Samadinia quindecimspina sp. nov.

[New Japanese name: Jyuugo-toge-tsuno-gani]

(Figures 4–8)

Material examined. Holotype. JAMSTEC 106777, male (31.6 × 19.5 mm), R/V Kaimei, KM20-10C cruise, KM-ROV dive #132, Ritto Seamount, northern West Mariana Ridge, 21°48.58'N, 142°02.66'E, 617 m, rock, suction sampler, 8 December 2020.

Diagnosis. Carapace with 15 prominent, sharp, upright dorsal spines (paired preorbital; paired hepatic; 3 gastric; 1 median cardiac; 1 median intestinal; on each side, 3 pairs branchial); hepatic spine conical. Male cheliped ischium unarmed on upper distal margin; merus rounded in cross section; carpus with blunt carina on upper outer margin; palm upper margin rounded, non-carinate. First ambulatory leg (second pereopod) merus armed with upper distal spine. First gonopod straight, distal part forming 3 unequal lobes, distolateral lobe well developed, directed distolaterally.

Description. Carapace (Figs. 4–7A) pyriform, 1.6 times longer than wide (except for pseudorostral spines); pseudorostral spines long (0.6 times as long as carapace), slender, slightly curved and prominently diverging outwards, cylindrical in cross-section. dorsal surface with regions defined by shallow sulcus, with dense fields of short curled setae on gastric region and lateral sides; dorsal spines also with covering of short pubescens, other parts almost glabrous; no covering of granules; 15 prominent spines present on carapace: obliquely erect preorbital spine on each side; gastric region with 3 erect spines (1 median and 2 shorter submedian spines, about 0.2 length of median spine, each on just lateral to median spine); hepatic spines remote from postocular tooth, slightly curved, directed dorsolaterally; branchial regions with 3 pairs of spines, 2 medial spines pointed dorsally, unequal in length, lateral epibranchial spines strongest, directed dorsolaterally; cardiac region elevated, with 1 median spine slightly shorter than gastric median spine; intestinal region with 1 median spine directed posterodorsally. Supraocular eave short; postocular tooth relatively short, anterior margin roundly truncate, inner surface shallowly excavate. No additional spine on hepatic regions; pair of low tubercles on gastric region anterior to submedian gastric spines. Suborbital margin confluent with lateral margin of basal antennal article. Pterygostomial area separated from subbranchial region by row of 3 tubercles.

Ocular peduncle (Figs. 4, 5) short, inflated basally with constriction proximal to base of cornea, surface covered with pubescens. Cornea rounded, darkly pigmented.

Inter-antennular septum with convex ventral margin. Epistome slightly depressed medially; anterior margin trilobate, median lobe extending anteriorly and connected with inter-antennular septum (Fig. 7A). Buccal cavity with anterior ridge divided by deep, slit-like median incision, either lateral ridge further divided by narrow incision; anterolateral margins each elevated into blunt lobe.

Antennules (Fig. 7A) folding vertically. Basal antennal article (Fig. 7A) distinctly longer than broad, with obtuse distolateral tooth; penultimate + ultimate peduncular articles slightly curved inwards, latter slightly shorter than former, with tuft of few stiff setae at distomesial angle, directed mesially; flagellum slightly longer than ultimate peduncular article.

Third maxillipeds (Figs. 7A, 8A) completely covering buccal cavity when closed; outer surface smooth, covered with short pubescence. Ischium slightly wider distally than proximally, with distinct, broad median sulcus. Merus subquadrate, depressed medially, anterolateral margin auriculiform. Exopod relatively stout, reaching to just proximal to distal edge of merus, with moderately long flagellum.

Chelipeds and ambulatory legs generally covered with short pubescens. Chelipeds (Figs. 4, 8B–D) subequal, chelae not particularly inflated. Merus subcylindrical without distinct ridges or crests, having conspicuous upper distal tooth ascending at about 45° against horizontal plane; upper surface with 2 low, blunt tubercles in proximal half; lower surface with 1 (right) or 2 (left) low, blunt tubercles proximal to midlength along outer side. Carpus short, cup-shaped; upper surface convex, with blunt carina along outer side. Chela 3.4 times as long as wide. Palm about twice as long as wide; upper margin rounded, non-carinate, proximal part expanded into prominent knob-like protuberance; lower margin also rounded, non-carinate, also with knob-like protuberance proximally; fixed finger nearly straight, not deflexed, glabrous, with row of small blunt teeth on occlusal margin. Dactylus 0.9 times as long as palm, slightly curved, glabrous; occlusal margin with row of low, blunt teeth.

Ambulatory legs (second to fifth pereopods) (Fig. 4) moderately long, slender; first leg longest, fourth leg shortest. Ischium unarmed on upper distal margin. Meri subcylindrical, with conspicuous, obliquely erect distal spine on upper distal angle in first leg, or blunt tubercle in second to fourth legs. Carpi short, upper surfaces slightly flattened. Propodi linear. Dactyli moderately slender, slightly curved, tapering to slender, naked corneous claws, 0.8 times as long as propodus in first leg.

Thoracic sternum (Fig. 7A, B) covered with short pubescence. First and second sternites completely fused, forming small triangular plate, fused with third sternite. Third and fourth sternites also fused but boundary between those sternites marked by shallow transverse groove; fourth sternite depressed anteriorly, anterolateral margins broadly concave to accommodate coxae of chelipeds, not markedly constricted. Sutures between posterior sternites deeply marked.

