Pleistacantha rubida ( Alcock, 1895 )

Ahyong, Shane T., Mitra, Santanu & Ng, Peter K. L., 2019, Redescription of Echinoplax rubida Alcock, 1895, a valid species of Pleistacantha from the Andaman Sea (Crustacea: Brachyura: Oregoniidae), Raffles Bulletin of Zoology 67, pp. 391-395 : 392-394

Pleistacantha rubida ( Alcock, 1895)

( Figs. 1 , 2A )

Echinoplax rubida Alcock, 1895: 179 . — Alcock & Anderson, 1896: pl. 17 fig. 2, 2a. — Sakai, 1963: 14, 17.

Pleistacantha rubida . — Serène & Lohavanijaya, 1973: 42, 43, 44. — Sakai, 1976: 173. — Guinot & Richer de Forges, 1982: 1101; 1986: 129, pl. VII fig. E. — Kazmi, 1997: 85. — Ahyong et al., 2005: 1. — Ng et al., 2008: 112.

Holotype. ZSI IM9480 /9, female (cl 33.8 mm, pcl 26. 2 mm, cw 20.9 mm), Andaman Sea, 90–177 fathoms (165–324 m), RIMSS Investigator .

Description. Carapace pyriform, postrostral carapace length 1.28× width. Rostral spines 0.26 pcl; widely separated basally, divergent; unarmed dorsally; with 2 ventral (excluding basal) spine; with 2 or 3 well-spaced lateral spines. Orbital margin with 5 or 6 well-spaced spines increasing in size posteriorly: 1 or 2 small preorbital spines near rostral base, 2 larger, slender supraorbital, slender intercalated and postorbital spines. Hepatic spine large, anterolaterally directed, with 2 or 3 small accessory spines basally. Branchial margins lined with short spines and 2 larger spines: 1 each between pereopod 1/pereopod 2, and pereopod 2/pereopod 3 articulations, respectively. Dorsal surface covered with short conical tubercles of similar size in addition to widely spaced, long, upright spines, longest on gastric and anterior branchial regions. Branchial regions neither markedly swollen nor contiguous in the carapace midline, distinctly separated by cardiac region; cardiac region width at least half length.

Epistome with small ventrally directed spine lateral to antennal gland aperture, and cluster of few acute tubercles midway between antennal aperture and anterolateral angle of buccal cavity; anterolateral angle of buccal produced to prominent anterolaterally directed spine, extending beyond hepatic margin beneath hepatic spine.

Eye, when folded back into ‘orbit’, extending posteriorly slightly beyond level of antennal gland aperture. Eyestalk with few, stiff setae on anterior margin.

Interantennular spine bifurcated in distal quarter, slightly divergent; distal margin of antennular sinus produced to slender, ventrolaterally directed spine. Antennal peduncle article 1 ventrally and distally spinose; article 2 distally and ventrally spinose; article 3 unarmed; flagellum extending slightly beyond rostral apices.

Maxilliped 3 merus as wide as ischium; meral surface spinose, with slender spine on either side of carpal articulation, anterolateral margin triangular with spinose margins; ischium margins dentate, surface with conic tubercles and shallow longitudinal groove.

Thoracic sternites 3 and 4 weakly tuberculate. Female abdomen forming operculum, with 6 somites and telson, widest at somite 6; surface spinulous.

Cheliped (pereopod 1) length 1.29 pcl; slender, markedly spinous; palm, carpus and merus with longitudinal rows of slender, upright spines, longest on lower and inner margins; propodus palm length 3.61× height; dactylus 0.53× propodus length; occlusal margins of dactylus and pollex dentate, with slight gape.

Walking legs (pereopods 2–5) long, slender, decreasing in length posteriorly; articles spinous, with longitudinal rows of long, widely spaced, upright spines along lower and lateral margins, particularly on merus; dactyli with dense setae and corneous tips; dactyli of pereopods 2 and 3 0.62× propodus length, those of pereopods 4 and 5 0.54× propodus length. Pereopod 2 2.84 pcl, merus 0.97 pcl. Pereopod 5 1.52 pcl; merus 0.62 pcl.

Remarks. Pleistacantha rubida closely resembles P. oryx , with which it has been synonymised, in the combination of the short distal bifurcation of the interantennular spine, basally divergent rostral spines, and dorsal carapace ornamentation consisting of several longer spines within a field of much shorter spines or acute granules. The two species, however, differ most clearly in the armature of the anterolateral margin of the buccal cavity, but also in the length and curvature of the rostral spines. The anterolateral margin of the buccal cavity is produced to a prominent anterolaterally directed spine in P. rubida ( Fig. 2A ), extending almost to the hepatic margin in the holotype. In contrast, the anterolateral buccal margin of P. oryx is only dentate but not produced to a long spine ( Fig. 2B ). The rostral spines of the holotype of P. rubida are distinctly shorter than similarly-sized P. oryx (0.26 versus> 0.39 pcl), in which the rostral spines are either straight or curved slightly outwards (cf. Fig. 1A, B , Sakai, 1965: fig. 10A).

Pleistacantha rubida remains known only from the single type specimen, the adult female holotype from the Andaman Sea ( Alcock, 1895). In exploring the possible validity of P. rubida, Guinot & Richer de Forges (1986) reported several specimens in the Natural History Museum, London, from the western Indian Ocean that were identified as P. rubida by W.T. Calman in 1916. Sakai’s (1963, 1976) purported examination of a cotype of E. rubida from the British Museum is evidently based on these western Indian Ocean specimens, rather than the holotype of E. rubida in the ZSI. Although similar in many respects to P. rubida and P. oryx in carapace spination and the shallowly bifurcate interantennular spine, the western Indian Ocean form represents a separate species, currently under study.

The only other species of Pleistacantha known from the Andaman Sea, P. pungens , is readily separated from P. rubida by the dense, even covering of short dorsal carapace spines ( Ahyong & Ng, 2007: fig. 6; Ng et al., 2017: fig. 4B) (versus several long spines interspersed among acute tubercles in P. rubida ; Fig. 1A, B ), the weakly dentate anterolateral margin of the buccal cavity ( Ng et al., 2017: fig. 7D, E) (versus produced to a strong spine in P. rubida ; Fig. 1C , 2E ), and the much larger adult size. Pleistacantha pungens matures at sizes exceeding pcl 60 mm, rather than less than pcl 30 mm in P. rubida . Males of P. rubida are as yet unknown.

Distribution. Presently known only from the Andaman Sea; 165– 324 m.

Figures

Fig. 1. Pleistacantha rubida (Alcock, 1895), female holotype, cl 33.8 mm, pcl 26.2 mm, cw 20.9 mm, Andaman Sea, ZSI IM9480/9. A, dorsal habitus; B, carapace, dorsal view; C, anterior cephalothorax and oral field, ventral view; D, anterior cephalothorax, anterior view; E, abdomen and ventral view.

Fig. 2. Anterior margin of buccal cavity. A, Pleistacantha rubida (Alcock, 1895), female holotype, cl 33.8 mm, pcl 26.2 mm, cw 20.9 mm, Andaman Sea, ZSI IM9480/9; B, Pleistacantha oryx Ortmann, 1893, male, cl 43.5 mm, pcl 30.1 mm, cw 22.5 mm, Taiwan, ZRC 2001.0065.