Hexapinus okinawa Sato, Reimer & Naruse, 2025, sp. nov.

Hexapinus okinawa sp. nov. [New Japanese name: Okinawa-mutsuashi-gani] (Figures 1, 2) Hexapinus simplex Rahayu & Ng, 2014: 418 (part, RUMF-ZC-2108). — Naruse et al. 2017: 19 (part, RUMF-ZC-2108) (not Hexapinus simplex Rahayu & Ng, 2014). Material examined. Holotype: RUMF-ZC-8244, female (12.0 × 17.6 mm), Red Beach, Kin Bay, Okinawa Island, Ryukyu Islands, Japan (26.446043, 127.911707), ca. - 10 m, SCUBA and yabbie pump from Chaetopterus tube, coll. T. Sato & H. Nakajima, 16 April 2022. Paratypes: RUMF-ZC-8245, 1 female (6.6 × 9.6 mm), same data as holotype; RUMF-ZC-2108, 1 ovigerous female (11.0 × 16.0 mm), off Camp Schwab, Oura Bay, Okinawa Island, Ryukyu Islands, Japan, coll. Naruse et al., 21 July 2010. Description. Carapace trapezoidal, CW 1.45–1.47 times as long as CL; dorsal surface convex strongly longitudinally, weakly transversely, sparsely pitted, with small granules beside lateral margins, with median shallow H-shaped depression, most of branchial region covered by very short, dense setae (Fig. 1A). Front bent ventrally, becoming triangular lobe in frontal view, forming part of interantennular septum; anterior most part of front, in dorsal view, fringed by relatively long setae, medially concave. Supra- and infraorbital margins smooth, entire, mesial end of infraorbital margin upwardly produced as subtriangular inner orbital tooth. Suborbital and subhepatic regions with scattered pits (Fig. 2B). Pterygostomial region with several oblique costae and small granules (Fig. 2B). Anterolateral margin entire, arcuate, with small granules, posterolateral margin having subtrapezoidal protuberance with bilobed lateral margin. Posterior margin weakly convex ventrally, lateral ends concave, accommodating small condyle of thoracic sternite 7 (Fig. 1A). Epistome with posterior margin keeled but interrupted medially, frontal surface sloping into buccal cavity in median part (Fig. 2B). Eye relatively small, superior surface covered by short setae, slightly movable; cornea pigmented. Antennular peduncle composed of three articles; basal antennular article set transversely, width as long as second article length (Fig. 2B). Antennal peduncle composed of five articles, set obliquely, entering orbit; basal antennal article largest, subrectangular, second and third articles slightly smaller than basal article, forth article elongated rectangular, fifth article thin, cylindrical (Fig. 2B). Mxp3 covering buccal cavity almost completely when folded (Fig. 2C). Ischium subrectangular, width as long as length, strongly convex mesiodistally; merus subrhomboidal, about 0.7 times as long as ischium; outer surface of ischium and merus with scattered small granules and covered by dense setae. Carpus, propodus and dactylus cylindrical; dactylus longest, tip of dactylus overreaching middle of ischium mesial margin when folded. Exopod relatively stout, width about 0.4 times of ischium width. Chelipeds stout, subequal (Figs. 1A–C, 2D). Merus short, unarmed, trigonal in cross-section; upper, lower and mesial margins with small granules, upper margin also fringed by setae; mesial surface smooth with line of setae parallel to margins. Carpus rhomboidal in dorsal view; distal half of inner margin fringed by small granules and dense setae; upper and lateral surface smooth, glabrous, roundly convex. Palm subrectangular, as long as movable finger; upper inner part forming weak ridge, proximal half fringed by granules and dense setae; upper surface smooth, glabrous, outer and lower surfaces covered by granules, outer surface also partially covered by setae, median part of inner surface with a few granules; movable finger straight, tip pointed; upper margin proximal half covered by spinules and long setae, inner surface with 1 shallow longitudinal groove and short, transverse costae, outer surface with 1 shallow longitudinal groove, occlusal margin with low, triangular teeth on proximal half; immovable finger straight, tip pointed, low, triangular teeth on proximal half of occlusal margin, outer surface with 1 shallow longitudinal groove fringed by setae, outer and mesial surfaces covered by dense setae proximally; narrow gap present between occlusal margins when movable finger closed. Ambulatory legs stout, P3 longest, P4 shortest, morphologically otherwise similar (Fig. 1A, B). Merus subrectangular, superior surface with scattered granules on extensor half, almost glabrous in P2, only median part glabrous in P3, almost covered by short setae in P4; inferior and flexor surfaces almost smooth and glabrous; extensor margin covered by short dense setae and sparsely fringed by long setae; flexor superior margin covered by short dense setae in P3 and P4, glabrous in P2, smooth in P2 and P4, granular in P3. Carpus subtriangular, about half-length of merus, entirely covered by short setae excepting flexor half of superior surface and median part of inferior surface, setae increase in density posteriorly. Propodus