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A new species of Lucinoma (Bivalvia: Lucinoidea) from the Oxygen Minimum Zone of the Oman Margin, Arabian Sea.
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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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... Lucinids occur at a wide range of latitudes (60°N-55°S) and at depths to around 2100 metres but they are most diverse and abundant in tropical, shallow-water habitats (Glover & Taylor 2007). Although some deeper water lucinids, particularly species of Lucinoma, have been known for a long time, new explorations of deep-sea habitats, in particular cold seeps and oxygen minimum zones, are revealing many new lucinid species (Okutani & Hashimoto 1997;Salas & Woodside 2002;Bouchet & Cosel 2004;Holmes, Oliver & Sellanes 2005;Cosel 2006;Oliver & Holmes 2006;Cosel & Bouchet in press). As yet, only a single lucinid, Bathyaustriella thionipta, has been recorded from a hydrothermal vent (Glover, Taylor & Rowden 2004). ...
... Elsewhere, Lucinoma species have been the most frequently recorded Lucinidae at cold seeps (Hashimoto et al. 1995, Salas & Woodside 2002, Holmes et al. 2005, Cosel, 2006, Oliver & Holmes 2006, although the "Lucinoma spectabilis" recorded by Hashimoto et al. (1995) was later described as Mesolinga soliditesta Okutani & Hashimoto, 1997. Other seep-associated lucinids include Graecina karinae, which occurs with Lucinoma myriamae Cosel, 2006 at seeps off West Africa, while Myrtea amorpha (Sturany, 1896) is recorded from mud volcanoes in the eastern Mediterranean (Olu-LeRoy et al. 2004), and Meganodontia acetabulum Bouchet & Cosel, 2004 occurs with Lucinoma at a likely hydrocarbon seep off Taiwan. ...
... data, pers. comm.), oxygen minimum zones (Cary et al. 1989, Levin et al. 2000, Oliver & Holmes 2006, hydrocarbon seeps and mud volcanoes. ...
... Thick hinge plates similar to that of 'C. karpatic'a is also similar to the ones of Nymphalucina (latest Cretaceous-Paleocene) from shallow water hydrocarbon seeps (Kiel, 2013), and Lucinoma (Oligocene-Recent) broadly distributed in shallow to deep water high-redox potential and low oxygen environments, especially in temperate water sandy and muddy bottoms (Dando et al., 1986;Salas and Woodside, 2002;Oliver and Holmes, 2006;Kiel, 2010;Taviani, 2017, 2018). We therefore argue that 'Cyprina karpatica' is a deep water lucinid. ...
... Among some of the means used to withstand low-oxygen conditions are nitrate-based respiration of the symbionts (Hentschel et al., 1993), and haemoglobins aiding oxygen transport (e.g. Oliver and Holmes, 2006); the latter is not confined to bivalves from lowoxygen environments and is in fact common in many chemosymbiotic bivalve species apart from bathymodiolin mussels (Childress and Girguis 2011). Perhaps the best documented examples of chemosymbiotic bivalves known from low-oxygen settings are lucinid species living within oxygen-minimum zones, such as Lucinoma heroica from off the coast of Mexico, where hypoxic conditions are present throughout the year (Zamorano and Hendrickx, 2012). ...
The paper reports rare occurrence of deep-water solemyid and lucinid bivalves from Cenozoic sequences of Outer Eastern Carpathians, Ukraine. The studied bivalves occur in the Eocene Pasichna Formation, represented by deep-water marls and pelagic limestones intercalated with thin-bedded calcareous sandstones yielding material re-deposited from shallow water. This find offered a chance to study the unaltered shell breakage pattern of these deep-burrowing solemyid bivalves buried in situ by turbidites. The breakage includes arcuate breaks representing likely syn-vivo damage related to active burrowing of the animal while telescopic breaks are most likely post-mortem distortions produced by the load of the heavy sediment covering sea bottom after a turbiditic event. Taxonomic study indicates that both solemyid and lucinid bivalves from the Eocene of the Pasichna Formation are most similar to deep-water members of their respective clades, known from roughly coeval deposits from the Tethys. The available evidence indicates that bivalves in question lived on the deep-sea muddy bottom, influenced by turbidite deposits, and were buried by them syn-vivo or early post-mortem. This is in contrast to the great majority of molluskan faunas from the Carpathian Paleogene, reworked during transport downbasin from shallow-marine settings by mass gravity flows. The detailed study of the occurrence mode of deep-water bivalves in the Carpathian part of the Tethys during the Paleogene, influenced by frequent build-up of oxygen-depleted waters, offers implications for the evolutionary histories of both groups in question. For both solemyids and lucinids, adaptations to oxygen-depleted environments are primordial, and are carried from the Silurian until today, as supported by the presented fossil and actualistic evidence. This is in contrast to their histories in high-redox potential environments, with solemyids known from cold seeps from the Devonian onwards and lucinid seep record only from the Jurassic.
... 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, , 2015Holmes, 2006 andTaylor, 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.
