Pilumnus okutanii, Zou & Jiang & Wang & Wei & Sun & Basu & Chen & Kotecha & Li & Liu & Patel & Chen, 2024

Zou, Wenjun, Jiang, Qin, Wang, Yanling, Wei, Wenbin, Sun, Xiaodong, Basu, Karen, Chen, Qiuyu, Kotecha, Aachal, Li, Sijing, Liu, Ruyuan, Patel, Vaibhavi & Chen, Youxin, 2024, One New and Two Rare Species of the Genus Pilumnus Leach, 1816 (Crustacea: Decapoda: Brachyura: Pilumnidae) from Takase Submarine Bank off the Izu Islands, South of Tokyo, Japan, Bulletin of the National Museum of Nature and Science. Series A, Zoology (Philadelphia, Pa.) 50 (4), pp. 145-153 : 148-151

Pilumnus okutanii sp. nov.

( Figs. 2–3 )

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Material examined. Takase submarine bank ( Fig. 1 ; 34°26.78′N, 139°11.27′E, 99 m depth), northwest off Nii-jima I., Izu Is., holotype Ə (NSMT-Cr 11043; cb 4.8×cl 3.8 mm), 11-IX-1989, dredged by TV Umitaka-Maru of Tokyo University of Fisheries (now, Tokyo University of Marine Science and Technology ) and donated by T. Okutani .

Diagnosis. Small species. Carapace dorsal surface convex, ill-defined, smooth, shining without granules or hairs; anterolateral margin armed with two tuberculate teeth behind obtuse external orbital angle, and with one vestigial tooth on posterior slope of outer margin of second tooth. Both chelipeds heavy, unequal in size, but similar in shape, armed with numerous spiniform tubercles on carpi and palms. Ambulatory legs armed with long spines along anterior margins of meri, carpi and propodi.

Description of holotype (male). Carapace ( Figs. 2A , 3A ) relatively narrow among congeners, with 1.3 times as wide as long, hexagonal rather than ovate, equally convex dorsally for most part, curved downwards around frontal and lateral margins; dorsal surface ( Figs. 2A–B , 3A ) quite ill-defined, only with median longitudinal shallow furrow separating frontal and gastric regions of both sides, wholly smooth, shining, without granules or spinules. Frontal margin ( Figs. 2A–B , 3A ) not produced forwards, divided into two truncated lobes by a median small, wide notch; lateral end of each lobe weakly angulated, with a small notch close to lateral end; outer margin of frontal lobe longitudinal behind angulated lateral end, shallowly separated posteriorly from obscure supraorbital angle. Supraorbital margin thin, narrowly raised to form deep orbit, with minute notch at median part; external orbital angle not formed; infraorbital margin thin, raised similarly in supraorbital margin; inner infraorbital angle strongly developed as triangular tooth along inner one third of infraorbital margin, sharply pointed at tip.

Anterolateral margin of carapace ( Figs. 2A , 3A ) armed with two spine-tipped teeth and one minute, but distinct tooth at posterior slope of outer margin of second tooth; first tooth narrow, weakly curved, directed obliquely forward, second tooth similar to first tooth in shape, but much stouter. Outer margin of second tooth directly retreating toward lateral end of carapace posterior margin, without distinct boundary of carapace anterolateral and posterolateral margins; distance from tip of second tooth to lateral end of carapace posterior margin more than two times of distance from tip of second tooth to external end of supraorbital margin. Subhepatic region unarmed, smooth, weakly expanded, separated from infraorbital margin.

Third maxilliped smooth on external surface, comparatively narrow; ischium 1.5 times width of exopod; merus 1.5 times as long as ischium, weakly becoming wider distally, with obtuse antero-external angle.

Both chelipeds ( Fig. 2 ) comparatively large, slightly different in size, similar in armature, right cheliped being larger. Merus short, smooth, with strongly edged upper margin. Outer surfaces of carpus and chela, even proximal halves of both fingers, thickly covered with strong, curved spiniform tubercles ( Fig. 3B ); tubercles more or less arranged in some lines, tubercles of larger chela stronger, but shorter than those of smaller chela; those on outer lower surfaces of larger chela becoming conical in shape. Fingers of larger chela armed with two triangular teeth at median part and subsided by a small tooth each at basal and distal parts on each cutting edge; fingers of smaller cheliped irregularly toothed on cutting edges.

Ambulatory legs ( Figs. 2 , 3C–D ) slender, with sparse silky hairs on meri, carpi and propodi, but dactyli with dense fringe of silky hairs of various lengths; each merus with some equidistant spinules on anterior margin, distal part of anterior margin obtusely tuberculated; each carpus and propodus armed with some long, strong spines along anterior margin; each carpus of first two pairs with two rows of four strong spines along anterior margin, carpus of third pair with a row of strong spines and a row of small tubercles, carpus of fourth pair unarmed on anterior margin.

