Pleistacantha sanctijohannis Miers, 1879, by original designation.

Holotype: male (cl 106.2 mm, pcl 87.4 mm, cw 87.0 mm) (CASAU), Pazhayar, coll. S. Ravichandran et al., 2017. Paratypes: 1 female (cl 83.9 mm, pcl 78.7 mm, cw 72.9 mm) (rostrum broken) (CASAU), 1 ovigerous female (cl 84.4 mm, pcl 69.8 mm, cw 71.5 mm) (all ambulatory legs broken off), 1 female (cl 79.5mm, pcl 65.21 mm, cw 66.9 mm) (each side two pairs of ambulatory legs broken) (CASAU), same data as holotype; 1 ovigerous female (cl 91.3 mm, pcl 81.2 mm, cw 75.5 mm), 2 females (cl 85.2 mm, pcl 75.0 mm, cw 70.4 mm; cl 87.3 mm, pcl 77.2 mm, cw 72.5 mm) (DABFUK), Tuticorin fishing port, India, coll. R. Ravinesh, March 2017; 1 female (cl 90.4 mm, pcl 80.2 mm, cw 73.9 mm) (DABFUK), Muttam Fishing Harbour, coll. A. B. Kumar, 14 October 2015. All localities from state of Tamil Nadu, India.

Pleistacantha moseleyi (Miers, 1885): 1 male (cl 82.5 mm, pcl 61.6 mm, cw 61.6 mm) (ZRC 2005.117), Maribohoc Bay, Panglao, Bohol. Philippines, 100–300 m, coll. T. J. Arbasto, November 2003–March 2004. Pleistacantha pungens (Wood-Mason, in Wood-Mason and Alcock 1891): 1 male (cl 94.0 mm, pcl 66.9 mm, cw 67.6 mm) (ZRC 2016.23), Ayeyarwady Delta, station 58, 14.3225°N, 93.7405°E, off Myanmar, Andaman Sea, 265–268.5 m, bottom trawl, coll. EAF-NANSEN Project (Myanmar Cruise), 9 May 2015; 1 female (cl 45.2 mm, pcl 30.8 mm, cw 29.8 mm) (ZRC 2016.24), Ayeyarwady Delta, station 68, 14.06216667°N, 94.31816667°E, off Myanmar, Andaman Sea, 455–457 m, bottom trawl, coll. EAF-NANSEN Project (Myanmar Cruise), 10 May 2015. Pleistacantha ori Ahyong & Ng, 2007: holotype male (cl 146.0 mm, pcl 129.1 mm, cw 106.3 mm) (ZRC 2006.158), off Durban, South Africa, coll. Oceanographic Research Institute, Durban, October 2004; paratypes: 1 male (cl 129.0 mm, pcl 115.0 mm, cw 92.7 mm), 1 ovigerous female (cl 120+ mm, pcl 106.6 mm, cw 83.9 mm), 1 spent female (cl 119.6 mm, pcl 104.9 mm, cw 84.5 mm) (ZRC 2006.0159), same data as holotype.

Name after the late Professor T. Kannupandi, an influential crustacean worker from the Centre of Advanced Study in Marine Biology in Annamalai University. The name, a shortened version of his family name, is used as a noun in apposition.

Carapace broadly pyriform, postrostral carapace length equal to or slightly longer than carapace width (ratio 1.0–1.1) (Figs 2A, B, 4C, D); dorsal carapace surface with short spines with relatively wider bases (Figs 2A, B, 4C, D, 5C, D); gastric regions strongly swollen (Figs 2A, B, 4C, D, 5C, D); branchial regions strongly swollen laterally and dorsally; medially separated by narrow space, area without spines, spines on margins of regions overlapping (Figs 2A, B, 4C, D, 5C, D); posterior carapace margin convex (Fig. 4C); rostrum relatively short; gently divergent, directly obliquely laterally, not curving upwards (Figs 2A, B, 4C, D, 5C, D, 6E); interantennular spine short, tip bifurcated with shallow concavity between short processes (Figs 6E, F); lateral margins of posterior margin of epistome strongly concave (Fig. 7G, H); ischium of third maxilliped short (Fig. 7I); adult male cheliped elongate, merus and chela slender (Figs 2A, 8F); surface of adult male chela mostly smooth, proximal part with short tubercles or granules, without long spines (Figs 2A, 8E, F); male anterior thoracic sternum relatively broad; surface with numerous blunt and sharp tubercles, never spines (Fig. 9E); male pleon transversely wide; distinctly trapezoidal; surface with numerous blunt and sharp tubercles, never spines (Fig. 9F); G1 relatively stout; distal part gently curved; subdistal dorsal papilla short (Fig. 10K–M).

