Sternostylus spiniger, Komai & Tsuchida & Fujiwara, 2023

Komai, Tomoyuki, Tsuchida, Shinji & Fujiwara, Yoshihiro, 2023, Squat lobsters of the superfamily Chirostyloidea (Decapoda: Anomura) from seamounts on the Nishi-Shichito and Mariana ridges, North-West Pacific off Japan, with descriptions of two new species, Zootaxa 5293 (1), pp. 45-73 : 61-68

Sternostylus spiniger n. sp.

[New Japanese name: Oni-tsuno-wara-ebi]

( Figs. 9–15)

Type material. Holotype. JAMSTEC 106770, male (cl 16.9 mm, DNA voucher), R / V Kaimei, KM20-10 C cruise, KM-ROV #132 , Ritto Seamount, 21°48.69’N, 142°02.67’E, 657 m, 8 December 2020, associated with antipatharian coral, suction sampler.

Paratype. JAMSTEC 108516, 1 female (cl 14.5 mm), R / V Natsushima, NT 07-17 cruise, ROV Hyper-Dolphin, dive #747, Bayonnaise Knoll, Izu-Ogasawara Arc, 802 m, suction sampler, 30 August 2007 , reared in Enoshima Aquarium (died on 15 June 2009).

Diagnosis. Carapace dorsally covered with prominent spines; gastric region with strong spines in hexagonal arrangement, with central spine and some additional small spines; cervical groove across about midlength of carapace. Sternite 4 with 2 pairs of prominent spines on anterior margin, ventral surface further with 3 pairs of unequal spines, posteriormost pair strongest. Pleon partly covered with spines; tergites of pleomeres 1 and 2 each with anterior row of spines; tergites of pleomeres 3 and 4 unarmed medially and armed laterally with field of spines adjacent to pleura; pleura of pleomeres 2–4 each with numerous spines, arranged in 2 main transverse rows, pleura 2 and 3 tapering to sharp point, pleura 4 tapering to subacute tip; pleomeres 5 and 6 tergites and pleura with numerous strong spines. Antennal scale strongly reduced to minute, basally articulated spine. Maxilliped 3 carpus extensor margin with small denticles, distal spine distinct; propodus unarmed. Pereopods 2–4 subequal in length; propodi each with row of movable spines along entire flexor margin, at most a single fixed spine proximally; dactyli 0.2–0.3 times as long as propodi.

Description. Carapace (excluding rostrum) ( Figs. 10A, 11A, B), 1.1 times as long as greatest width at midlength, narrowed toward anterior from midlength. Dorsal surface covered with prominent spines of various sizes across entire surface, more numerous on posterolateral portions. Rostrum about 0.3 times as long as carapace, spiniform, strongly ascending; rostral base laterally ridged, merging into orbital margin.Anterolateral spine small, submarginal. Gastric region convex, distinctly separated from cardiac region by deep transverse depression corresponding to posterior cervical groove, which is situated at about midlength of carapace; 6 spines in hexagonal arrangement with another spine in center, and additional 3 smaller spines, arranged in transverse row, posterior to central spine. Hepatic region with few spinules. Anterior branchial region separated by faint depression from posterior branchial region; large spine situated at centre of anterior branchial region. Posterior branchial region with a few large spines, more numerous and slightly smaller spines in lateral portion. Cardiac region with 2 pairs of prominent submedian spines (anterior pair larger than posterior pair) and few much smaller spines on anterior and lateral portions. Lateral margin of orbit rounded, not produced. Posterodorsal margin with 7 small spines. Pterygostomial flap covered with scattered small spines, and with strongly sinuous ridge (dorsally convex portion just above coxae of cheliped).

Thoracic sternum ( Figs. 11C, 12) about 1.3 times as wide as long. Sternite 3 with pair of small spines on anterior part. Sternite 4 with 2 pairs of large spines along anterolateral margin, posterior pair longer than anterior pair; surface grooved medially, with 1 pair of prominent spines preceded by 3 additional pairs of much smaller spines anterior to prominent spines. Sternite 5 with 4 pairs of small spines on ridged anterior margin; surface with shallow median groove. Sternite 6 with row of small spines or tubercles on anterior ridge; anterior ridge slightly produced at each lateral angle; surface with deep median pit. Posterior part of sternite 7 nearly perpendicular against horizontal plane of sternum.