Pleon six-segmented, third to fifth pleonites becoming narrow, but lateral margins of sixth pleonite to proximal halves of telson subparallel to each other; telson 1.5 times as long as sixth pleonite, obtusely rounded along distal margin. G1 ( Fig. 3E ) not strongly curved, with small, strongly recurved sharp tip; marginal hairs at subterminal part not so long.

Remarks. The present new species is impressive in having the smooth and shining carapace and the ambulatory legs armed with long spines on the meri, carpi and propodi. The carapace dorsal surface completely lacking granules, spinules and hairs suggests that the new species may be referred to the genus Glabropilumnus Balss, 1932 and some related genera (cf. Galil and Takeda, 1988). In all the species of these genera, however, the carapace anterolateral teeth are less developed, not spiniform nor tubercular, and the ambulatory legs are entirely unarmed unlike the most of the Pilumnus species.

The vestigial third tooth of the carapace anterolateral margin of the new species is also seen in the genus Viaderiana Ward, 1942 , but the small third tooth is distinctly isolated from the second tooth not on the margin of the second tooth. In all the known Viaderiana species, the carapace are covered with longish single hairs or bundles of plumose hairs, and the long ambulatory legs are armed with some spines only on the merus anterior margin.

The present new species is somewhat aberrant in the genus Pilumnus in the carapace dorsal surface being entirely naked and the third anterolateral tooth being vestigial on the slope of the second tooth, but it seems reasonable for the present that the taxonomic position is in the genus Pilumnus rather than the other pilumnid genera until the detailed revision of Pilumnus and some related genera is made on the basis of adequate comparison of the known species.

The distinct rows of long spines along the anterior margins of the ambulatory meri, carpi and propodi link Pilumnus okutanii sp. nov. to P. acanthosoma Ng, 2000 from Taiwan (Kaohsiung County), P. armatus Komai and Motoh, 2012 from the Philippines ( Bohol Sea ), P. bohol Komai and Motoh, 2012 from the Philippines (Panglao), P. curvipenis Komai and Motoh, 2012 from Japan (Izu Islands), and P. dofleini Balss, 1933 from Japan (Sagami Sea and Ishikawa Pre- fecture in the Sea of Japan). All of these species are closely related to each other, but distinguished by the elaborate descriptions and figures by Komai and Motoh (2012). In P. dofleini and the related congeneric species mentioned above, the carapace dorsal surface is shallowly but rather distinctly divided into regions and covered with scattered obtuse or conical granules (vs. smooth and shining in the new species), and the anterolateral margin is armed with three, more or less lobate teeth or spines with some accessory spinules (vs. two spiniform teeth and a vestigial spinule in the new species).

In addition to the species of the P. dofleini group mentioned above, P. acutifrons Rathbun, 1906 is also close to the new species in the general appearance of the carapace, chelipeds and ambulatory legs. This species is known only by the holotype male from the French Frigate Shoals in the northwestern Hawaiian Islands, 30.5–31.5 m depth, and is also small in size (cb 3.8×cl 3 mm); the chelipeds and the ambulatory meri and carpi are armed with strong spines somewhat similar to those of the new species, although the anterior margins of the ambulatory propodi is unarmed unlike the strongly armed propodi of the new species. According to the original description of P. acutifrons by Rathbun (1906), the carapace dorsal surface is smooth and weakly areolated, with scattered tufts of hairs; each carapace anterolateral margin is armed with three spines, the first two of which are large and have broad bases and slender tips; the last is explained as to be small and bifid at the tip; the frontal lobes are deflexed, with the very oblique and subtruncated margins. The new species differs from P. acutifrons , as mentioned above, in the smooth, ill-defined dorsal surface of the carapace, the frontal lobes not extended forward, the vestigial third anterolateral tooth of the carapace, and the strongly armed propodi of the ambulatory legs.

Etymology. This species is named after Dr. Takashi Okutani, Professor Emeritus of the Tokyo University of Fisheries (now, Tokyo University of Marine Science and Technology), who enriched the National Museum of Nature and Science, Tokyo, with various marine invertebrate specimens from offshore banks.

Distribution. Known only by the holotype male from Takase submarine bank, 99 m depth, off the Izu Islands, south of Tokyo, Japan. One of the related species mentioned in the text, P. curvipenis Komai and Motoh, 2002 was also obtained from Takase not far from the type locality of P. okutanii .

Figures

Fig. 2. Pilumnus okutanii sp. nov., holotype, Ə (NSMT-Cr 11043; cb 4.8×cl 3.8mm). Habitus; dorsal (A), frontal (B) and ventral (C) views.

Fig. 3. Pilumnus okutanii sp. nov., holotype, Ə (NSMT-Cr 11043; cb 4.8×cl 3.8mm). Carapace, dorsal view (A); left chela, dorsal view (B); carpus and propodus of right second ambulatory leg, anterior view (C); right third ambulatory leg, dorsal view (D); left G1, sternal view (E). Scales A–B=2 mm; C–E=1mm.

Fig. 1. Map of the Izu Islands, showing submarine banks (broken lines).