Bacteriocytes in the mantle cavity of Lurifax vitreus Warén & Bouchet, 2001 (Orbitestellidae): the first case among heterobranch gastropoda

Symbiotic chemoautotrophic (usually sulphuror methaneoxidizing) bacteria housed in vacuoles of so-called bacteriocytes occur frequently in the gills or gut derivatives of molluscs inhabiting hydrothermal vents or cold seeps, or that live in or close to reducing sediments. Such bacteriocytes have been reported in the mantle epithelium of Laevipilina antartica (Tryblidia: Neopilinidae; Haszprunar et al., 1995) and in the gills of representatives of all major clades of the Bivalvia (e.g. Krueger, Dubilier & Cavanaugh, 1996; Distel & Roberts 1997; Duperron 2010; Brissac et al., 2011). Except for several gastropod taxa that inhabit hydrothermal vents or other reducing habitats (reviewed by Sasaki et al., 2010), true bacteriocytes have only been found in the oesophageal glands of the ‘scaly foot gastropod’ (not yet formally named; Neomphalida: Peltospiridae; Goffredi et al., 2005), in a glandular tissue occupying most of the body and mantle haemocoel in lepetellid limpets (Vetigastropoda: Lepetelloidea; cf. Judge & Haszprunar, 2014), in the epithelium of the gill of Hirtopelta (Neomphalida: Peltospiridae; cf. Beck, 2002) and in one family of Caenogastropoda (Loxosomatoidea: Provannidae; Stein et al., 1988; Windoffer & Giere, 1997; Urakawa et al., 2005; Suzuki et al., 2005a, b). As part of our studies on various families of basal Heterobranchia (Haszprunar et al., 2011; Hawe, Hes & Haszprunar, 2013), we recently examined individuals of the orbitestellid Lurifax vitreus Waren & Bouchet, 2001, which live in the vicinity of hydrothermal vents. The posterior mantle cavity of this species shows a granular and thickened epithelial structure. Since the histology of these structures does not appear to be glandular and is unusual in its position, we investigated its ultrastructure to see if it contained bacteriocytes. We studied one specimen of Lurifax vitreus (SMNH 43242; Lucky Strike vent field, Mid-Atlantic ridge; 378170N, 0328170W; 1,620–1,720 m deep; 3–9 July 1998), which was fixed in 80% ethanol—suboptimal for electron microscopy studies. This specimen was treated in a descending ethanol series, the shell dissolved in 1% ascorbic acid, dehydrated in an ascending acetone series and embedded in pure Epon A, as described by Hawe & Haszprunar (2013). The plastic block was cut with an MT-7000 ultra-microtome in 70-nm sections at the region of the posterior mantle cavity. The resulting ultrathin sections were stained with uranyl acetate (4% solution) and lead citrate (saturated solution) for 4 min each. Afterwards the sections were examined with a FEI MORGAGNI transmission electron microscope (TEM) at 80 kV. In addition, serial semithin section series of three other orbitestellids, Orbitestella wareni Ponder, 1990 (Shelly Beach, Manly, N.S.W. Australia, “beneath rocks at low tide”; 10 and 17 July 1987, see Ponder, 1990),Microdiscula cf subcanaliculata (E.A. Smith, 1875) andMicrodiscula vanhoeffeni Thiele, 1912 were examined with light microscopy. The investigated ‘glandular’ epithelium covers the posterior end of the mantle cavity (Fig. 1A), lying above parts of the gonoduct, oesophagus, stomach and rectum, but not of the kidney. The epithelium is composed entirely of bacteriocytes that have many mitochondria in the distal regions and central or basal nuclei. Several large vacuoles per cell contain the bacteria (Fig. 2). However, due to the osmotic stress during collection and/or because of poor fixation (in 80% ethanol), the surface of the epithelium is badly damaged—therefore further characterization of the bacteriocytes (e.g. microvilli, cilia and cell openings into the mantle cavity) cannot be reported (Fig. 2A). Depending on the section plane, the rod-shaped bacteria appear palisade-like (longitudinal sections) or round (crosssections) (Fig. 2B–D). The bacteria are enclosed in smaller compartments within an additional membrane that is not always closed (Fig. 2C, D). Two morphotypes of bacteria can be distinguished (Fig. 2C, D): (I) single rod-shaped ones with a length of 1 mm and a width of 0.25 mm; and (II) thicker and rounder bacteria (maximum length 1 mm and maximum width 0.5 mm). Type I have thick membranes and are filled with heavily contrasted material showing no or only small amounts of cytoplasm. Bacteria of type II have even thicker membranes and a single, heavily electron-dense dot and more cytoplasm. The structure of these bacteria is homogeneous throughout the epithelium. Different stages of cleavage can be found in both bacterial types (Fig. 2B). Bacteriophagous vacuoles could not be detected, likely due to poor fixation. Bacteria were found in various stages of degeneration (Fig. 2D), but the location and structure of these stages did not show any regular pattern.