Actumnus ngankeeae, Takeda & Komatsu, 2024

Takeda, Masatsune & Komatsu, Hironori, 2024, Actumnus ngankeeae sp. nov. and Pilumnus swajayai Ng & Rahayu, 2021, pilumnid crabs from Japan (Crustacea: Decapoda: Brachyura), Zootaxa 5476 (1), pp. 424-433 : 425-428

Actumnus ngankeeae sp. nov.

( Figs. 1 , 2 , 3A ⎼C, 4)

Type material. Holotype: male (cb 4.6 mm, cl 3.5 mm), NSMT-Cr 31014, diving site Zakuzaku, 26°35'19"N, 142°08'41"E, south of Haha-jima Island , Ogasawara Islands, scuba diving, coll. by H. Komatsu, 4 July 2015.

Diagnosis. Small species. Carapace transversely ovate, strongly convex, uniformly covered with short setae; dorsal surface microscopically granulated, with regions sharply defined by shallow, narrow furrows; front convex, medially developed, divided into two lobes by median deep fissure; anterolateral margin fringed with narrow crest, divided into four lobes by three V-shaped notches. Both chelipeds robust, subequal, similar in shape, with short, strongly curved fingers.

Description of holotype. Carapace ( Figs. 1 , 4 ) transversely ovate, narrow, ca. 1.3 times as long as wide, strongly convex longitudinally and transversely; dorsal surface uniformly covered with microscopic granules, sparsely with short setae; regions sharply defined by shallow, narrow furrows. Carapace anterolateral margin ( Figs. 1 , 4 ) narrowly crested along its whole length, cut into four lobes by three V-shaped notches; first lobe as long as third lobe, longer than second and fourth lobes, anterior end of first lobe forming external orbital angle; margin of each lobe lined with some small granules, with sharpened anterior end. Greatest breadth of carapace at fourth lobes of both sides. Carapace posterolateral margin ( Figs. 1A , 4 ) strongly convergent toward lateral end of carapace posterior margin. Carapace posterior margin ( Figs. 1A , 4 ) as wide as frontal margin. Front ( Figs. 1 , 4 ) directed obliquely downward, bilobed, lobes separated by median narrow, deep slit; each lobe fringed with sharp, close-set granules, most prominently developed close to median slit, outer one-third weakly concave, ending as small lateral angle directed obliquely downward, widely separated from inner angle of supraorbital margin. Orbit ( Fig. 1 ) deep, with inner part of supraorbital margin strongly concave; supraorbital margin narrowly raised, fringed with small closelyset granules, separated to three parts by two V-shaped notches; infraorbital margin transverse, thinly edged, without interruption; inner infraorbital angle not developed.

Both chelipeds ( Figs. 1 , 2 , 3A , 4 ) robust, subequal. Merus short, entirely concealed under carapace, when viewed dorsally; both with lower margins armed with rows of several obtuse granules. Carpus thickly covered with small sharp granules sparsely interspaced with short setae, with inner surface touching with carapace epimeral surface; inner angle blunt, without tooth. Palm ( Figs. 2A , 3A ) short, not strongly bulged outward; outer surface covered with small conical granules of various sizes; upper margin sharp, with row of several larger, sharper granules; outer surface thickly covered with short setae sparsely interspaced with long setae. Both fingers ( Figs. 2A , 3A ) short, stout, with prominent tuft of setae on each external surface; immovable finger one-third as long as palm, with triangular tooth occupying more than half length of cutting edge; movable finger stout, length slightly less than upper margin of palm, strongly curved at distal one-third toward to tip of immovable finger; dorsal surface of movable finger armed with several sharp, tuberculate granules.

Ambulatory legs ( Figs. 1 , 3B , 4 ) stout, moderate in length, covered with silky setae of various lengths on dorsal and ventral margins of meri, carpi and propodi of all pairs; dorsal margins of meri, carpi and propodi subacute, not sharply crested; both lower margins of meri of first three pairs sharp-edged, armed with several minute, sharp granules; merus of last pair with denser setae and smaller granules along lower margins.

Male pleon narrow ( Fig. 2B ) throughout six somites, first two somites setose; telson as long as sixth somite, weakly tapering distally. G1 ( Fig. 3C ) of typical pilumnid shape, long, weakly curved, occupying whole space of abdominal trench; tip sharply pointed. G2 short.

Color in life ( Fig. 4 ). Yellowish tan, sparsely speckled with blurry reddish spots; setae on carapace, chelipeds and ambulatory legs reddish to yellow; cornea reddish.

Etymology. This new species is named after the late Dr. Ng Ngan Kee, who was our friend and colleague.

