Ochetoctena tomasi, Oliver, P. Graham, 2014

Oliver, P. Graham, 2014, " TUBULAR GILLS " Extreme gill modification in the Thyasiroidea with the description of Ochetoctena tomasi gen. et sp. nov. (Bivalvia: Thyasiroidea), Zoosystematics and Evolution 90 (2), pp. 121-132 : 126-127

Taxon classification Animalia Veneroida Thyasiridae

Ochetoctena tomasi sp. nov.

Thyasira n. sp. Regab - Rodrigues & Duperron, 2011.

Material examined.

2 specimens (1 broken) in ethanol. Regab Pockmark, Gulf of Guinea, West Africa, RV Meteor GUINECO cruise, M76/3, Dive 215, 05°47.84' S. 09°42.64' E. 3167m

Holotype, entire shell from above, NMW.Z.2014.014.00001

Description of holotype.

14.4 mm long, 13.4 mm high, 4.3 mm half t. Equivalve, Equilateral. Outline subcircular. Anterior dorsal (lunule) margin concave, lunule slightly depressed, ovate, smooth, demarcated by a low ridge. Posterior dorsal (escutcheon) margin almost straight; escutcheon narrow, very shallow, auricle lacking. Posterior ventral margin weakly indented corresponding with a distinct but shallow posterior sulcus. Median area slightly flattened corresponding with slightly straightened ventral margin. Anterior ventral and anterior a continuous broad curve. Ligament prominent but deeply sunken, about half the length of the escutcheon. Hinge edentulous. Sculpture of irregular commarginal lines and growth stops, overall with microscopic conical, calcareous spines emerging through the periostracum, these as isolated individuals, in small clumps or connected by weak sub-concentric ridges. Periostracum relatively thick, brown in colour, shell white. Muscle scars prominent, anterior adductor scar elongate not diverging from the broad pallial line; posterior adductor scar oval.

Anatomy.

Mantle edge thick, free except for a junction demarcating the posterior aperture. Foot large, vermiform, toe elongate, heel very small. Lateral pouches large, multi-lobed, lobes cuboid. Ctenidium of two demibranchs, the outer about half the length of the inner; both with fully reflected filaments, these of Type 5T, composed of tubules fused to a median tube with only a small dorsal inter lamellar space.

Etymology.

Named for my son Tomas.

Discussion.

The shell of Ochetoctena has a weak posterior sulcus and the escutcheon is excavated but lacks any auricle to support the sunken ligament. In this it differs from Thyasira sensu stricto where there is a well developed auricle and from Conchocele which is oblique with a very strong posterior sulcus. Species lacking an auricle are often placed in the genus Parathyasira ( Oliver and Killeen 2002; Payne and Allen 1991). The type species of Parathyasira is Parathyasira resupina Iredale, 1930 (Fig. 9F) and has a shell microsculpture of radial rows of calcareous spines. This character is also seen in the Atlantic species Parathyasira granulosa (Monterosato, 1874), Parathyasira subcircularis Payne & Allen, 1991 (Fig. 9 G–I) and an un-named species from the Arabian Sea (Figs 9 A–E). The spines in these species are angular with strongly developed basal connections between the rows (Figs 9 C–D). In contrast the spines in Ochetoctena are not arranged in rows, are conical and the basal connections are weak or absent (Figs 8 C–E). The genus Spinaxinus is characterised by having a spiny microsculpture but this is entirely periostracal ( Oliver and Holmes 2006; Oliver et al 2013).

Ochetoctena is the only thyasirid known to have ctenidia of the Type 5T structure; Conchocele has a partly tubular gills of the Type 4TL morphology. The ctenidia of these Parathyasira species show poorly developed abfrontal extension resulting in a flimsy open structure. Dufour (2005) reported that the gills of Parathyasira granulosa are of her type 2 (type 2aR above) suggesting that the genus Parathyasira is characterised by a weak symbiont partnership.

There are, therefore, shell and anatomical synapomorphies that separate Ochetoctena from all other known thyasirids.