Thyasira anassa, Oliver, P. Graham, 2015

Oliver, P. Graham, 2015, Deep-water Thyasiridae (Mollusca: Bivalvia) from the Oman Margin, Arabian Sea, new species and examples of endemism and cosmopolitanism, Zootaxa 3995 (1), pp. 252-263 : 255

Thyasira anassa sp. nov.

Figs 2A–C

Type material. Holotype, single live collected specimen, Off Ras Madrakah, southern Oman, Discovery cruise 211 st. 12714#1, 19°10.74´N 58°20.33´E to 19°10.10´N 59°19.50´E, 919–967 m, collected P.G. Oliver, 28 October 1994. NMW.Z. 1995.009.15. Paratypes, one fragmented valve and one shell. Discovery cruise 211 st. 12685#6, 19°18.88´N 58°15.46´E, 688 m, collected P.G. Oliver, 15 October 1994. NMW.Z. 1995.009.16.

Description. Shell ( Figs 2A–B ). Holotype 21.7 mm in length, 20.7 mm in height. Thin, fragile. Equivalve. ~ Inflated. ~Equilateral, beaks prosogyrous, just anterior of midline. Outline polygonal, slightly longer than high. Posterior dorsal margin long, sloping steeply, trisinuate with a distinct posterior sinus, sub-marginal sinus and auricular sinus; anterior dorsal margin long, slightly concave; anterior narrow, rounded, ventral curved. Posterior area deeply bi-sulcate. Posterior sulcus deep, demarcated by a sharp radial ridge. Sub-marginal sulcus long, narrow, sunken, with a high auricle extending ~half the length of sms. Lunule large, flattened, not depressed, defined by a weak ridge. Anterior slope slightly flattened set off by a weak radial ridge. Hinge edentulous. Sculpture of commarginal growth lines and weak ridges. Adductor muscle scars small, anterior elongate, ~half free from the pallial line; posterior oval set on the posterior angle.

Anatomy ( Fig. 2C ) Mantle thin; edges un-fused except for the formation of a small exhalant aperture Anterior adductor scar slender, elongate and approximately 4 times longer than posterior adductor scar; free from mantle edge for about one-half of its ventral edge; posterior adductor muscle oval. Foot vermiform very long, tip noticeably expanded, heel obsolete, protractor muscles very slender. Labial palps small, with grooved lamellae leading to a short tubular portion leading to the mouth. Lateral body pouches large, arborescent, terminations forked and bluntly pointed. Ctenidium large, both demibranchs with fully reflected filaments; outer demibranch about half the depth of the inner demibranch. Filaments fleshy.

Habitat and ecology. Taken from soft, greenish mud on the upper slope in hypoxic conditions, the live collected holotype from bottom water at a temperature of 9°C and an oxygen concentration of 0.27 ml l-1. The associated bivalve fauna was limited, the most frequent being the large lucinid Lucinoma gagei along with the large nuculid Acila fultoni, Smith, 1892 .

Etymology. anassa Greek “queen” ( Brown 1956) for the elegance of the shell and memories of a certain card game, played at the Dove Marine Laboratory, that Roger never lost!

Remarks. The deeply sulcate shell is reminiscent of polygonal species of Thyasira ss and Channelaxinus Valentich-Scott & Coan , 201 2. In the Indian Ocean only two species warrant comparison: Channelaxinus investigatoris Smith, 1895 ( Figs 2 D–E) and Thyasira acuticarinata Smith, 1895 ( Figs 2 F–H). The former is strongly oblique with a deeply excavated ligament demarcated by a strong ridge and has a distinct flattened median zone, all such characters are absent in T. anassa sp. nov. Thyasira acuticarinata is inequilateral with the anterior extended laterally, the auricle is low and does not extend above the submarginal sulcus; it has been taken only from the type locality off the east coast of Sri Lanka at 1114 m, while C. investigatoris has been found off the western coasts of Sri Lanka and southern India at depths between 260 and 1114 m.

Figures

FIGURE 2. A – C, Thyasira anassa sp. nov. Holotype: A – B, external and internal of the left valve; C, gross anatomy viewed from the right side and right mantle removed; D – E, Channelaxinus investigatoris Smith, off Sri Lanka, National Museum of Wales, NMWZ 1955.158.11399; F – H, Thyasira acuticarinata Smith, reproduced from Annandale & Stewart, 1909. aa, anterior adductor muscle; au, auricle; aus auricular sinus; f, foot; id, inner demibranch; lbp, lateral body pouch; lun, lunule; me, mantle edge; od, outer demibranch; pa, posterior adductor muscle; ps, posterior sulcus; psn, posterior sinus; sms, submarginal sulcus; smsn, submarginal sinus.