subrectangular, as long as carpus; almost entirely smooth and covered by short setae, increasing in density posteriorly; distal half of mesial margin serrate. Dactylus spiniform, slightly recurved, extensor margin with spinules, other part smooth, flexor margin fringed by relatively long setae, remaining part covered by short setae excepting glabrous superior surface. Ventral surface of female thoracic sternites sparsely pitted, sternite 1 to anterior part of sternite 4 covered by dense setae but glabrous in posterior part of sternite 4 to sternite 8; sternites 1 and 2 fused, subtriangular, medially slightly concave, gently sloping into buccal cavity; sternite 3 broad, separated from sternites 2 and 4 by transverse ridges; sternites 4–7 very broad, separated from other somites by distinct sutures; sternite 8 (Fig. 2E) small, triangular, set beside pleonal somite 1 (Fig. 1B, D). Sternopleonal cavity ovate, anterior end rounded, reaching ridge separating sternites 3/4 (Fig. 1D). Female vulva elliptical, posterolateral margin elevated and forming a hoodshaped protrusion (Fig. 1D). Female pleon separated into six pleonal somites and telson, fringed by relatively long setae, somite 4 widest, somite 6 longest; somites 1–5 transversely subrectangular, each somite of similar length, somite 1 longer than sternite 8; somite 6 subtrapezoidal, convergent distally, lateral margin weakly concave; telson roundly subtriangular, as long as somite 6 (Figs. 1B, 2E). Etymology. The specific epithet is named after Okinawa Island, the type locality of the new species. The name is used as a noun in apposition. Distribution. Hexapinus okinawa sp. nov. is so far known only from Kin Bay and Oura Bay, eastern coast of Okinawa Island, Ryukyu Islands, southwestern Japan. Ecology. The holotype and paratype of Hexapinus okinawa sp. nov. from Kin Bay were collected from a tube of a large individual of Chaetopterus sp. on a muddy bottom at approximately 10 m depth in an inner bay. Remarks. Hexapinus currently contains six species. Hexapinus okinawa sp. nov. is morphologically closest to Hn. simplex in having the posterolateral margin of the carapace with bilobed protuberances (Rahayu & Ng 2014: figs. 18, 20A). The present new species can be distinguished from Hn. simplex by its relatively narrow carapace with CW/CL ratio = 1.45–1.47 in Hn. okinawa sp. nov. (versus CW/CL ratio = 1.56–1.75 in Hn. simplex); dorsal surface of the carapace being partially covered by very short and dense setae in Hn. okinawa sp. nov. (Figs. 1A, 2A) (versus glabrous in Hn. simplex; Fig. 3B); and the superior surface of P4 merus being almost covered by velvety setae in Hn. okinawa sp. nov. (Fig. 1A) (versus partially covered in Hn. simplex; Fig. 3B). The results of molecular phylogenetic analyses also supported the difference between these two species where the K2P distances between the two species are 4.4–4.7% (Fig. 5; Table 2; also see Polyphyly of Hexapinus below). The only specimen of Hn. simplex recorded from Okinawa Island was RUMF-ZC-2108 (Rahayu & Ng 2014; Naruse et al. 2017). Our examination found that this specimen is actually Hn. okinawa sp. nov. Therefore, Okinawa Island is excluded from the distribution of Hn. simplex. Both the new species and Hn. simplex, the closest congener, were collected from the tubes of parchment tube worms of the genus Chaetopterus (Annelida). Their hosts appear to belong to two different species (T. Sato, personal observation). The host species of Hn. okinawa sp. nov. has a larger body size and tube itself (ca. 10 mm in diameter of the opening and ca. 30 mm in diameter of the widest point), and inhabits muddier bottoms in inner bays, whereas the host of Hn. simplex has a medium sized body and tube (ca. 5 mm in diameter of opening and ca. 15 mm in diameter of widest point), and inhabits generally sandy to sandy-mud bottoms (T. Sato, personal observation). The identification of each host Chaetopterus species should be a subject for future research. In the genus Hexapinus, Hn. latipes, the type species of the genus, was also collected from a tube of the large annelid (De Man 1888), although the host annelid species has never been identified. Hexapinus okinawa sp. nov. also appears similar to Hn. latipes as these two species have relatively large-sized bodies (CW easily overreaching 15 mm in adults) and the same general carapace shape. Hexapinus okinawa sp. nov., however, can be distinguished from Hn. latipes by having the dorsal surface of the carapace only sparsely pitted (Figs. 1A, 2A) (versus densely pitted in Hn. latipes; Fig. 3A); dorsal surface of the carapace being partially covered by very short and dense setae in Hn. okinawa sp. nov. (Figs. 1A, 2A) (versus glabrous in Hn. latipes; Fig. 3A); superior surface of P4 merus being almost entirely covered by velvety setae (Fig. 1A) (versus only partially covered in Hn. latipes; Fig. 3A); and a weakly bilobed lateral protuberance on posterolateral margin of the carapace in Hn. okinawa sp. nov. (Figs. 1A, 2A) (v