Margarites manusensis, 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 : 445

Margarites manusensis sp. nov.

( Figures 3 , 4 )

Type material. Holotype: MBM283049 (height: 9.1 mm, width 10.4 mm), from type locality. Paratype: MBM283050, one specimen (height (excluding protoconch): 7.2 mm, width 10.6 mm), 3°42′S 151°52E, at depth of about 1940 m, June 18, 2015.

One additional immature specimen, collected with holotype (type locality), was also observed.

Type locality. A hydrothermal vent at a depth of about 1740 m, 3°43′S 151°40′E, Manus Back-Arc Basin.

Etymology. The name of new species refers to its type locality, Manus Back-Arc Basin.

Diagnosis. Shell helicoid, small, fragile and moderately elevated. Shell surface olivaceous with iridescence. Teleoconch whorls convex, sculptured with pronounced spiral cords and microscopic growth lines. Suture deeply incised. Basal disk with fine, flattened spiral threads. Aperture subquadratic. Radula rhipidoglossate, with a formula of (ca. 20)+8+1+8+(ca. 20).

Description. Shell ( Figs 3 A–E) helicoid, small and fragile, height up to 9.1 mm. Protoconch eroded. Spire small and moderately elevated, last adult whorl large and wide. Suture deeply incised. Shell surface olivaceous with iridescence. Teleoconch whorl convex, axial sculpture of oblique microscopic growth lines and spiral sculpture of irregularly developed cords. Penultimate whorl with 5 spiral cords, separated by wide grooves; last adult whorl with 7 slightly raised spiral cords, divided by relatively wide grooves. Basal disk with 15 fine, flattened spiral threads. Umbilicus narrow and deep. Aperture subquadratic. Outer lip simple, inner lip slightly calloused. Basal lip and columellar lip forming a rounded angle. Operculum thin, corneous, yellowish and mutispiral.

Radula ( Figs 3 F–K) rhipidoglossate, formula (ca. 20)+8+1+8+(ca. 20). Rachidian tooth with a blunt, serrated overhanging cusp and a very broad base. Lateral teeth gradually increasing in size outwardly, with serrated overlapping cusps and relatively narrow base. Lateromarginal plate (see arrows in Figs 3 G–H) nearly quadrate in shape with a truncated top, inner side longitudinally keeled. Marginal teeth thin, with long shaft and serrated cutting edges.

Soft body ( Fig. 4 ). Foot whitish in colour, large, well developed. Head cylindrical. One pair of cephalic tentacles very long and slender, of similar size to each other. Eye lobes stout, without visible papillae and eyes. Each side of foot equipped with 9 epipodial tentacles. The first four epipodial tentacles clustered together, decreasing in size; the other ones subequal in size. Bipectinate ctenidium large, attached to left part of the mantle. Kidney large, located on dorsal midline of the body. Stomach located at behind of the kidney and partly surrounded by digestive gland.

Remarks. Margarites manusensis sp. nov. most closely resembles Margarites huloti Vilvens & Sellanes, 2006 in general shell shape. But the latter differs from the new species in having an elevated rather than a low spire, more regularly spaced and raised spiral cords, a much thinner radula and a different shape of the lateromarginal plate (triangular vs. nearly quadratic). The new species closely resembles Margarites shinkai Okutani, Tsuchida & Fujikura, 1992 in having a similar radula formula. However, the new species has a more elevated shell that is sculptured with widely spaced spiral cords. Moreover, the rachidian tooth of the radula has a much broader base.

Molecular analyses. One sequence for the COI region of each new species was obtained. The length of the partial COI sequence of the new species is 620 bp. The Neighbor-joining (NJ) tree ( Fig. 5 ) was reconstructed using available COI sequences from this study and GenBank. The alignment of COI had a total 468 bp, without insertions and deletions. The NJ tree shows that the new species fall into the genus Margarites in which Margarites similis sp. nov. and Margarites manusensis sp. nov. together with Margarites groenlandicus and M. pupillus form a well-supported clade, ascertaining their placements within the genus Margarites . Within the genus Margarites , the two new species are the species closest related to each other, with 2% pairwise distance; with the available sequences from GenBank, the pairwise distances among Margarites similis sp. nov. and other species of Margarites are 18.9–26.1%, whereas the pairwise distances among Margarites manusensis sp. nov. and its other congeners are 19.2–26.8%.

Figures

FIGURE 3. Margarites manusensis sp. nov. A – D. Holotype. A. Apertural view; B. Apical view; C. Sculpture of the last adult whorl, scale bar = 500 µm; D. Basal view. E. Apertural view of paratype; F – K. Radula. F. Dorsal view of radula; G. Rachidian and lateral teeth of radula, scale bar = 50 µm; H. Enlargement of lateromarginal portion, showing lateromarginal plate, scale bar = 50 µm; I. Enlargement of lateral teeth; J. Marginal teeth, scale bar = 50 µm; K. Enlargement of marginal teeth, scale bar = 10 µm. White arrows indicate the lateromarginal plate.

FIGURE 4. Soft part of Margarites manusensis sp. nov. Left view of holotype. Abbreviations. - (bct) bipectinate ctenidium; (ct) cephalic tentacle; (dg) digestive gland; (el) eye lobe; (et) epipodial tentacle; (f) foot; (h) head; (k) kidney; (op) operculum; (pm) pillial margin; (sto) stomach.

FIGURE 5. Neighbour-joining tree for Margaritidae based on available COI sequences from this study and GenBank. Numbers above branches indicate the bootstrap values.