Pleistacantha stilipes, Ahyong, Shane T., Chen, Huilian & Ng, Peter K. L., 2005

Ahyong, Shane T., Chen, Huilian & Ng, Peter K. L., 2005, Pleistacantha stilipes, a new species of spider crab from the South China Sea (Decapoda: Brachyura: Majidae)., Zootaxa 822, pp. 1-10 : 2-7

Pleistacantha stilipes sp. nov. ( Figs. 1 , 2 )

Type material. HOLOTYPE: IOCAS­SSBV25­4, male (18.1 mm / 11.8 mm), Nansha Islands, South China Sea, 5°35.94’S, 112°05.93’E, 170 m, muddy sand, 31 July 1998. PARATYPES: ZRC 1968.2.14.7, 1 male (19.3mm / 12.8 mm), 1 female (17.5/ 14.9 mm), northern South China Sea, Hong Kong Fisheries Research Station Cruise 4/64, Stn 26, 97– 105 fms [177–192 m], mud, Granton trawl, RV Cape St. Mary, O. T. Chan, 4 August 1964.

Comparative material of Pleistacantha sanctijohannis Miers, 1879 : AM G5169, 1 ovigerous female (15.9 mm /13.0 mm), Japan; AM P3066, ovigerous female (16.4 mm / 13.5 mm), Sagami Bay, Japan, 183 m; AM P34572, 1 male (19.0 mm/ 14.3 mm), 2 ovigerous females (18.0/ 14.6–18.4 mm / 14.1 mm), Okinose, Sagami Bay, Japan, 180 m, Mortensen Pacific Expedition, 23 June 1914; AM P34573, 2 males (18.0/ 15.3–19.5 mm / 15.5 mm), 1 ovigerous female (16.3 mm /14.0 mm), Sagami Bay, Japan, 549 m, Mortensen Pacific Expedition, 29 June 1914.

Diagnosis. Rostral spines appressed medially for almost two­thirds length, slightly inclined dorsally; longer than half postrostral carapace length in males, almost half postrostral carapace length in female. Pereopod 5 merus distinctly longer than postrostral carapace length (1.3–1.4); dactylus sparsely setose.

Description. Carapace pyriform, postrostral carapace length longer than width, length 1.29–1.38 times width in males, 1.24 times width in female. Rostral spines 0.6 postrostral carapace length (male) or 0.4 postrostral carapace length (female); appressed for proximal two­thirds, distally divergent; slightly inclined dorsally; with 3 ventral (excluding basal) spines; with strong lateral basal spine and 3–4 smaller spines along lateral margin; orbital margin with series of short spines or acute tubercles, with strong preorbital spine at rostral base; supraorbital, intercalated and postorbital spines slender, widely spaced; dorsal surface covered with upright, slender spines and acute tubercles; with 3 anterior gastric (in transverse row), 1 mesogastric, paired cardiac, paired intestinal, cluster of 3 or 4 hepatic, and 3 spines distributed across anterior and posterior branchial margins, all stouter and standing almost twice as high as surrounding spines.

Epistome with spine lateral to antennal gland, and spine midway between antennal gland and anterolateral angle of buccal cavity; anterolateral angle of buccal cavity dentate.

Eyestalk with elongate, stiff setae and 2 or 3 small spines on anterior margin; interantennular spine bifurcated in distal third; distal margin of antennular sinus produced to stout, triangular ventrolaterally directed spine.

Basal antennular segment with row of 3 spines; basal antennal peduncle segment ventrally and distally spinose; penultimate segment distally and ventrally spinose; ultimate segment with ventral spine at midlength; flagellum extending slightly beyond rostral apices.

Third maxilliped merus as wide as ischium; meral surface spinose, with slender spine on either side of carpal articulation, anterolateral margin triangular with spinose margins; ischium with dentate margins, surface with conic tubercles and shallow longitudinal groove.

Third sternite of male spinous either side of midline; remaining sternites spinose laterally. Female sternites granulate. Male abdomen with 6 free segments; telson and sixth somite fused, suture not visible; widest at somite 2; all segments distinctly spinose. First pleopod of male gently curving outwards, subdistal papilla on inner margin distinctly longer than margin between papilla base and pleopod apex; obtuse triangular lobe proximal to subdistal papilla. Female abdomen with 6 free segments, surface spinulous; telson and sixth somite fused forming ‘operculum’; widest at somite 6.

Male and female chelipeds (P1) slender, similar; propodus, carpus and merus covered with well­spaced upright spines and long, stiff setae; distal spines of merus markedly longer than others; occlusal margins of dactylus and pollex dentate, with slight gape; cheliped length 1.6 postrostral carapace length (male), 1.4 postrostral carapace length (females); dactylus 0.6 propodus length.