Carapace broadly pyriform, postrostral carapace length almost equal to carapace width (Figs 2A, 4C). Rostral spines short, 0.2 times postrostral carapace length; basal half completely fused, medially gently divergent laterally, not curving upwards; dorsal surface with three small low dorsal spinules; lateral margin with one or two ventral (excluding basal) spines; with three equally spaced lateral spines, median one largest, at junction of diverging spine (Fig. 3C). Interantennular spine strongly bent downwards, surface concave, bifurcated distally, depth of bifurcation shallow, processes short; distal margin of antennular sinus produced to form prominent ventrolaterally directed spine (Figs 6E, F, 7G). Orbital margin with three large spines increasing in size posteriorly, including intercalated and postorbital spines (Fig. 4C). Hepatic spine large, anteriorly directed, with 2–4 small accessory spines (Fig. 4C). Dorsal surface covered with short conical spines with broad bases and acute tubercles (Figs 2A, 4C, 5C). Branchial regions markedly swollen dorsally and laterally, inner margins almost touching along carapace midline, with associated spines overlapping opposite region, regions separated by narrow longitudinal channel; cardiac region not prominently swollen, with two rows of six short spines in total; intestinal region not well demarcated, region appears depressed (Figs 2A, 4C, 5C). Posterior carapace margin convex (Fig. 4C). Sub-branchial region covered with short, stout spinules; pteryogostomial region with scattered sharp tubercles (Fig. 5C).

Proepistome with ventrally directed, laterally flattened tooth, tip rounded; margin lateral to antennal gland aperture (infraorbital margin) with 2 long spines; anterolateral angle of buccal cavity flared, lobiform, margin with two or three low teeth (Fig. 7G, H). Eye short, when folded back into ‘orbit’, not reaching antennal gland aperture; ocular peduncle short, with 2 granules on subdistal surface adjacent to cornea. Basal antennular article with two short spines. Basal antennal article elongate, rectangular, outer margin with two short spines, mesial margin with two short spines; next article elongate, with two sharp tubercles (Fig. 7G). Epistome wide; posterior margin with lateral margins strongly concave; median lobe subtruncate, separated by deep median fissure, separated from lateral margin by V-shaped cleft (Fig. 7G, H).

Third maxilliped merus almost as wide as ischium; meral surface spinose, with long slender spines on either side of carpal articulation, anterolateral angle triangular, produced, with spinose margins; ischium short, with dentate margins, surface with two longitudinal rows of tubercles separated by shallow median sulcus; exopod relatively slender, outer surface and outer margin each with row of short spines (Fig. 7I).

Male cheliped elongate, slender, symmetrical (Fig. 2A). Male chela elongated, stout, not distinctly inflated; distal two-thirds relatively smooth, proximal third with tubercles and low spines; occlusal margins of dactylus and pollex with blunt, obtuse teeth, not forming distinct gape when closed; carpus and merus with numerous sharp tubercles and granules along margins and surfaces; merus elongate, slender with distal half wider than proximal part (Figs 2A, 8E, F). Ambulatory legs (P2–5) long, slender, decreasing in length posteriorly (Fig. 2A). Surfaces of propodus, carpus and merus of P2–4 granular, with short tubercles or granules, not spinose; dactylus covered with dense soft setae and corneous tip (Fig. 2A).

