Shell Morphology, Anatomy and Gill Histology of the Deep-Sea Bivalve Elliptiolucina ingens and Molecular Phylogenetic Reconstruction of the Chemosynthetic Family Lucinidae

Abstract

From 2008 to 2012, twenty-four specimens of the lucinid bivalve Elliptiolucina ingens were collected alive by beam trawl surveys of the T/S Nagasaki-Maru, Faculty of Fisheries, Nagasaki University, from a chemosynthetically nourished community west of Amami-Oshima Island, Japan between depths of 601 and 646 m. Based on conchological examination of the new material, the species diagnosis provided in the original description was critically revised with respect to the general shell shape and conditions of the muscle scars, lunule and ligament. The gross anatomy of the mantle characters revealed the presence of a long mantle fusion below the inhalant aperture and the absence of mantle gills and apertural papillae. Histological examination of the gill filaments confirmed the presence of a bacteriocyte containing endosymbiotic bacteria; the granule cells were also found in the inner part of the lateral zone. Phylogenetic reconstruction of the Lucinidae based on two nuclear rRNA gene sequences confirmed the previous assumption that Elliptiolucina belongs to the subfamily Myrteinae. The same dataset, in combination with previously published results, suggested that the Lucinidae is divided into two reciprocal sister clades, one consisting of Lucinidae + Codakiinae, the other of (Leucosphaerinae + Pegophyseminae) (Fimbriinae (Monitilorinae + Myrteinae)).