Remarks. Actumnus ngankeeae sp. nov. is close to A. digitalis ( Rathbun, 1907) and its synonym, A. carinatus Bouvier, 1915 , in the general appearance of the carapace, chelipeds and ambulatory legs. Actumnus digitalis was originally referred to the genus Platypodia Bell, 1835 , but is really a pilumnid species and accommodated in the genus Actumnus due to the dorsally convex carapace with the regions defined by narrow furrows, the stout chelipeds having the short and convex fingers, the sharp but not crested dorsal margins of the ambulatory legs, the narrow thoracic sternum, the slender and freely segmented male pleon, and the pilumnid type G1. As figured by Rathbun (1907: pl. 1 fig. 6, as P. digitalis from Kusaie, now Kosrae in the Caroline Islands, and Papeete, Tahiti), Bouvier (1915: fig. 26, as A. carinatus from Port Louis in Mauritius) and Takeda & Miyake (1969: fig. 3, as A. digitalis from Kamiyama-jima Island in the Ryukyu Islands), the supraorbital and anterolateral margins of the carapace are sharply crested, with two closed, deep fissures on the supraorbital margin and three fissures on the anterolateral margin.

In Actumnus ngankeeae , the carapace anterolateral margin is narrowly crested throughout the length, and divided into four distinctly isolated lobes by three deep, V-shaped notches. Furthermore, in the new species, the carapace dorsal surface is uniformly covered only with short setae, but in A. digitalis , the setae are restricted to the regions which are weakly convex dorsally and isolated from each other.

It is noteworthy that Platypodia andamania Deb, 1992 , may be synonymous with Actumnus digitalis . This species, described from the Andaman Islands, was left behind the recent studies since its original description. The original figure is so schematic that it is difficult to make a definitive conclusion, but the figure is similar to A. carinatus .

Actumnus ngankeeae is somewhat similar to A. calypso ( Herbst, 1801) in the general appearance of the carapace. The original figure of A. calypso is schematic, but indicates the characteristic formation of the proto- and mesogastric regions that is distinguished in the photograph of the type specimen of A. calypso given by K. Sakai (1999: pl. 18 fig. E), and also in the illustration of the synonymous species, A. verrucosus Henderson, 1893 , given by Henderson (1893: pl. 36 fig. 15). In A. calypso , the protogastric region is imperfectly subdivided into three by two oblique furrows, with three-forked image, and the mesogastric region is symmetrically subdivided into anterior one and posterior two by wide furrows.

Actumnus obesus Dana, 1852 , reported by Rathbun (1906: pl. 11 fig. 2) and Edmondson (1962: fig. 28a) from the Hawaiian Islands, seem to be close to A. ngankeeae in the general form of the carapace, chelipeds and ambulatory legs. Dana (1852) described the specimen from Maui Island, the Hawaiian Islands, as “Anterolateral margin arcuate, almost entire, very faintly four-lobed, lobes minutely denticulate,” but as Rathbun (1906: 865) mentioned, no lobes are indicated by Dana (1855: pl. 14 fig. 3a). In spite of some discrepancies with the original description, Rathbun (1906) recorded the specimens from Molokai and Maui Islands as A. obesus , with a fine photograph that is apparently different from the Dana’s original figure. Later, A. obesus was recorded, without figures, by many authors from the wide area of the Indo-Pacific waters, most remarkably by Balss (1933) based on the specimens from Madagascar, the Persian Gulf, Western Australia, the Andamans, New Guinea, Samoa, Fiji and the Marquesas. It is highly probable that most of the old records followed Rathbun (1906). The holotype of A. ngankeeae is rather small, but such characters as the fully developed G1, the distinct carapace dorsal areolation and anterolateral armature, the stout chelipeds and the ambulatory legs embody the adult form and are valid for the subsequent identification of the species.

Distribution. The type locality is Haha-jima Island, Ogasawara (Bonin) Islands (26°35'19"N, 142°08'41"E), southern Japan.

Figures

FIGURE 1. Actumnus ngankeeae sp. nov., holotype, male (cb 4.6 mm, cl 3.5 mm), Ogasawara Islands, NSMT-Cr 31014. A, habitus, dorsal view; B, habitus, oblique dorsal view.

FIGURE 2. Actumnus ngankeeae sp. nov., holotype, male (cb 4.6 mm, cl 3.5 mm), Ogasawara Islands, NSMT-Cr 31014. A, chelae, outer view; B, pleon, ventral view.

FIGURE 3. A–C, Actumnus ngankeeae sp. nov., holotype, male (cb 4.6 mm, cl 3.5 mm), Ogasawara Islands, NSMT-Cr 31014. D, Pilumnus swajayai Ng & Rahayu, 2021, male (cb 5.8 mm, cl 5.0 mm), Ryukyu Islands, NSMT-Cr 31015.A, right chela, outer view; B, right fourth ambulatory leg, dorsal view; C–D, left G1, inner view.

FIGURE 4. Actumnus ngankeeae sp. nov., holotype, male (cb 4.6 mm, cl 3.5 mm), Ogasawara Islands, NSMT-Cr 31014. Color in life.