Pleon ( Figs. 10B, 11D) partly covered with spines. Pleomere 1 tergite with transverse row of spines of various sizes; pleura poorly developed. Pleomere 2 tergite with anterior ridge bearing row of spines, extending to pleura as double row; posterior part spineless, with scattered stiff setae; pleura each tapering to acute tip. Pleomeres 3 tergite with cluster of spines on lateral portions, median portion unarmed, but with scattered stiff setae; pleura terminating in spine, surface with numerous scattered spines. Pleomere 4 tergite with few spines on lateral parts, medially unarmed; pleuon terminating in spine, surface with numerous spines arranged in 2 main rows. Pleomere 5 tergite covered with numerous spines, terminal margin of pleuron with few small spines. Pleomere 6 with numerous spines on surface, including 2 pairs of submedian spines, posterior margin with 3 large spines. Telson ( Fig. 11E) divided into two plates, laterally lobe-like; 1.1–1.2 times broader than long; posterior plate 1.1–1.3 times longer than anterior plate, terminal margin slightly emarginate medially.

Ocular peduncle ( Fig. 11A, B) not reaching rostral tip, 1.5 longer than wide; cornea globular, moderately dilated, subequal in length to ocular peduncle.

Antennular peduncle moderately slender. Basal article moderately inflated, distally with small blunt projection. Ultimate article slightly compressed, 1.8 times as long as penultimate article.

Antennal peduncle ( Fig. 11F) article 2 with small distolateral spine. Article 4 slightly widened distally, with small distoventral spine. Article 5 with small distoventral spine, 1.5 times as long as article 4. Antennal acicle greatly reduced to minute blunt piece. Flagellum falling slightly short of tip of extended antennular flagellum.

Maxilliped 3 ( Fig. 14A, B) endopod relatively stout. Coxa with distinct ventrolateral distal spine. Basis with few denticles on mesial face dorsally. Ischium with 15 small corneous spines along entire length of crista dentata. Merus with small distolateral spine; mesial face forming deep concavity. Carpus with prominent distolateral spine; lateral face with and small proximal denticle; flexor surface deeply concave. Propodus smooth along extensor margin; flexor surface with prominent swelling having cluster of stiff setae. Dactylus slightly shorter than propodus. Exopod

Chelipeds ( Figs. 9, 13A, 14C, D) elongate, 7.7–7.8 times as long as carapace, only sparsely setose, bearing numerous procurved spines arranged in longitudinal rows along entire length; in holotype, left cheliped slightly longer and stouter than right; more stout in male than in female. Coxa ( Fig. 12) with strong dorsolateral distal spine and ventromesial distal spine, and with scattered small spines on ventral surface. Ischium with strong dorsodistal and ventrodistal spines, with numerous small spines along ventral and dorsal margins and scattered over lateral surface. Merus distinctly longer than carpus (ischium and merus combined 1.7 carpus length), with 5 strong distal spines. Carpus subequal to palm in length, with 6 distal spines (dorsal spines strongest). Palm very slender, exceeding 10 times as long as wide; several small spines along proximal portion of fixed finger. Dactylus 0.5 times as long as palm; with small spines on proximal portion, otherwise unarmed; occlusal margins gaping proximally, with row of numerous tufts of stiff setae and row of distally diminishing teeth, proximal tooth prominent on each side (when closed, that of dactylus fitting to concavity formed by 2 opposite proximal teeth (proximal smaller) on fixed finger.

Pereopods 2–4 ( Figs. 9, 13B) slender, subcylindrical, subequal in length, with numerous spines arranged in longitudinal rows along entire length, also with sparse coarse setae. Coxae ( Fig. 12) with 1 dorsodistal and 2 ventromesial spines, both prominent; ventral surface with some scattered small spines; lateral and mesial surfaces with only few minute spinules. Ischia with 1 prominent dorsodistal and 1 subterminal spine on ventral margin. Meri about twice as long as carpi; distal spines strongest. Carpi 0.7–0.8 times as long as propodi. Propodi with spines along entire extensor margin; 2 rows of spines on mesial and lateral faces; flexor margin with subdistal pair of movable spines, followed by single row of movable spines in distal 0.7–0.8 length (10 or 15 on pereopod 2, 12 or 16 on pereopod 3, 12 or 17 on pereopod 4), proximal spines fixed, distal 7–9 spines longer than others, arranged in comb-like row. Dactyli ( Fig. 14E–G) short, compressed laterally, 0.2–0.3 length of propodi, flexor margin nearly straight, ending in strong, curved corneous claw preceded by 8 inclined spines along entire length, successively diminishing in size proximally.

