Saturday 11 October 2014

A new species of Thyasiroid Clam from British Columbia.

Thyasiroid Clams are a group of Veneroid Bivalves found in deep waters around the world. They are not well understood as a group, as most species are known only from their shells, which allows for only a limited understanding of their soft tissue anatomy and lifestyle.

In a paper published in the journal Zootaxa on 2 October 2014, Graham Oliver of the National Museum of Wales and Mellisa Frey of the Royal British Columbia Museum describe a new species of Thyasiroid Clam from off the coast of Quatsino Sound on Vancouver Island in British Columbia.

The new species is named Ascetoaxinus quatsinoensis, where ‘Ascetoaxinus’ means ‘curiusly-fashioned axe’, a reference to the shape of the shell, and ‘quatsinoensis’ means ‘from Quatsino’. The species is described from a single specimen collected from a depth of 1086–1318m off Quatsino Sound in 2009. This was originally thought to be a possible aberrant specimen of Conchocele bisecta, due to its large size (31.3 mm long by 24.3 mm high) but closer examination showed both the shell and soft anatomy to be quite different.

Ascetoaxinus quatsinoensis. (A) External of left valve; (B) oblique dorsal view showing posterior and submarginal sulci; (C) internal of left valve; (D) oblique view of the lunule.lig,ligament; lun, lunule; ps, posterior sulcus; sms, submarginal sulcus. Oliver & Frey (2014).

The shell of Ascetoaxinus quatsinoensis is thin and brittle, and scalloped around the edges. Its mantle is also thin, and unfused except for a small exhalent aperture. It has a long, worm-like foot, and a digestive tract comprising a short oesophagus and small stomach connected to large lateral body pouches. Its kidneys are large and packed with gold coloured granules.

Photomicrographs of anatomical details of Ascetoaxinus quatsinoensis. (A) Mouth and oral tube; (B) labial palps andprotrusions of the digestive gland; (C) branches of the lateral body pouch; (D) stomach and style sac/mid gut. Scale bars are 1mm. Abreviations: dd, duct to digestive gland; dg, protrusions of digestive gland; lp, labial palps; mo, mouth; ss/mg, style sac and mid gut; np, neck of lateral body pouch; ot, oral tube; s, stomach. Oliver & Frey (2014).

A search for similar species to Ascetoaxinus quatsinoensis yielded a single specimen of a shell with similar scalloping in the collection of the Smithsonian National Museum of Natural History. This was collected off the coast of Cape Fear in North Carolina at a depth of 646 m, and described as Cryptodon ovoideus by William Dall in 1890. Since this shares the scalloped shell of Ascetoaxinus quatsinoensis, which is not seen in any other member of the genus Cryptodon or other Thyasiroid Clam, the species is redescribed by Oliver & Frey as Ascetoaxinus ovoidea (the species name changes because Ascetoaxinus is feminine while Cryptodon and ovoideus are masculine; the species name must be the same gender as the genus name). The soft anatomy of this species is unknown.

Ascetoaxinus ovoidea. (A) External of left valve; (B) oblique dorsal viewshowing posterior and submarginal sulci; (C) internal of left valve; (D) oblique view of the lunule.lig, ligament; lun, lunule; ps,posterior sulcus; sms, submarginal sulcus. Oliver & Frey (2014).

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