Anterior thoracic sternum relatively wide transversely (Fig. 9E). Thoracic sternites 1 and 2 fused, forming acutely triangular process; separated from sternite 3 by prominent ridge with concave surface; sternites 3 and 4 fused, anterior part constricted at junction of sternites; sternite 4 with low obliquely transverse ridge lined with tubercles, anterior surface with more prominent tubercles; surfaces of sternites 5–7 with scattered tubercles, some relatively sharp; sternopleonal cavity reaching to suture between sternites 4 and 5 (Fig. 9E, F). Male pleon with six free somites and telson; somites 4–6 trapezoidal, wide; widest at somites 2 and 3; surface tuberculate but not spinose (Fig. 9E, F).

G1 gently curving outwards, relatively shorter, with distal tenth more distinctly curved; subdistal papilla on inner margin short, triangular, shorter than length between papilla base and tip (Fig. 10K–M). G2 short, with basal part dilated; distal part approximately bifurcate, short (Fig. 10N).

The adult females differ from the holotype male in possessing a proportionately shorter rostrum and chelipeds (Figs 1B, 2B, 11A). In addition, the surfaces of the chelipeds and ambulatory legs are covered with more prominent spines and sharper tubercles, with these structures all appearing distinctly spinose (Figs 1B, 2B, 11A, B). The spines and tubercles on the carapace of females (Fig. 4D, 5D) also tend to be relatively more acute compared to those on the male (Fig. 4C, 5C). Pleistacantha species are typically sexually dimorphic in these respects (see Grindley 1961; Ahyong and Ng 2007). The female pleon is very broad, and while all the somites and telson appear to be free, they are quite rigid due to their strongly convex shape, forming a dome-like structure (Fig. 11D). The vulvae are large, subovate and positioned on the anterior surface of sternite 6 (Fig. 11D).

Most of dorsal carapace surface of carapace orange to orange-red (Fig. 1); male chelipeds and ambulatory legs reddish-brown on dorsal surface except for white fingers (Fig. 1A); female chelipeds and ambulatory legs orange and white (Fig. 1B); ventral surfaces dirty white.

Ahyong and Ng (2007: 72) recognized a group of three large species of Pleistacantha readily distinguished from congeners by possessing a “relatively uniform dorsal carapace spination, in which the dorsal spines are of similar length rather than having several gastric and branchial spines markedly longer than the remainder, in combination with divergent rather than medially appressed rostral spines, and a deeply bifurcate interantennular spine.” Pleistacantha moseleyi (Miers, 1885) is known from the Philippines in the western Pacific (Guinot and Richer de Forges 1982, 1986; Ahyong and Lee 2006) while P. pungens Wood-Mason, in Wood-Mason and Alcock 1891) and and P. ori Ahyong & Ng, 2007, are known from the eastern and western Indian Ocean basins, respectively (Guinot and Richer de Forges 1982, 1986; Ahyong and Ng 2006, 2007). Although most extant literature cite the Challenger material as “Miers, 1886”, a number of names like Pleistacantha were actually validated by the same author a year earlier (see Froglia and Clark 2011).

