Elliptiolucina subovalis, Jiao & Wang & Guo & Zhang, 2023

Jiao, Yingyi, Wang, Minxiao, Guo, Yang & Zhang, Junlong, 2023, A new deep-sea species of Elliptiolucina Cosel & Bouchet, 2008 (Bivalvia, Lucinida, Lucinidae) from cold seep of the South China Sea, Zoosystematics and Evolution 99 (1), pp. 261-271 : 261

Elliptiolucina subovalis sp. nov.

Figs 2 , 3 , 4F

Material examined.

Holotype: MBM229033, one complete specimen collected on Aug. 3, 2018, by a TV grab, deposited in the Marine Biological Museum , Chinese Academy of Sciences (MBMCAS), Qingdao.

Description.

Shell medium-sized, 44.7 mm long, thick, elongate (SH/SL 0.78), sub-rectangular oval, inequilateral, nearly equivalve, slightly inflated (SW/SL 0.44). Umbones slightly prominent and prosogyrous, situated in front of the vertical midline, anterior umbonal slope slightly concave, postero-dorsal margin straight. Anterior margin narrowly rounded, posterior margin broad, rounded-truncate, and the vertical part slightly convex. Postero-dorsal corner obtuse. Ventral margin broadly rounded but slopes distinctly upswept in front.

Shell white, flaky periostracum pale-brownish, dense, and slightly wrinkled towards the margin of the shell. Exterior with dense, irregular commarginal growth ribs, fine commarginal striae, and few irregular vertical lines which diverge from the commarginal sculpture and running in a more upward direction on the postero-dorsal area (Fig. 3A ), visible on 5 × magnification. Both of anterior and posterior dorsal area with one very slight radial depression. Lunule rather long, generally symmetrical, deeply sunken, lanceolate. Escutcheon long, completely covered by ligament which almost occupies the whole postero-dorsal margin and situated on a strong nymph.

Inner shell surface dull white to slightly orange inside pallial line, and glossy at the margin. Dense radial stria from umbonal cavity to the shell margin. Hinge plate narrow, with only very faint and low indications of cardinal teeth in both valves, but a strong anterior lateral tooth in right valve, and a corresponding socket in left valve. Anterior adductor scar elongate, detached from pallial line for less than 1/2 of length. A distinct but shallow pedal retractor scar above the anterior adductor scar. Posterior adductor scar reniform, with a dorsal notch, open to the anterior. Pallial line entire. Shell margin smooth.

Etymology.

The specific name Elliptiolucina subovalis was derived from the Latin sub + oval in reference to the shape of the shell, sub-rectangular oval but more ovate than most congeners.

Type locality.

An active seepage site (22.1159°N, 119.2854°E), site F, in southwest Taiwan, South China Sea, 1146 m depth (Fig. 1 ). Buried in the muddy bottom near the seep.

Remarks.

The new species possesses thick, elongated shells with fine commarginal sculptures, a relatively short anterior adductor muscle scar, and a narrow hinge, especially a dorsal notch in the posterior adductor scar, which are in accord with the key characteristics of Myrteinae . It corresponds to the genus Elliptiolucina in the elongated shells and fine irregular commarginal sculptures. The new species was found from IWP. The West-Atlantic genus Jorgenia shares similar general morphology of the outer shell with Elliptiolucina . But the limited distribution combined with the different hinge features (small but distinct cardinal teeth in both valves of Jorgenia ) can differentiate the two genera.

Elliptiolucina subovalis sp. nov. is distinct from other congeners by possessing a strong anterior lateral tooth on the right valve and anterior tapering, subrectangular-oval shells (Table 1 , Fig. 4 ). The new species is morphologically similar to E. magnifica Cosel & Bouchet, 2008 in outline but can be distinguished by the more prominent and prosogyrous umbones and the lateral tooth. Elliptiolucina subovalis sp. nov. is the second species after E. williamsae known to possess a lateral tooth. The distinct cardinal teeth of E. williamsae can be used to separate it from the new species. In addition, the tapering anterior shell of E. subovalis differentiates the two species. E. virginiae Cosel & Bouchet, 2008 can be distinguished from the new species by its almost rectangular outline. It has a straight dorsal margin and vertical posterior margin, while the shell shape of E. subovalis sp. nov. is rather subovate. The more compressed shells of E. virginiae (SW/SL ratio = 26-29%) and E. labeyriei Cosel & Bouchet, 2008 (SW/SL ratio = 24-33%) are distinct from that of E. subovalis sp. nov. (SW/SL ratio = 44%). E. ingens can be separated from the new species by its ridges in the inner surface running from the umbonal cavity to both posterior and anterior adductor scars and the absence of a posterior dorsal corner on the external surface.

