Benthomodiolus abyssicola (Knudsen, 1970)

Oliver, P. Graham, 2015, Description and morphology of the " Juan de Fuca vent mussel ", Benthomodioluserebus sp. n. (Bivalvia, Mytilidae, Bathymodiolinae): " Phylogenetically basal but morphologically advanced ", Zoosystematics and Evolution 91 (2), pp. 151-165 : 160

Taxon classification Animalia Mytiloida Mytilidae

Benthomodiolus abyssicola (Knudsen, 1970)

Modiolus abyssicola Knudsen, 1970: 92-94, text figs 55-57, pl. 14, fig. 6.

Benthomodiolus abyssicola (Knudsen) - Dell 1987: 31-33.

Benthomodiolus abyssicola (Knudsen) - Coan et al. 2000: 162, pl. 24.

Benthomodiolus abyssicola (Knudsen) - Coan and Valentich-Scott 2012, 123, pl. 39.

Material examined.

Holotype. Gulf of Panama, E. Pacific, 05°49'N 78°52'W, 3670-3270m. ZMUC, not examined.

Paratype. From type locality, 10 specimens examined, ZMUC-BIV-30.

Illustration.Paratypes were dissected by Vita Kenk and her drawing was published in Coan and Valentich-Scott (2012). A copy of the original drawing was sent to me by Paul Valentich-Scott and is reproduced here (Fig. 9f).

Shell.

The shells have a maximum length of 17.2 mm and the dissected specimen was 17.1 mm in length (Fig. 9 a–b). The shell is weakly umbonate, wedge shaped with the posterior distinctly deeper than anterior in outline and medially narrowed but weakly sulcate. The beaks are close the anterior margin with a total length /anterior length of 0.22. The sculpture is weak of commarginal lines only. Fine periostracal hairs are present (Fig. 9c) but worn off in the larger specimens.

Pedal byssus musculature.

The byssal retractors (pbr2 and pbr1) are widely separated with pbr2 attached immediately behind the umbonal cavity (Fig. 9 e–f). The angle between the byssal retractors is acute. The anterior pedal retractor (apr) is more slender than the posterior byssal retractor (pbr1). The posterior protractor muscle (ppr) is slender and incorporated into the anterior byssal retractor (Fig. 9 e–f).

Ctenidium and labial palps.

The ctenidium consists of both demibranchs with relatively short strap-like filaments and has a flimsy appearance with weak inter-filamentar junctions (Figs 9d, 10f). The ascending and descending arms of the filaments are fused for only a short distance (Fig. 10f). The filaments have a single row of ciliary junctions on the ascending and descending arms (Fig. 10g). The frontal cilia are present but the abfrontal surfaces are largely devoid of epithelium, probably a consequence of poor fixation. Where present a weak polygonal pattern can be seen (Fig. 10h).

Mantle edge and apertures.

The mantle edge is free along its entire length except for a small junction with the gill axis of the ctenidium to form a small exhalant aperture (Fig. 9d, f). The mantle edge bears weak folds posteriorly but smooth ventrally and anteriorly.

Alimentary system.

The gut follows a simple route with the oesophagus, stomach and mid gut in a line running across the dorsal surface of the visceral mass (Fig. 9f). The hindgut descends into the visceral mass and returns to form a distinct loop (Fig. 9f, hgl) before passing through the heart.