Symmetriapelta Beck, 2023

Chen, Chong & Sigwart, Julia D., 2023, The lost vent gastropod species of Lothar A. Beck, Zootaxa 5270 (3), pp. 401-436 : 426-428

Symmetriapelta Beck , gen. nov.

Figs 18 , 19

[ZooBank LSID: urn:lsid:zoobank.org:act:97289DCB-A600-4E88-8713-0984E32FFDFA ]

Type species. Symmetriapelta wareni sp. nov.

Diagnosis. Rhipidoglossate ["]archaeogastropods["] with limpet-shaped shell of small size. Apex slightly left of mid-line, nearly at center of shell in adults, in juveniles near posterior margin of shell; in juveniles with the first teleoconch whorl present; in adults it is eroded at apex; sculpture of beaded radial ribs of varying thickness and number; aperture oval with lateral parts running almost parallel in adults; periostracum not overhanging; shell interior opaque to white with fine, coalescing, concentric lines corresponding to the shell structure, no nacre; muscle scars barely marked; depth of shell interior with minute remainder of teleoconch whorl even in adults with eroded apex. Radula with formula about 18 x 4 x 1 x about 18; of peltospirid type, first and second lateral tooth bases each with additional ridge (or node), cups of fourth lateral with delicate denticulation on inner side whereas outer side has four conspicuous denticles.

Animals with eyeless head, snout with greatly thickened circumoral roll which is divided ventrally, cephalic tentacles of equal size, very thick, directed anteriorly, extending beyond margin of snout; neck short, passing over to a deep and large concave depression in which the visceral mass is embedded; depression translucent as foot sole is very thin; foot sole oval, with continuous marginal flap that is broadened and elongated anteriorly and that bears a moderate-sized slit of the anterior foot gland; epipodium well developed, it consists of a continuous basal ridge which bears about 25 anterio-lateral epipodial tentacles and two slightly solitary posterior tentacles. Pallial cavity is of moderate depth reaching to the left on half of animal length; deeper and larger on left side where it contains the gill with long afferent membrane at ventral-right side; efferent membrane fused to the pallial skirt on 80% of gill length, free tip tapering; dorsal gill lamellae short but ventral lamellae large and triangular. Anterior right part of pallial cavity short, containing anus and openings of kidney and gonad. Pallial margin adhering to the shell throughout its length, pallial skirt with micropapillae that fit into the shell pores. Visceral mass small, not filling the concave depression on dorsal side of the foot. Gonad is mostly covered by the digestive gland. Operculum absent.

Etymology. Genus name has Latin roots, symmetria for symmetrical and pelta for shield.

Remarks. Symmetriapelta is the first limpet-shaped peltospirid from the western and southwestern Pacific. However, the main area of [neomphaloidean] distribution is at EPR, where 11 genera are recognized (with coiled shell: Depressigyra , Lirapex , Melanodrymia , Pachydermia , Planorbidella ; with limpet-shaped shell: Peltospira , Rhynchopelta , Hirtopelta , Ctenopelta , Nodopelta , Echinopelta ) ( Warén & Bouchet 1993). Symmetriapelta gen. nov. differs from all these genera in having an almost central apex, however, there are some characters which point to a close relationship with Nodopelta and foremost, with Echinopelta : aperture of shell with lateral margins running parallel, anatomy of the epipodium, and the presence of ridges or nodes at the bases of lateral radular teeth. While Symmetriapelta gen. nov. is different from Nodopelta in having no vestige of an operculum and no glandular pallial margin, the new taxon substantially differs from Echinopelta in the following characters: apex of shell is almost positioned centrally (as opposed to being near the shell margin), shell margin smooth, following the line of the radial ribs on the shell surface (compared with an undulating shell margin), number of epipodial tentacles about 50 (in contrast to of more than 100), periostracum with beaded radial ribs of different size (instead of hollow spines).

Figures

FIGURE 18. Symmetriapelta wareni gen. et sp. nov. A–C. Paratype 1 (shell length 4.0 mm). A. Dorsal view. B. Viewed from right. C. Shell interior; anterior on the left. D–E. Holotype (shell length 4.1 mm). D. Ventral view. E. Dorsal view. F.A specimen from BIOLAU, BL03 (shell length 2.2 mm) with extremely thick radial ribs; calcareous layers partly dissolved, ventral view.

FIGURE 19. Symmetriapelta wareni gen. et sp. nov. A. Paratype 2 (body length 3.1 mm; shell fragmented), animal removed from shell, in dorsal view. B. Critical point dried specimen in ventral view. C–D. Radula. C. Overview. D. Half-row of radula teeth, arrows indicate nubs on the bases of first and second lateral teeth. [Scale bars: B, 1 mm; C, 50 μm; D, 20 μm.]