Molecular analysis.

The obtained sequences were uploaded in GenBank (see Suppl. material 1). The entire dataset included sequences of 219 individuals from 146 species. Because markers are not always available for the same species, somewhat different taxon sets are employed in three-gene and 18S trees. In particular, for E. williamsae , only the 18S rRNA gene was available. Phylogenetic frameworks of ML and BI methods showed similar topologies on each dataset. Thus, four trees were presented in two summary cartoon trees in our study (Figs 5 , 6 ). Our results based on three-gene (Fig. 5 ) have divided the species of Lucinidae into seven clades, corresponding to seven subfamilies. Phylogenetic results of two datasets displayed similar topologies within Myrteinae . Our new species Elliptiolucina subovalis sp. nov. was found to be a member of Myrteinae . E. williamsae and E. subovalis sp. nov. formed a stable sister group in 18S trees. A close relationship between the group E. subovalis sp. nov. (+ E. williamsae ) and Rostrilucina garuda Cosel & Bouchet, 2008 was shown in all trees. The new species formed a sister clade of all other Myrteinae species in conjunction with Rostrilucina garuda and Myrtea flabelliformis (Prashad, 1932) in concatenated trees. In 18S trees, the three species mentioned above, together with the E. williamsae , were also basal to other Myrteinae species. The monophyletic Elliptiolucina and Myrtea were not supported. E. ingens involved in a clade composed of Gloverina Cosel & Bouchet, 2008, Myrtea catonii (Glover & J. D. Taylor, 2016), Myrtea vincentia (Glover & J. D. Taylor, 2007), and another undescribed Myrtea species in all trees. Deep-sea species of Myrteinae formed three monophyletic clades in the concatenated dataset (Fig. 5 ). Two stable monophyletic deep-sea clades (clade A and C) represented by species of Elliptiolucina , Gloverina , Myrtea and Rostrilucina Cosel & Bouchet, 2008. The polyphyletic genus Elliptiolucina and Myrtea were shared in the two clades. One species of Taylorina Cosel & Bouchet, 2008, T. solomonensis Cosel & Bouchet, 2008, formed a separate clade, as the sister group of clade A plus all of shallow-sea Myrteinae .

Figures

Figure 1. The locations of Elliptiolucina species. Blue triangle, E. subovalis sp. nov.; green circle: E. magnifica; red circle: E. labeyriei; yellow circle: E. williamsae; black circle, E. virginiae; white circle: E. ingens.

Figure 2. Elliptiolucina subovalis sp. nov. A, B. Exterior and interior of the right valve; C, D. Exterior and interior of the left valve; E. Dorsal view; F, G. Detail of the hinge of the right and left valves.

Figure 3. Shell sculptures of Elliptiolucina subovalis sp. nov. A. Posterior surface (~ 5 x); B. Anterior surface (~ 5 x); C. Middle surface (~ 5 x).

Figure 4. Outline drawing of shells interiors of Elliptiolucina. A. E. magnifica; B. E. labeyriei; C. E. virginiae; D. E. ingens; E. E. williamsae; F. E. subovalis sp. nov. Sketches of A-E. were adapted from Kuhara (2014, fig. 4) and Glover and Taylor (2016, figs 19, 20).

Figure 5. Phylogenetic tree inferred by Maximum likelihood (ML) and Bayesian inference (BI) based on concatenated dataset of 18 S, 28 S and CytB genes. The species marked with an inverted triangle are deep-sea species.

Figure 6. Phylogenetic tree inferred by Bayesian inference (BI) and Maximum likelihood (ML) based on 18 S rRNA gene.