Male with 2 pairs of gonopods (pleopods 1 and 2) similar to those illustrated for Sternostylus cavimurus ( Baba, 1977b) (cf. Baba 1977b: fig. 3b–e, as Gastroptychus ).

Colouration in life. Carapace, anterior 4 pleomeres and pereopods orange-red, rostral base of carapace whitish; pleomeres 5 and 6 whitish, orange-tinged, telson and uropods white; corneas black ( Fig. 9).

Distribution. Known only from West Mariana Ridge and Izu-Ogasawara Arc; 657–802 m deep.

Ecology. Schnabel (2020) overviewed ecological information on species of Sternostylus , which appear to be common associates of deep-sea corals. During the dive of KM-ROV #132 , we sometimes encountered individuals probably referrable to the present new species, which were on colonies of various octocoral species. The holotype was clinging on an octocoral colony before caught by suction sampler ( Fig. 15) .

Remarks. The present new species is most similar to Sternostylus hendersoni ( Alcock & Anderson, 1899a) , originally described from the eastern Arabian Sea. The latter species is supposedly widely distributed in the Indo-West Pacific, including Japan (cf. Baba et al. 1986; Baba 2005). As remarked by Schnabel (2020), however, there are some inconsistencies between the original description of S. hendersoni by Alcock & Anderson (1899a; as Ptychogaster ) and subsequent reports ( Baba 1988, 1991, 2005; Ahyong & Poore 2004; as Gastroptychus ) that require reappraisal. In reference to this, we rely only on the original description by Alcock & Anderson (1899a) and Alcock & Anderson’s (1899b: pl. 45, fig. 2) illustration of the holotype dorsal habitus for comparison. Alcock & Anderson (1899a: p. 23) clearly stated that there were two rows of carinae on the pleonal tergite 2 and “two transverse series of spines” on tergites 4 and 5 ( Alcock & Anderson 1899a: p. 23), and those features are depicted in the illustration of the holotype of S. hendersoni (cf. Alcock & Anderson 1899b: pl. 45, fig. 2). These features of S. hendersoni differ from those seen in S. spiniger n. sp., which has only one anterior row of spines on the tergite 2, no spines on the tergite 3 (except for lateral clusters of spines near pleura) and only 1 pair of small spines on the tergite 4 (also except for lateral clusters of spines near pleura). Secondly, Alcock & Anderson (1899a: 24) stated that the maxilliped 3 was unarmed. However, in our two specimens, the merus and carpus of the maxilliped 3 each bear a small spine on the dorsolateral margin. Furthermore, Alcock & Anderson (1899a: 24) mentioned the relative proportions of the walking legs, with the last walking leg (pereopod 4) being the longest “owing to the elongation of their propodite” and the propodus being 5 × as long as the dactylus. This was used to distinguish between S. hendersoni and S. investigatoris (as Ptychogaster ), with the latter having the first walking leg (pereopod 2) the longest and the dactyli “hardly more than a quarter the length of the propodites”. In S. spiniger n. sp., the pereopods 2–4 are subequal in the length; and the dactylus of the pereopod 4 is 0.2–0.3 times as long as the propodus.

Three COI gene sequences attributed to three species of Sternostylus , i.e., S. formosus ( Filhol, 1884) , S. iocasicus Dong, Gan & Li, 2021 , and S. rogeri ( Baba, 2000) , are available in the GenBank database ( Table 1). We sequenced the barcoding region of the COI gene from the holotype of S. spiniger n. sp. Genetic divergence among the four sequences is 13.9–23.7%, and that between the sequence of S. spiniger n. sp. and those of the other three species is 15.0–23.7% ( Table 4), supporting the four voucher specimens as representing four species. No genetic data is available for S. hendersoni thus far.

Four 16S rRNA gene sequences attributed to four species of Sternostylus ( S. formosus , S. iocasicus , S. investigatoris ( Alcock & Anderson, 1899a) and S. rogeri ) are available in the GenBank database ( Table 1). We sequenced a partial fragment of the 16S rRNA gene from the holotype of S. spiniger n. sp. Genetic divergence among the five sequences is 2.8–6.4% ( Table 5) and between the sequence of the new species and those of the other four species is 2.8–4.7%.