The present new species most closely resembles P. moseleyi and P. pungens in having the branchial regions relatively more swollen, such that the inner margins are close to each other in the midline of the carapace with the gastric and cardiac regions forming an approximate “hour-glass” shape (cf. Ahyong and Ng 2007: 73). In P. kannu sp. n., however, the branchial regions are more prominently swollen laterally and dorsally, so much so that the inner margins are now almost adjacent to each other, forming a narrow channel between them (Fig. 4C, D). The margins are so close that the adjacent spines overlap each other (Fig. 4C, D). As the specimens are all comparable in size, the difference in carapace inflation is not size-related. In addition, P. kannu can be distinguished from these three congeners in its relatively broader carapace (Figs 2A, B, 4C, D), the spines on the carapace are relatively broader and shorter even in large specimens (Figs 2A, B, 4C, D, 5C, D), the posterior carapace margin is distinctly convex (Fig. 4C), the lateral margins of the posterior margin of the epistome are prominently more concave (Fig. 7G, H), the ischium of the third maxilliped is proportionately shorter (Fig. 7I), the rostral spines are proportionately shorter, gently divergent, not curving upwards (Figs 2A, B, 4C, D, 5C, D, 6E), the interantennular spine is short and while the tip is bifurcated, the processes are short (Figs 6E, F), the male anterior thoracic sternum is proportionately broader (Fig. 9E), the adult male chelipeds are proportionately more slender and longer, with the chela elongate rather than distinctly inflated and mostly smooth (Figs 2A, 8F), the male pleon is broader and more trapezoidal in shape (Fig. 9F), and G1 is not elongate with the distal part not sharply curved and the subdistal dorsal papilla is short (Fig. 10K–M) (Table 1).

Pleistacantha kannu may be conspecific with a taxon discussed in Kazmi (1997) and identified as “Pleistacantha adenicus”. In a review of Pakistani spider crabs, Kazmi (1997) discussed the identity of a berried female of Pleistacantha measuring 68 mm in carapace length collected from the mouth of the Gulf of Aden in her collection. In her abstract, she noted that “The occurrence of the genus Pleistacantha in the Arabian Sea and its adjacent gulfs is discussed in detail due to presence of a unique female which was taken by Fridtjof Nansen Cruise in 1977. This seems to be an undescribed species. At the moment the female is just given as Pleistacantha sp1 till more specimens obtained determine its position.” (Kazmi 1997: 79). Later in the paper, the heading for her discussion was written as “DESCRIPTION OF UNDESCRIBED PLEISTACANTHA. Pleistacantha sp. 1” and she commented that “My specimen is still unnamed and is given here as P. sp. 1” (Kazmi 1997: 82). She described and compared the specimen with related congeners at length but she clearly opted not to name the taxon due to a lack of additional material. In her caption for her second figure, she wrote “Fig. 2. Pleistacantha sp.1, cl.68mm” (Kazmi 1997: 84) while in her table of species discussed, she listed the taxon as “Pleistacantha sp.1” (Kazmi 1997: 86). However, in her caption for the first figure of this species, she wrote “Fig. 1. Pleistacantha adenicus n. sp., holotype, cl.68 mm” (Kazmi 1997: 83). In the context of her comments, “Pleistacantha adenicus” cannot be regarded as an available name. The use of this new name is clearly an accident as her published intentions in the paper are clear. She probably originally wanted to name the new species but decided against this later on before publication, but forgot to remove the name from the first caption. Under the terms of reference for Article 15.1 which states that names regarded as conditionally published after 1960 are not available (ICZN 1999), “Pleistacantha adenicus Kazmi, 1997” must therefore be regarded as a nomen nudum.

Pleistacantha kannu is probably the same species as Kazmi’s “P. adenicus”. Both taxa share the character of the highly inflated branchial regions with the inner margins almost meeting along the midline of the carapace (Kazmi 1997: fig. 1), and their rostral, epistomal, and third maxilliped features (Kazmi 1997: fig. 2A–C) also agree. However, in lieu of examining specimens, and given that her specimen was from the Arabian Sea (the present material is all from southern India), we cannot be certain.

The prominently swollen branchial regions of Pleistacantha kannu are noteworthy, and may suggest that they also live in low oxygen deep-sea habitats, an area known as the “oxygen minimum zone” in the Indian Ocean (see Creasey et al. 1997). This is the habitat apparently favoured by the inachid Encephaloides armstrongi Wood-Mason, in Wood-Mason and Alcock 1891, which has even more disproportionately swollen branchial regions, presumably to aid in respiration in such zones (see also Kazmi and Moazzam 2014; Dash et al. 2017). This may also be true of a recently described deep-water homolid, Moloha tumida Ng & Kumar, 2015, described also from the western Indian Ocean (Ng and Kumar 2015).