Margarites similis, Zhang & Zhang, 2017

Zhang, Shuqian & Zhang, Suping, 2017, wo new species of Margarites (Gastropoda: Margaritidae) from hydrothermal vent areas, Western Pacific, Zootaxa 4299 (3), pp. 441-450 : 443-445

Margarites similis sp. nov.

( Figures 1 , 2 )

Type specimens. Holotype: MBM283047 (height 6.5 mm, width 8.6 mm), from the type locality, April 16, 2014; Paratypes: MBM283048, three specimens, collected with the holotype at the type locality, April 16, 2014.

Type locality. A hydrothermal vent area at a depth of 1243 m, 27°32′N 126°58′E, Okinawa Trough.

Etymology. The name of the new species refers to its conchological similarity to Margarites shinkai Okutani, Tsuchida & Fujikura, 1992 .

Diagnosis. Shell helicoid, small, fragile and depressed. Shell surface olivaceous with iridescence, sculptured with developed spiral cords and microscopic growth lines. Suture moderately constricted. Basal disk with numerous fine spiral and axial threads. Umbilicus narrow and deep. Aperture squarish. Radula rhipidoglossate, with a formula of (ca. 20)+10+1+10+(ca. 20)

Description. Shell ( Figs 1 A–E) helicoid, small and fragile. Protoconch eroded and leaving a pin-hole at apex. Spire small and depressed, last adult whorl large and wide. Suture moderately constricted. Shell surface olivaceous with iridescence. Teleoconch whorls slightly convex, sculptured with pronounced spiral cords crossing with microscopic growth lines, giving shell surface a rough appearance. Penultimate whorl with 6 flattened spiral cords, separated by narrow grooves; last adult whorl with 9 slightly raised spiral cords, separated by relatively wider grooves. Basal disk with numerous fine spiral and axial threads. Umbilicus narrow and deep. Aperture squarish, outer lip simple, inner lip thinly calloused, basal lip and columellar lip forming a rounded angle. Operculum thin, corneous, yellowish and mutispiral.

Radula ( Figs 1 F–I) rhipidoglossate, formula (ca. 20)+10+1+10+(ca. 20). Rachidian tooth with a blunt, serrated overhanging cusp and a broadly ovate base. Lateral teeth gradually increasing in size outwardly, with overlapping cusps and relatively narrow base. Lateromarginal plate present, triangular in shape, bilobated by a deep incision (see arrows in Fig 1 F). Marginal teeth thin, with long shaft and serrated cutting edge.

Soft part ( Figs 2 A–B). Foot pale orange and the rest pale white in color. Foot large, well developed. Head quite large, snout cylindrical. One pair of cephalic tentacles very long and slender, of similar size to each other. Eye lobes stout, without visible papillae and eyes. Neck lobes arising from both basal sides of the head. Right neck lobe forming a tentacle-like protuberance and left neck lobe forming a tentacle of neck lobe. Foot equipped with four and six epipodial tentacles on left and right side respectively. Left side: four epipodial tentacles subequal in size, of which the first and second ones clustered together. Right side: first, second and third epipodial tentacles clustered together, decreasing in size; the remaining ones subequal in size. Ctenidium bipectinate and large, attached to left part of the mantle. Kidney large, located on dorsal midline of the body. Intestine situated at lower right of the kidney. Posterior part of intestine forming a loop in front of stomach. Stomach located behind the kidney and partly surrounded by digestive gland. The stomach containing grey fluffy materials.

Remarks. Margarites similis sp. nov. superficially resembles Margarites shinkai Okutani, Tsuchida & Fujikura, 1992 in shell shape, but differs in having fewer spiral cords with wider interspaces and in having a much smaller shell size (6.5× 8.6 mm vs. 11.0× 15.1 mm). In addition, the new species can be distinguished from Margarites shinkai in having a central tooth with a much broader base and 10 instead of 8 pairs of lateral teeth. Margarites similis sp. nov. is similar to another hydrothermal vent species, Margarites ryukyuensis Okutani, Sasaki & Tsuchida, 2000 , with which it shares the same radula formula. However, Margarites ryukyuensis differs from the new species in having much more convex teleoconch whorls with weaker spiral cords, a narrower base of the rachidian teeth and a different shape of the lateromarginal plate (subquadrate vs. triangular).

Figures

FIGURE 1. Margarites similis sp. nov. A – D. Holotype. A. Apertural view; B. Apical view; C. Sculpture of the last adult whorl; D. Basal view. E. Apertural view of paratype; F. Dorsal view of radula, arrows indicate the lateromarginal plate. G. Rachidian and lateral teeth; H. Marginal teeth; I. Enlargement of marginal tooth, scale bar = 50 µm.

FIGURE 2. Soft part of Margarites similis sp. nov. Right view (A) and left view (B) of paratype, respectively. Abbreviations. (bct) bipectinate ctenidium; (ct) cephalic tentacle; (dg) digestive gland; (el) eye lobe; (et) epipodial tentacle; (f) foot; (in) intestine; (k) kidney; (nl) neck lobe; (op) operculum; (pm) pillial margin; (sto) stomach; (t) tentacle of neck lobe.