Gigantopelta Chen & Linse & Roterman & Copley & Rogers 2015, GEN. NOV.

Abstract

GIGANTOPELTA GEN. NOV. <p> <i>Type species</i></p> <p> <i>Gigantopelta chessoia</i> sp. nov., by original designation.</p> <p> <i>Etymology</i></p> <p>Giganteus (Latin), gigantic; Pelta (Latin), shield. This refers to the extremely large adult shell size of the species in this genus for the family Peltospiridae. The genus name is feminine.</p> <p> <i>Zoobank registration</i> urn:lsid:zoobank.org:act: C25960CA-B974-452C-AE24- B128FF1CEA0F</p> <p> <i>Diagnosis</i></p> <p>Shell extremely large for family, reaching 45 mm in adult shell length. Shell globose, rather loosely coiled with deep suture, three to four whorls. Spire depressed. Protoconch consisting of 0.5 whorls. Aperture very large, circular, expanding rapidly. Thick, dark olive periostracum enveloping edge of aperture. Shell milky white and thin, not nacreous. Columellar folds lacking. Concentric, multispiral operculum present. Foot large. Cephalic tentacles thick, broad, triangular, thinning towards tips. Eyes lacking. Snout tapering and thick. Oesophageal gland hypertrophied. Single, bipectinate ctenidium. Sexes separate. Epipodial tentacles present surrounding operculum. Radula rhipidoglossate, formula ∼50 + 4 + 1 + 4 + ∼50. Central, lateral teeth strong, solid with smooth cusps. Marginal teeth long, slender, truncate, divided into about 20 toothlets towards distal end.</p> <p> <i>Remarks</i></p> <p> Adult <i>Gigantopelta</i> are easily distinguished from all other described peltospirids by their extremely</p> <p> <i>COI</i>, cytochrome c oxidase subunit I</p> <p> large shell size. Furthermore, <i>Gigantopelta</i> can be distinguished from the limpet-like peltospirid genera <i>Ctenopelta</i> Warén & Bouchet, 1993, <i>Echinopelta</i> McLean, 1989, <i>Hirtopelta</i> McLean, 1989, <i>Nodopelta</i> McLean, 1989, and <i>Rhynchopelta</i> McLean, 1989, by having a coiled shell with three to four whorls. It can be distinguished from the three skeneiform genera, <i>Pachydermia</i> Warén & Bouchet, 1989, <i>Depressigyra</i> Warén & Bouchet, 1989, and <i>Lirapex</i> Warén & Bouchet, 1989, by its inflated form with a much more depressed spire and larger aperture. The shell surface is nearly smooth, which differs from all peltospirid genera except <i>Depressigyra</i>. The shell roughly resembles that of <i>Peltospira</i>, but has a more tightly coiled initial whorl, and lacks lamellar sculpture. Analysis of the soft parts shows an enlarged oesophageal gland, a feature previously only known from the ‘scaly-foot gastropod’ <i>Chrysomallon squamiferum</i> Chen <i>et al</i>., 2015 (Warén <i>et al</i>., 2003; Chen <i>et al</i>., 2015), which is also the only other known peltospirid to attain a similar size. In <i>C. squamiferum</i> the oesophageal gland houses symbiotic bacteria, but it is unclear whether this is also the case for <i>Gigantopelta</i>. <i>Gigantopelta</i> can be distinguished from <i>Chrysomallon</i> easily as it does not possess dermal sclerites, has a large operculum, and a shell that is less vertically compressed, with a more circular aperture. The shell of <i>Gigantopelta</i> may be coated in a layer of sulphide, which is frequent amongst vent gastropods including the neomphalins (Hickman, 1984; Warén & Bouchet, 2001). <i>Gigantopelta</i> is also comparable to the Oligocene fossil genus <i>Elmira</i> Cooke, 1919, from a seep deposit near Bejucal, Cuba whose possible affinity to Neomphalina based on resemblance to <i>Chrysomallon</i> was remarked upon by Kiel & Peckmann (2007). Although the type species <i>Elmira cornuarietis</i> Cooke, 1919, is approximately the same size as <i>Gigantopelta</i> (> 40 mm in shell length), it carries broad revolving grooves, which <i>Gigantopelta</i> lacks. The true taxonomic affinity of <i>Elmira</i> is still unclear.</p>Published as part of <i>Chen, Chong, Linse, Katrin, Roterman, Christopher N., Copley, Jonathan T. & Rogers, Alex D., 2015, A new genus of large hydrothermal vent-endemic gastropod (Neomphalina: Peltospiridae), pp. 319-335 in Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) (Zool. J. Linn. Soc.) 175 (2)</i> on pages 322-323, DOI: 10.1111/zoj.12279, <a href="http://zenodo.org/record/5338416">http://zenodo.org/record/5338416</a&gt