Dorhynchus rostratus (Sakai, 1932)

(Fig. 5C–D)

Material examined. RV Hakuhō Maru KH-72-1 cruise, sta. 26 (Timor Sea; 09°27.0′S, 127°58.6′E –09°285.0′S, 127°56.1′E, 610–690 m depth), 3 m beam trawl; June 19, 1972; 1 ♀ (NSMT-Cr 29261: CL 15.2 mm excluding pseudorostral spines, CB 15.0 mm).

Remarks. Manning and Holthuis (1981) suggested that Stenorhynchus ramusculus Baker, 1906 (southern Australia and New Zealand; Ahyong, 2008, Farrelly and Ahyong, 2019) and Achaeopsis rostrata Sakai, 1932 (West Pacific) are congeneric with Dorhynchus thomsoni Wyville Thomson, 1873 from the Atlantic and Indo-Pacific. Griffin and Tranter (1986a) agreed with Manning and Holthuis (1981) in considering the congeneric identity of the three species, but referred them to the genus Achaeopsis Stimpson, 1857, without mention about the relationship to the type species, A. spinulosa Stimpson, 1857 . In recent studies (Davie, 2002; Poore, 2004; Ng et al., 2008; Carmona-Suárez and Poupin, 2016), the three species in question were assigned to the genus Dorhynchus, and thus Achaepsis is monotypic. As a result, Dorhynchus is comprised of two western Pacific and three Atlantic species: D. rostratus (Sakai, 1932) from Japan and the Kai Islands, D. ramusculus (Baker, 1906) from southern Australia and New Zealand, D. basi Macpherson, 1983 from Namibia, D. furcillatus A. Milne-Edwards, 1881 from the Sea of the Antilles, and D. thomsoni Wyville Thomson, 1873 from the western and eastern Atlantic and the Indo-Pacific.

A fully-grown female at hand agrees with the original description of Achaeopsis rostrata by Sakai (1932), though the protogastric and epibranchial spines are not remarkable as shown in the figures (Fig. 2B vs. Sakai 1932, fig. 3; 1938, fig. 17). Dorhynchus rostratus can be distinguished from the other two congeners from the western Pacific by the relatively short, slightly incurved pseudorostral spines not exceeding the distal end of the antennal peduncles and the lack of distal spine on the ambulatory legs meri (Griffin and Tranter, 1986a).

Distribution. Hitherto known from Japan and the Kai Islands, Indonesia; 170–366 m depth. The present bathymetric record, 650 m in the Timor Sea, is the deepest for this species.