Sphenia elongata, Zhang, Jun-Long, Xu, Feng-Shan & Liu, Rui-Yu, 2012

Zhang, Jun-Long, Xu, Feng-Shan & Liu, Rui-Yu, 2012, The Myidae (Mollusca, Bivalvia) from Chinese waters with description of a new species, Zootaxa 3383, pp. 39-60 : 55-57

Sphenia elongata sp. nov.

Figure 8 D–L

Type material. Holotype. MBM265367 (1 articutate shell), The holotype was collected on May 8th, 2010.

Type locality. Sanniang Bay, Beibu Gulf, Guangxi Province, China; Mid-tide zone.

Etymology. The new species is named in reference to its elongate shell, which is a marked difference with other species in this genus.

Description. Length to 9 mm; shell small, moderately inflated, elongate, distorted, strongly inequivalve, inequilateral, posterior gape small; umbo depressed, opisthodetic and situated at anterior third of shell, umbo of right valve more tumid than of left valve; anterodorsal shell margin convex; posterodorsal margin more straight in right valve than in left valve; ventral margin rather straight; the end of right valve slightly distorted.

Sculpture of fine, commarginal growth lines and wider spaced low growth steps, medially weaker, anteriorly consisting of dense commarginal growth lines, commarginal features becoming coarser on very weakly developed posterior area and on right valve more irregular; colour grey lime-white and with slight yellowish periostracum externally, greyish with irregular greyish-white blotches interiorly.

Hinge of left valve with a large, trigonal, flattened chondrophore extending posteriorward; hinge of right valve with an obliquely ovate concave resilifer, with a blunt tooth-like process on anterior end.

Interior shell margin with yellowish periostracum; anterior adductor muscle scar thin and elongate, arched, near the anteroventral shell margin; posterior adductor scar oval; pallial sinus deep, broad, extending almost to middle of shell; pallial line obscure, discontinuous, barely visible.

Measurement. Holotype (MBM265367): length 8.92 mm, height 4.51 mm, width of both valves 4.17 mm.

Remarks. Hydroids are found on the shell surface, which may indicate a nestling behaviour ( Yonge 1951a). Because of the small size, nestling habit and variation in shape, species distinctions are rather difficult and many names probably synonymous. For example, Coan (1999), Coan & Valentich-Scott (2012) and Pastorino & Bagur (2011) assumed that S. fragilis ( H. Adams & A. Adams, 1854) is the senior synonym of Sphenia pacificensis Folin, 1867 , Corbula iheringiana Pilsbry, 1897 , S. antillensis Dall & Stimpson, 1901 , and S. trunculus Dall, 1916 . Sphenia luticola (Valenciennes, 1846) is the senior synonym of S. pholadidea Dall, 1916 , Cuspidaria nana Oldroyd, 1918 and S. globula Dall, 1919 ( Coan 1999) .

Sphenia elongata sp. nov. differs from both species complexes and from the type species S. binghami Turton, 1822 , by its rather straight and almost parallel dorsal and ventral margins, and its much more depressed umbo. We also exclude S. coreanica Habe, 1951 , which has distinct growth lines, surface wrinkles, and a somewhat crenulated anterior and ventral margin.

Sphenia sincera Hanks & Packer, 1985 , is the only Sphenia species without a distorted shell, but with a prominent umbo and somewhat round ventral margin. S. ruppellii A. Adams, 1851 , develops a few radial ribs on the anterior end of the valves, and S. philippinarum A. Adams, 1851 , has radial ribs posteriorly. The Indo-Pacific S. perversa Blanford, 1867 (= S. inaequalis Smith, 1893 ) differs from the new species by its posteriorly truncated shell, and S. sowerbyi Smith, 1893 (= S. similis Smith, 1893 ) differs by its rather prominent umbo and rather blunt posterior end.

Figures

FIGURE 8. A – C. Sphenia coreanica Habe, 1951. MBM 300740, length = 5.8 mm. A. Exterior of right valve. B. Interior of right valve. Scale bar (A, B) = 1 mm. C. Close up of surface sculpture. Scale bar = 1 mm. D – L. Sphenia elongata sp. nov., D – L, holotype, MBM 267367, length = 8.92 mm. D. Exterior of right valve. E. Exterior of left valve. Scale bar (D, E) = 1 mm. F. Hinge of right valve. Scale bar = 1 mm. G. Interior of right valve. H. Interior of left valve. Scale bar (G, H) = 1 mm. I. Hinge of left valve. Scale bar = 1 mm. J. Close up of umbo on right valve. K. Close up of umbo on left valve. Scale bar (J, K) = 1 mm. L. Close up of surface sculpture. Scale bar = 0.5 mm.