Diversification of chemosymbiotic bivalves: origins and relationships of deeper water Lucinidae

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 2500m. New molecular analyses including species from habitats down to 2000m 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 200m. Most Myrteinae inhabit water depths below 100m, 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 200m. 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.