Walking legs (P2–5) long, slender, descreasing in length posteriorly. P2 about 4 times postrostral carapace length. P2–4 propodus, carpus and merus with short, widely spaced, upright spines and stiff setae, longest and densest ventrally; dactyli of P2–3 with dense stiff setae, 0.6 propodus length; dactylus of P4 with short, soft setae; 0.8 propodus length. P5 merus 1.3–1.4 times postrostral carapace length (male), 1.3 times postrostral carapace length (female), with short, upright spines or acute tubercles; propodus and carpus relatively smooth; dactylus with sparse, short, soft setae; 0.8 propodus length (male), 0.7 propodus length (female).

Etymology. Named stilipes , derived from the Latin stilus, stake, pen, and pes, foot, alluding to the long, slender walking legs in comparison to other members of the genus.

Remarks. Of the eight species of Pleistacantha recognised by Griffin & Tranter (1986), Pleistacantha stilipes sp. nov. most closely resembles P. sanctijohannis Miers, 1879 , described from Japan, in having the rostral spines medially appressed for the proximal half to two­thirds, in the comparatively long walking legs in which the male P5 merus is longer than the postrostral carapace length, and in the morphology of the first gonopod in bearing the long subdistal papilla. The new species differs subtly but consistently from P. sanctijohannis in having generally longer walking legs as measured by the relative length of the P5 merus, in the inflation of the male chela, in having a relatively more elongate carapace, and the length of the rostrum. The P5 merus measures 1.3–1.4 times (male) or 1.3 times (female) postrostral carapace length in P. stilipes vs 1.2 times (male) or 0.9– 1.0 times (female) postrostral carapace length in P. sanctijohannis .

The inflation of the chelae is also different between P. stilipes and P. sanctijohannis . In both males of P. stilipes , the chela is slender and uninflated as in females. However, in the smallest male of P. sanctijohannis examined (14.3 mm postrostal carapace length, AM P34572), which is only marginally larger than the male paratype of P. stilipes (12.8 mm postrostral carapace length), the chelae are already distinctly inflated. Whether sexual dimorphism in cheliped size in P. stilipes occurs at all, or is manifested later than in P. sanctijohannis , remains to be determined.

The postrostral carapace length in P. stilipes is also proportionally greater than in similar sized P. sanctijohannis , having a slightly more slender appearance that is most pronounced in males. In male P. stilipes , the postorbital carapace length/width ratio is 1.29– 1.38 and 1.24 in the female, in contrast to 1.14–1.17 for males and 1.13–1.17 for females of P. sanctijohannis .

Lastly, the length of the rostral spines differs between P. stilipes and P. sanctijohannis . As observed in P. o r y x (see Griffin & Tranter 1986), the rostral length in P. stilipes and P. sanctijohannis appears to be sexually dimorphic, being relatively longer in males. In males of P. stilipes , the rostral spines are distinctly longer than half the postrostral carapace length and in the female measures almost half the postrostral carapace length. Conversely the rostral spines in P. sanctijohannis are about 0.33–0.40 postrostral carapace length in males and about 0.25–0.33 in females. The rostrum of the male holotype of P. sanctijohannis was reported as almost half the postrostral carapace length ( Miers 1879), and Parisi (1916) reported a rostral length of 0.25–0.50 postrostral carapace length for a series of males and females from Japan. Further study is required to determine the extent of rostral length variation between P. stilipes and P. sanctijohannis . Guinot & Richer de Forges (1982) provisionally recognised P. e re c t a Ihle & Ihle­Landenberg, 1931, described from Indonesia, as distinct from P. sanctijohannis based on the strongly upraised, near vertical inclination of the rostral spines. Irrespective of whether P. erecta is distinct from (Guinot & Richer de Forges 1982) or synonymous with P. sanctijohannis (see Griffin & Tranter 1986), it cannot be regarded as identical to P. stilipes owing to marked differences in rostral inclination.

Apart from sexual differences already noted, the type series is generally uniform. The right P5 of the holotype is shorter than the left P5; it appears to be regenerating.

Distribution. Presently known only from the Nansha Islands and northern South China Sea; 170–192 m on mud or muddy sand.

Figures

FIGURE 1. Pleistacantha stilipes sp. nov., male holotype (18.1 mm / 11.8 mm), Nansha Islands (IOCAS­SSBV 25 ­ 4). A, dorsal view. B, right lateral view. C, dorsal carapace. D, anterior ventral view.

FIGURE 2. Pleistacantha stilipes sp. nov., male holotype (18.1 mm / 11.8 mm), Nansha Islands (IOCAS­SSBV 25 ­ 4). A, cephalic region, ventral. B, cephalic region, dorsal. C, interantennular spine, anterior. D, right third maxilliped. E, right chela. F, abdomen. G, left P 4 dactylus. H, right P 5 dactylus. I, right first gonopod, abdominal view. J, apex of right first gonopod, sternal view. Scale A – G = 2 mm, H – J = 1 mm.