... Deep-sea malacofaunal research in the Arabian Sea is infrequent although the fauna has been explored since the late 19th century. Several bivalve species have been described from the bathyal zone of Arabian Sea includes Amygdalum anoxicolum P. G. from the northern Arabian Sea, Lucinoma gagei P. G. Oliver & Holmes, 2006, Nucinella owenensis P. G. Oliver & Taylor, 2012, Huxleyia habooba P. G. Oliver & Taylor, 2012, Thyasira anassa P. G. Oliver, 2015, and Parathyasira bamberi P. G. Oliver, 2015 from the Oman margin, Leptaxinus indusarium P. G. Oliver & Levin, 2006 from off Pakistan, Ascetoaxinus ravichandrani Ravinesh et al., 2024 from off Southern India, etc. The family Mytilidae Rafinesque, 1815 represented by a total of 138 genera and the genus Amygdalum Megerle von Mühlfeld, 1811, comprising about 14 species that are known to have cosmopolitan distribution with a bathymetric range from the sublittoral to the abyssal plain . ...
This paper reports one mytiloid species, Amygdalum anoxicolum P. G. Oliver, 2001 from the summit of a seamount located in the eastern Arabian Sea. This species was encountered from the core of the Arabian Sea oxygen minimum zone at a depth of 340 m and possessed several adaptations to low-oxygen concentration. This paper presents the first-ever DNA barcodes for this species.
... All investigated species of Recent lucinid bivalves possess chemosynthetic sulfur-oxidizing bacteria in their gills (Taylor and Glover, 2000), and dense populations of lucinid bivalves are generally found at cold seeps (Callender and Powell, 1997) and in oxygen-minimum zones (Bottjer et al., 1995;Oliver and Holmes, 2006). Acharax are also known to harbor chemosynthetic bacteria in their gills (Reid, 1990) and are generally present in seeps, vents, whale-falls, and other oxygen-depleted environments (Reid, 1990;Dufour, 2005;Braby et al., 2007). ...
We report a single whale bone associated with many molluscan fossils from the Omma Formation, Lower Pleistocene shallow marine deposits, along the Sai-gawa River, Kanazawa City, in central Japan. Most molluscan species which are commonly found in the Omma Formation show disarticulated and/or damaged shells, indicating semi-autochthonous or allochthonous modes of occurrence. However, the assemblage contained chemosynthetic bivalves, such as lucinid, solemyid and thyasirid bivalves, which are rare in the Omma Formation. The lucinids and solemyids show a high articulation ratio, along with some predatory and scavenging gastropods, such as naticids, nassariids and borsoniids whose well-preserved shells indicate an autochthonous mode of occurrence. In addition, most of the lucinid bivalves show an umbo-upward position similar to the life position of Recent species. Recent lucinid, solemyid and most thyasirid bivalves harbor chemosymbiotic bacteria in their gills and are well known members of the chemosynthetic community. These lines of evidence indicate that the community, mainly comprising lucinid bivalves and other autochthonous molluscan species associated with the whale bone, is an ancient whale-fall community. This shallowest fossil whale-fall community differs from deep-water cases in the dominance of infaunal bivalves, such as lucinids, and in the lack of epifaunal and semi-infaunal chemosynthetic bivalves, such as bathymodiolins and vesicomyids. This community supports a previous suggestion that the difference in characteristic species of the whale-fall communities depends on the water depth.
... 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., 2011(Taylor 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.
From the Philippines at the centre of Indo-West Pacific marine diversity we record 78 species and 42 genera from 7 subfamilies of
the chemosymbiotic bivalve family Lucinidae. Sixty species were identified from over 16,000 lucinid specimens collected in the central
Philippines from the PANGLAO 2004 Biodiversity Project and the PANGLAO 2005 Deep-Sea Cruise. Other species were identified
from samples collected by other expeditions to the area (ESTASE 2, MUSORSTOM 2 & 3 and AURORA 2007) and from existing
museum collections. Twenty-six new species and three new genera, Opalocina, Easmithia and Jallenia are described. Notable are the
five species of Myrtina, and seven Notomyrtea recognised and Pseudolucinisca recorded for the first time outside of South and Western
Australia. Species range in size from 1.5 to 87 mm, with 37 species smaller than 15 mm and 11 less than 5 mm, including some of the
most abundant. Lucinidae have a wide habitat distribution and a total bathymetric range from the intertidal to 2570 m with up to 27 species
occurring between 100-300 m. Deeper water assemblages, dominated by species of Leucosphaerinae, Myrteinae and Cardiolucina,
begin at depths of ca. 100 m with a turnover of species to 1000 m with 3 species recorded from 2570 m. Protoconchs of 53 species were
examined and these demonstrate different strategies of larval development. Deeper water species of Leucosphaerinae, Myrteinae and
Cardiolucina tend to have large P1 with a very narrow PII stage indicating a short non-feeding pre-settlement stage while many shallow
water species in the Lucininae have larger PII stages, often with many growth increments, indicating a longer, feeding planktonic stage.
Microsculpture characters at magnifications to 2000x, examined for the first time, indicate a strong taxonomic signal with Myrteinae
possessing mesh-like pitted structures and Codakiinae linear pitting, while Leucosphaerinae lack these structures. The Philippine lucinid
fauna is by far the most diverse yet recorded demonstrating that chemosymbiosis is a common and widespread nutritional strategy
and not confined to extreme or unusual habitats.
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.
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.
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 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.