Pleon (Fig. 8E) narrowly triangular, with 6 free somites + telson, midline of each somite elevated into low protuberance or tubercle. Third and fourth somites each trapezoidal. Fifth and sixth somites subrectangular, subequal in length. Telson triangular, subequal in length to sixth somite, with almost straight lateral margins; proximal margin distinctly narrower than distal margin of sixth somite.

First gonopod (Fig. 8F–I) nearly straight, flattened, slightly tapering; distal part forming 3 lobes; distolateral lobe triangular with blunt apex, directed distolaterally, median lobe broadly convex with microscopic setae, distomesial lobe obtuse. Second gonopod (Fig. 8J) short, about a quarter length of first gonopod, tapering to rounded tip, subterminally with faint convexity on lateral margin; no distal segment or flagellum.

Colouration in life. Carapace dorsal surface generally whitish; spines on carapace, pseudorostral spines and pereopods pinkish, cheliped fingers white in distal parts (Fig. 4).

Genetic data. COI gene: LC856568 (659 bp); 16S rRNA gene: LC856564 (548 bp).

Etymology. The name is derived from combination of the Latin words, quindecim (= fifteen) and spina (= spine) in reference to the 15 prominent spines on the carapace of this new species. The name is used as a noun in apposition.

Remarks. The genus Samadinia was originally established for a single species S. longispinosa Ng & Richer de Forges, 2013 from French Polynesia and New Caledonia. A review by Lee et al. (2021) of species assigned to Rochinia resulted in the reassignment of 25 Indo-West Pacific species to Samadinia, significantly altering the original generic diagnosis. Since then, 11 new species have been described from various Indo-West Pacific localities in Samadinia (Richer de Forges et al. 2021; Takeda et al. 2022; Ahyong et al. 2023; Davie & Lee 2023; Lee et al. 2023) and one species has been further reassigned to the genus (Richer de Forges et al. 2021). Currently, Samadinia is represented by 38 species (DecaNet 2025).

Samadinia quindecimspina sp. nov. is similar to members of the S. pulchra group as discussed by Lee et al. (2021) and Ahyong et al. (2023), namely S. griffini (Davie & Short, 1989), S. jimlowryi Ahyong, Lee & Ng, 2023, S. livermorii (Wood-Mason in Wood-Mason & Alcock, 1891), S. paulayi (Ng & Richer de Forges, 2007), S. pulchra (Miers in Tizard, Moseley, Buchanan & Murray, 1885), and S. riversandersoni (Alcock, 1895), species which are characterised by the numerous long and sharp spines pointed outwards or upright on the carapace. The present new species has 15 long spines on the carapace (including preorbital spines), and in this regard, it appears closest to S. griffini, known from off Queensland, Australia. Other species have a different number of spines: 17 spines in S. paulayi (cf. Ng & Richer de Forges 2007: 1 A–C) and S. riversandersoni (cf. Alcock & Anderson 1896: pl. 22); and 20 or more spines in S. jimlowryi (cf. Ahyong et al. 2023: figs. 4B, C, H, 5A, B), S. livermorii (cf. Ahyong et al. 2023: fig. 6A, C, F), S. pulchra cf. (Miers 1886: pl. 4 fig. 1; Ahyong et al. 2023: figs. 2, 3), and S. spinosa (cf. Lee et al. 2023: fig. 13A, C). Samadinia quindecimspina sp. nov. is distinguishable from S. griffini by the following particulars: the submedian gastric spines are short, about 0.25 length of the median gastric spine in S. quindecimspina sp. nov. (Figs. 5B, 6A), rather than about 0.5 length in S. griffini (cf. Davie & Short 1989: fig. 12A); the cheliped ischium is unarmed on the upper distal margin in S. quindecimspina sp. nov. (Fig. 8B), rather than armed with one spine in S. griffini (cf. Davie & Short 1989: 181); and the first gonopod is straight with a more produced distolateral lobe in S. quindecimspina sp. nov. (Fig. 8F–I), whereas it is slightly curved mesially in the distal half with a less developed distolateral lobe in S. griffini (cf. Davie & Short 1989: 11 f, g). In addition, the followings may serve as distinguishing characters, although they might exhibit ontogenetic variation (note that the size of the holotypes of S. quindecimspinosa sp. nov. (cw 19.5 mm) and S. griffini (cw 18.5 mm) is relatively similar): the cheliped carpus has only a blunt carina only on the upper outer margin in S. quindecimspina sp. nov. (Fig. 8C), whereas it is provided with sharp carinae on the upper outer and the lower inner margins in S. griffini (cf. Davie & Short 1989: 181, figs. 11d, 12); the cheliped dactylus is proportionally longer at 0.9 times as long as the palm in S. quindecimspina sp. nov. (Fig. 8D) (versus 0.75 times as long as the palm in S. griffini; Davie & Short 1989: fig. 11D); and the male pleon is relatively broader in S. quindecimspina sp. nov. (Fig. 8E) than in S. griffini (Davie & Short 1989: fig. 11e). According to Davie & Short (1989: fig.11h), two prominent processes are figured anterior to the postocular tooth in the holotype of S. griffini, but in the holotype of S. quindecimspina sp. nov. there is only one process (= preorbital spine) (Fig. 7A). There is a need to verify if the illustration of Davie & Short (1989: fig. 11e) is correct.

Genetic data is still scarce for Samadinia. In this study, partial segments of the COI gene and the 16S rRNA gene were generated for S. quindecimspina sp. nov. The pairwise distances between the new species and available congeneric species (S. galathea (Griffin & Tranter, 1986b) and S. pulchra) based on the Kimura 2-parameter model are provided for information (Tables 1, 2).

Distribution. Presently known only from Ritto Seamount, West Mariana Ridge, at a depth of 617 m.