The chemosymbiotic bivalves collected from the mud volcanoes of the Gulf of Cadiz are reviewed. Of the thirteen species closely associated with chemosynthetic settings two Solemyidae, Solemya (Petrasma) elarraichensis sp. n. and Acharax gadirae sp. n., one Lucinidae, Lucinoma asapheus sp. n., and one Vesicomyidae, Isorropodon megadesmus sp. n. are described and compared to close relatives of their respective families. The biodiversity and distribution of the chemosymbiotic bivalves in the Gulf of Cadiz are discussed and compared to the available information from other cold seeps in the Eastern Atlantic and Mediterranean. Although there is considerable similarity at the genus level between seep/mud volcano fields in the Eastern Atlantic and Mediterranean, there is little overlap at the species level. This indicates a high degree of endemism within chemosymbiotic bivalve assemblages.

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Species of two genera, Bathyarca Kobelt (1891) and Bentharca Verrill and Bush (1898), occur beyond the shelf-slope break of the continental margins of the Atlantic. Little is known of these species and descriptions of the morphology of Bathyarca glacialis, B. pectunculoides, B. inaequisculpta, B. corpulenta, Bentharca nodulosa and B. asperula are given. The species of Bathyarca show a progression in form from an epibyssate nestling habit to an infaunal endobyssate habit. Correlated with this are various morphological changes. Thus the byssus, which is short and stout and controlled by strong retractor muscles in the epibyssate B. glacialis , is long and slender with weak associated musculature in endobyssate B. inaequisculpta . The mantle/shell form is increasingly heteromyarian in character in deeper-living species, with the increasing development of mantle flaps and muscles of the gill axis. In contrast, the species of Bentharca have largely retained their epibyssate habit. Their morphology is conservative and differs little from shallow-water arcids. Features are found in both genera that are related to the small quantity of suspended food present in the deep sea. These include the lack of glands in mantle tissue, the reduced palps and lack of sorting areas in the stomach. In Bathyarca corpulenta alone the (body biomass): (shell volume) ratio is reduced.

[1] Hydrothermal circulation at ultramafic-hosted sites supports a large variety of high-and low-temperature hydrothermal vents and associated ecosystems. The discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest, approximately 2.5 km east of the active Rainbow vent field on the Mid-Atlantic Ridge (MAR, 36°13′N), increased our knowledge regarding the diversity of vent communities at slow spreading ridges. Bivalve molluscs of the family Vesicomyidae were represented by the genus Phreagena. Here we present the first record of this genus in the Atlantic Ocean. This second vesicomyid species known from the MAR, Phreagena sp., was found to be associated with a Thyasira species that is affiliated with T. southwardae (at the Logatchev vent field on the MAR) and with T. vulcolutre (in the Gulf of Cadiz). These two clams have close relationships with seep taxa along the continental margin, and were likely associated with sedimented vent fields. d 18 O and d 13 C analyses of the shells suggested that the burrowing bivalve Thyasira could incorporate isotopically light carbon, derived from the oxidation of methane in the sediment, while the signature of Phreagena sp. shells denoted a different carbonate source. 14 C dating of the shells denoted that the hydrothermal activity in the Rainbow area began at least ∼25.5 kyr BP, which is similar to the model of the hydrothermal vent field distribution that was proposed for the Logatchev hydrothermal site. The results provide new insight regarding the diversity of chemosynthetic fauna on the MAR over geologic time. Ultramafic-hosted, on-axis sedimented vent fields extend the range of habitats for chemosynthetic communities, underlying the need to further explore the geology of these types of environments on slow-spreading ridges and to determine their role in the ecology of deep-sea vent communities.

The functional and adaptive morphology of six species of the genus Limopsis from deep water of the Atlantic is investigated and the following species and subspecies are described: L. aurita, L. surinamensis (new species), L. tenella, L. minuta, L. cristata cristata, L. cristata affinis, L. cristata intermedia (new subspeices), L. cristata lanceolata (new subspecies), L. galathea . Species of the genus Limopsis are among the relatively few suspension-feeding bivalves of the deep sea. The genus is not represented in shelf faunas. It is byssate, but the species described here show a change in habit from the exobyssate to the endobyssate condition and a corresponding reduction in the number and coarseness of the byssus threads produced. While the morphology of the species is similar in all the species described, differences being restricted to minor variation in the gills and palps, the viscera are confined to a small dorsal portion of the mantle cavity. Thus, while small body size is related to the paucity of food, production of a large thick shell may require a relatively small proportion of the energy input and yet confer advantage possibly in protection and in feeding mechanism. Eggs are relatively few and of a size indicating that the larvae are planktonic but non-feeding. The widely occurring species L. tenella shows differences in the shell characters of different populations. In L. cristata variation is such that it has been possible to describe subspecies.