the lucinid bivalve Lucinorna aequizonata, with sulfur-oxidizing endosym-biotic bacteria in the gills, is restricted to a narrow depth range (500 -f-10m) on the slope of the Santa Barbara Basin, California, USA. In this zone, the seawater just above the substratum is sub-oxic ([ 0 2 ] < 2 0 F M) . The organically rich sediments in which these clams live are well mixed by bioturbation, which appears to maintain a redox condition Limiting the extensive accumulation of hydrogen sulfide. However, thiol levels in the blood of the clams indicate an exposure to significant amounts of sulfide and/or thiosulfate apparently from randomly &spersed short-lived pockets of sulfidic mud that can b e reached by the clam's burrowing vermiform foot. When the bivalves are incubated in the presence of hydrogen sulfide, thiosulfate is concentrated in the blood and apparently utilized by the bacteria for metabohc energy and the production of intracellular elemental sulfur. Laboratory growth experiments demonstrated that sulfide concentrations greater than 10 F M were detrimental to the host, even though the bacteria continued to accumulate elemental sulfur. The utilization of thiosulfate under near anaerobic conditions and the accumulation of intracellular elemental sulfur by the endosymbiotic bacteria coupled with the high availability of environmental nitrate and low molecular oxygen suggests a metabolic strategy analogous to the free-living sulfur oxidizer Thlobacillus denjtrificans. The 6 ' 3 ~ values of the purified endosymbiont bacteria (-34.0 5 0.8%) were significantly lighter than those of the host tissue (-29.0 + 0.7 %) suggesting that in addition to the nutrition provided by the bacteria, over 25 % of the host carbon may b e attributed to exogenous dissolved carbon which is in high concen-trations in its natural habitat.
- A cold-seep invertebrate assemblage consisting mainly of the large bivalve Lucinoma sp. is exposed at the Kuge Shrine and surrounding area in Shintomi Town, Koyu County, Miyazaki Prefecture, Kyushu Island, southwestern Japan. The outcrops with this assemblage are in the upper part of the Upper Pliocene Takanabe Formation of the Miyazaki Group. The Takanabe Formation exposed in Shintomi Town is a cyclical sequence of upward-coarsening and upward-fining sediments, which reflects sea-level change: the paleobathymetry inferred from molluscs implies a shallowing upward from 160 m-200 m to 50 m in water depth, followed by a deepening back to 160 m-200 m. The cold-seep assemblage occurs in the coarsest (muddy sandstone) and bathymetrically shallowest (50 m in water depth) horizon, an outer shelf environment near the average storm-wave base, in this shallowing and deepening cycle. The cold-seep assemblage around the Kuge Shrine occurs sporadically in area of about 100 m in a north-south direction and about 60 m in an east-west direction, and is about 12 m in stratigraphic thickness. Many articulated and disarticulated bivalves are preserved with their commissure planes parallel to bedding, but some articulated bivalves are oriented normal to it, in the characteristic life position of lucinids. The muddy sandstone containing this assemblage is cemented strongly to weakly by authigenic carbonate depleted greatly in 13C (δ13C = -30.69‰ to -53.21‰ vs. PDB), which suggests the influence of methane seepage. The preserved life positions of the lucinid bivalves and the carbon isotope ratios of the associated carbonates suggest that this assemblage is autochthonous. The bedding-parallel orientation of many shells is inferred to have resulted from in-situ reworking by storm-induced waves and currents that are prevalent in such a shallow environment.
- A cold-seep assemblage is present in slope to outer-shelf strata of the Upper Pliocene to Lower Pleistocene Ofuna Formation and the Lower Pleistocene Koshiba Formation of the Kazusa Group in Yokohama City, on the Pacific side of central Japan. Four cores were taken in order to learn the three-dimensional extent of the assemblage: one core was parallel to bedding and three were normal to bedding. Study of the cores and outcrops shows that: (1) the assemblage is at least 37 m thick stratigraphically and extends across the boundary between the Koshiba Formation and the underlying Ofuna Formation; (2) parallel to bedding, the assemblage is at least 16 m wide in a north-south direction and 30 m long in an east-west direction; (3) the assemblage consists mostly of large (up to 10 cm in maximum diameter), articulated bivalves of the genera Lucinoma, Conchocele, and Acharax, in aggregations or as sporadic shells; (4) authigenic carbonate cement in the matrix enclosing the assemblage ranges from dense to sparse and is greatly depleted in 13C (δ13C = -47.99‰ to -55.06‰ , indicating that the carbonate resulted from the microbial oxidation of methane; (5) there is a subsurface horizon about 10 m thick that is characterized by: (a) scoria beds in which the matrix consists entirely of pure, white carbonate, (b) abundant brecciated clasts of the host sediment occurring along with extremely abundant bivalve fragments, many of which are very small, and (c) lithologic boundaries greatly discordant with the overall dip within the study area. Observations 5a-c together suggest very active seepage and/or an explosive effusion of subsurface material, and the presence of "pockmarks" in the subsurface of the study area. Twenty tuff beds are observable and accurately correlatable among outcrops where no cold-seep assemblage occurs. However, these tuff beds cannot be correlated with those in the cores. This is probably due to highly active bioturbation and by severe seepage and/or the explosive effusion of seep materials. We infer that these activities and events dispersed, disturbed, brecciated and/or reconcentrated the originally deposited tuff beds.
- All Lucinidae species studied so far possess sulphide-oxidizing. chemosymbiotic bacteria housed in bacteriocytes of gill filaments. The ecology, functional anatomy and evolution of the Lucinidae must be considered in relation to this symbiosis. The ctenidia have been extensively studied but other anatomical structures peculiar to lucinids have received much less attention. Reviewed are the morphological diversity of living lucinids, highlighting features of their anatomy including ctenidia, pallial apertures, anterior adductor muscle, pallial blood vessel and mantle gills. The latter are much more complex than previously understood and are here redescribed, They comprise Folded structures located near the anterior adductor muscle in Codakia, Phacoides and Lucina, and on the septum of Anodontin. These are interpreted as secondary respiratory surfaces, their location enabling the separation of the anterior inflow of oxygenated water from sulphide-containing water. The latter is released from the sediment by the probing activities of the highly extensible foot and is pumped over the gill through the pedal gape and perhaps also via the exhalant tube. The shell Features of Ilionia from the Silurian Period suggests that the lucinid chemosymbiosis is an ancient association.
