Parathyasira Iredale 1930

Parathyasira Iredale, 1930

Type species. Parathyasira resupina Iredale, 1930

Diagnosis. Small to medium sized thyasirids with a weakly defined posterior area, a well-defined supramarginal sulcus without an auricle, a microsculpture of calcareous spines arranged in dense radial rows. Hinge lacking teeth, ligament weak, in a shallow resilifer. Gills of two demibranchs, delicate, Type 2aR of Oliver (2014).

Remarks. The diagnosis here is limited to those species with a spiny micro-sculpture arranged in radial rows. There are many other species with a similar overall shell form such as the North Atlantic Thyasira equalis Verrill & Bush, 1898 that have been placed in Parathyasira by previous studies (Payne & Allen 1991; Oliver & Killeen 2002; Zelaya 2009). There remains many uncertainties concerning the relationships of these species in conjunction with many others currently placed in Thyasira but not agreeing in shell form or anatomy with the type species of Thyasira, T. flexuosa (Montagu, 1803). Further research is required to resolve how many genera should be recognized, while those with a distinct radial micro-sculpture form a coherent group.

Parathyasira bamberi sp. nov. Figs 1A–E

Type material. Holotype - single live collected specimen, Off Ras Madrakah, southern Oman, Discovery cruise 211 st. 12687#2, 18°55.85´N 58°52.17´E to 18°52.16´N 59°07.24´E, 3356 m, collected P.G. Oliver, 19 October 1994. NMW.Z. 1995.009.13. Paratype, one fragmented shell as holotype, NMW.Z. 1995.009.14.

Description. Shell (Figs 1A–D). Holotype 18.7 mm in length, 18.4 mm in height. Thin, fragile. Equivalve. Compressed. ~Equilateral, beaks prosogyrous, just anterior of midline. Outline subcircular, slightly longer than high. Posterior dorsal margin long, almost straight, sloping steeply; anterior dorsal margin short, slightly concave; anterior and ventral margins a continuous curve. Posterior area flattened not sulcate, demarcated by a weak radial ridge. Sub-marginal sulcus (sms = escutcheon) long, narrow, sunken, edges vertical. Ligament slender not sunken but not set on an auricle, ~half the length of sms. Lunule a flattened area, not depressed. Hinge edentulous. Sculpture of commarginal growth lines and weak ridges with a micro-sculpture of dense radial rows of blunt calcareous spines. Muscle scars indistinct.

Anatomy (Fig. 1E). Mantle thin; edges un-fused except for the formation of a small exhalant aperture Anterior adductor scar slender, elongate and approximately two 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 distinctly grooved lamellae leading to a short tubular portion leading to the mouth. Lateral body pouches large, thin walled, branches few, terminations simple, rounded. Kidney large with golden coloured granules. Ctenidium large, both demibranchs with fully reflected filaments; outer demibranch about half the depth of the inner demibranch. Filaments delicate.

Habitat and ecology. Taken from abyssal depths in ooze along with a high diversity of protobranchs and large numbers of suspension feeding Limopsis tenella Jeffreys and Bentharca asperula (Dall). The water temperature was 1.7°C and the oxygen concentration was 2.99 ml l -1. The delicate gill structure indicates that the abfrontal extension is poor and that any bacterial symbiosis is limited and this would be in keeping with the observations made on Parathyasira granulosa, (Monterosato, 1874) by Dufour (2005).

Etymology. Named for Roger Bamber, fellow student from our days at the Dove Marine Laboratory, Cullercoats, Northumberland, sadly departed February, 2015.

Comparisons. The number of species of Thyasiridae with a radial microsculpture of calcareous spines is few. The type species P. resupina (Fig. 1 F) and the closely related P. neozelanica Iredale, 1930 and P. verconis (Cotton & Godfrey, 1938) inhabit shelf depths off southern Australia and New Zealand. These are much smaller, not exceeding 8mm, are higher than long in outline with a much stronger posterior sinus. The other known species are from the North Atlantic, P. granulosa (Monterosato, 1874) (Fig. 1 G) is known from the lower shelf and upper slope while P. subcircularis (Payne & Allen, 1991) (Fig. 1 H) is known from the lower slope and abyssal rise. Parathyasira granulosa is very similar to the Australian species while P. subcircularis has a more poorly defined posterior area. The latter is however higher than long in outline and has a less defined lunule and sub-marginal sulcus.