Actumnus elegans De Man, 1887

Actumnus elegans De Man, 1887 View in CoL

( Figs. 5–7 View Fig )

Actumnus elegans De Man, 1887 View in CoL (1887–1888), p. 47. — Alcock, 1898, p. 206. — Chopra & Das, 1937, p. 408, figs. 12–13. — Takeda & Miyake, 1969a, pp. 95 (in list), 96 (in key), 97, figs. 1, 2d–f. — Ng et al., 2008, p. 139 (in list).

Material examined.ɹOse-zaki, Suruga Bay, 6 m depth, 1 Ə(cb 9.3 mm, cl 7.5 mm), NSMT-Cr 32415, 24-VI-2023, coll. H. Takakura; 4.5 m depth, 1 Ə (cb 9.3 mm, cl 7.2 mm), NSMT-Cr 32416, 29-VII-2023, coll. H. Takakura; 4.5 m depth, 1 Ə (cb 10.8 mm, cl 8.5 mm), NSMT-Cr 32717, 15-VI-2024, coll. H. Takakura.

Remarks.ɹThree males examined agree well with the notes on Actumnus elegans (De Man, 1887) by Takeda and Miyake (1969a) in which two males and one female from Kyushu, Japan were recorded hesitantly. Takeda and Miyake (1969a) wrote that, in their specimens, the carapace anterolateral margin is rather distinctly cut into four teeth and each tooth is bordered with four or five granules, one or two of which are more prominent than the others. In the present specimens, the four teeth of the carapace anterolateral margin may be better noted as lobate teeth, but each tooth marginally armed with some granules of different sizes is the same form with that of the specimens from Kyushu. The original description (De Man, 1887) noted that A. elegans is distinguished at first sight from A. obesus Dana, 1852 , by the carapace anterolateral margin being armed with six acute spiniform granules, behind the acute granuliform external angle, arranged in three groups, two together. According to Alcock (1898), the anterolateral margin is armed with seven acute spinuliform granules, in three pairs, with an odd one between the first pair and the orbital angle.

Alcock (1898) left the comments that A. elegans seems to be better placed in Pilumnus rather than Actumnus , because it might be close to P. scabriusculus Adams and White, 1849 . Chopra and Das (1937) and Takeda and Miyake (1969a) followed the precedent literature as for the generic affiliation to the genus Actumnus .

In the present specimens, the carapace is rather narrow, with 1.24 (NSMT-Cr 32415), 1.29 (NSMT-Cr 32416) and 1.27 (NSMT-Cr 32717) in the ratio of cb to cl, and strongly convex anteriorly and rather concave in the posterolateral dorsal surface, showing the close affinity to Actumnus . The carapace dorsal surface is ill-defined and covered thickly and uniformly with yellowish stiff hairs mixed with scant longish, clubshaped soft hairs for the most part, being possibly referred to the generic characters of both of Actumnus and Pilumnus ( Figs. 5–7 View Fig ). The armature of the carapace anterolateral margin is, as mentioned above, shows its systematic position in Actumnus , because the Pilumnus species are characterized by the carapace anterolateral margin armed with stout triangular teeth in dorsal view, or with narrower, more or tubercular teeth each terminating in a horny tip, or with sharp spiniform teeth directed obliquely forwards. In Actumnus , the carapace is typically much narrower and higher, and the thoracic sternum is narrower, with the rather elongated male pleon. In the present specimens ( Fig. 6C), the male pleon is not remarkably narrow and somewhat like the Pilumnus - type rather than the Actumnus - type. Otherwise, in the present specimens, the carapace front ( Fig. 7B–D View Fig ) is markedly developed forwards and obliquely downwards, and the inner half of the supraorbital margin is longitudinal in dorsal view, as specially mentioned by Takeda and Miyake (1969a). In the present paper, the specimens examined are placed in the genus Actumnus and identified to A. elegans as the preceding studies.

Distribution. Mergui Archipelago, Myanmar ( Burma); Ariake Sea and Okino-shima Island in the west and north of Kyushu, and Suruga Bay, Japan.