Phasmon typhlops sp. nov. Figs 1, 2, 3, 4

Type material.

Holotype: SYSBM 001982, male (14.4 × 9.0 mm), Leiping Town, Daxin County, Chongzuo City, Guangxi Province, China, 22.65°N, 107.10° E, subterranean karst stream in cave, baited trap, coll. local collector, December 2019. Paratype: AM P.105524, female (22.1 × 13.7 mm), Leiping Town, Daxin County, Chongzuo City, Guangxi Province, China, karst spring, baited trap, coll. local collector, September 2018.

Description.

Carapace broad, about 1.6 times as wide as long; fronto-orbital width about twice width of posterior margin; regions indistinct, dorsal surface slightly convex; surface finely pitted (Fig. 1A). Frontal margin weakly sinuous, continuous with supraorbital margin, forming almost straight transverse margin in dorsal view (Figs 1A, B). Epigastric cristae and postorbital cristae almost indiscernible (Figs 1A, 2A). Branchial regions slightly swollen (Figs 1A, 2A). Cervical groove shallow (Fig. 1A). Mesogastric region slightly convex (Fig. 1A). External orbital angle obsolete, outer margin convex, almost indistinguishable from anterolateral margin (Figs 1A, 2A). Epibranchial tooth granular, inconspicuous (Fig. 1A). Anterolateral margin lined with 15-20 small, single or partially fused granules. Posterolateral margin posteriorly convergent (Fig. 1A); posterolateral surface generally smooth (Fig. 1A). Orbits shallow; supraorbital margins weakly cristate, infraorbital margins lined with granules (Fig. 2A). Eyes almost immobile, greatly reduced, tapering, length about half orbital width; peduncle short, stout; cornea vestigial, surface without facets, unpigmented (Figs 2A, 3F). Sub-orbital, pterygostomial and sub-hepatic regions generally smooth, pitted (Fig. 2A). Antennules large, folded within broad fossae; antennae very short (Fig. 2A). Median lobe of epistome posterior margin broadly triangular, lateral margins sinuous (Fig. 2A).

Maxilliped 3 merus subtrapezoidal, with median depression, width about 1.2 × length; ischium subtrapezoidal with shallow median sulcus, distomesial margin rounded, width about 0.6 × length. Exopod reaching proximal one-third of merus; flagellum longer than half ischium length (Fig. 3A).

Chelipeds (pereiopod 1) subequal (Figs 1, 3D, E). Merus trigonal in cross section; margins slightly crenulated, surface generally smooth (Figs 1A, 2A). Carpus with sharp spine at inner-distal angle (Fig. 1 A). Major cheliped palm length about 1.5 × height; dactylus 0.9 × palm length (male) (Fig. 3D, E), as long as palm (female). Palm surface pitted (Fig. 3D, E). Dactylus as long as pollex (Fig. 3D, E). Occlusal margin of fingers with 18-20 irregular blunt teeth, without gape when closed (Fig. 3D, E).

Ambulatory legs (pereiopods 2-5) slender with very sparse short setae (Fig. 1). Pereiopod 3 merus 0.9 × CL (male) (Fig. 1A), 0.8 × CL (female) (Fig. 1B). Pereiopod 5 propodus length 2.8 × height (male) (Fig. 1A), 3.4 height (female) (Fig. 1B), shorter than dactylus; dactylus length 6.1 × height (male) (Fig. 1A), 6.2 × height (female) (Fig. 1B).

Male thoracic sternum generally smooth, pitted; sternites 1-4 width about 2.3 × length; sternites 1, 2 forming indistinguishably fused, broad triangle; fused sternites 1, 2 demarcated from sternite 3 by shallow transverse sulcus; sternites 3, 4 fused without indication of demarcation except for shallow lateral notch (Fig. 2B). Male sterno-pleonal cavity reaching anteriorly slightly beyond level of cheliped coxa articular condyle (Fig. 2B); deep median longitudinal groove between sternites 7, 8 (Fig. 2D). Male pleonal locking tubercle positioned at mid-length of sternite 5 (Fig. 2D). Female vulvae reaching sutures of sternites 5/6 anteriorly but not posteriorly to sutures of sternites 6/7, positioned widely apart from each other (Fig. 2F).

Male pleon broadly triangular; somites 3-6 progressively narrower; somite 6 width approximately 2.7 × length; telson width 1.6 × length; lateral margins slightly convex, apex rounded (Fig. 2C). Female pleon subovate (Fig. 2E).

G1 tapering, slightly sinuous, tip exceeding pleonal locking tubercle but not reaching suture between thoracic sternites 4/5 in situ (Fig. 2D); proximal segment length about 2.3 × length of distal segment (Figs 3C, 4A, B). Distal segment slender, tapering anteriorly, slightly inclined towards midline; tip pointed upwards in dissected view (Figs 3C, 4A, B). G2 slender, almost straight, proximal portion with distal two-thirds subcylindrical, length about 2.4 × length of distal portion (Figs 3B, 4C, D); distal portion flattened, apex acute, proximally with small triangular lobe.

Etymology.

The species name is derived from the Greek words “typhlos” and “ops”, meaning “blind” and “eyes”, respectively. It refers to the greatly reduced and non-functional eyes of this species.

Colour in life.

Pale yellowish-white all over (Fig. 3F).

Habitat.

Phasmon typhlops gen. nov. et sp. nov. occurs in subterranean karst streams, but little is currently known about its precise habitat. According to the collector, subterranean streams in the dark zone of caves appear to be the primary habitat of P. typhlops sp. nov., where it has been found in shallow and still water as well as flowing streamways. However, some specimens have also been captured at night from a karstic spring that is immediately connected to the more extensive subterranean karst system. We only examined the two type specimens, of which the holotype was collected from the former habitat and the paratype from the latter. An epigean species, Lacunipotamon cymatile, inhabits the areas immediately adjacent to the spring and has been observed to prey on Phasmon typhlops gen. nov. et sp. nov. (Huang et al. 2020c).

Distribution.

Chongzuo City, Guangxi Province, China.

Remarks.

Phasmon typhlops gen. nov. et sp. nov. can be considered a true stygobite owing to its stygomorphic features, in particular the strong reduction of the eyes, body depigmentation and slightly elongated appendages, which are consistent with its subterranean lifestyle (Holthuis 1986; Ng and Goh 1987). Apart from P. typhlops gen. nov. et sp. nov., Diyutamon cereum and Cerberusa caeca are the only other apparently blind stygomorphic potamid crabs known. We have not directly examined the eyes of C. caeca, but those of D. cereum and P. typhlops are unpigmented and the cornea is vestigial and without facets. Although we cannot exclude the possibility that the eyes of D. cereum and P. typhlops are capable of light detection, the absence of pigmentation or ommatidial facets indicates that the eyes are incapable of image formation. The enlarged antennules as present in Phasmon gen. nov. are otherwise seen in only a few cavernicolous freshwater crabs such as the gecarcinucids Sundathelphusa waray Husana, Naruse & Kase, 2009, and S. lobo Husana, Naruse & Kase, 2009 (Husana et al. 2009: figs 2B, 5B), and are likely a sensory compensation for the loss of vision (Culver et al. 1995). Other than these two species, there are other stygomorphic gecarcinucids from Asia, but these can be separated from the new species by obvious family-level characters.

Sexual dimorphism is evident in our two specimens of P. typhlops: the smaller male holotype has proportionally longer but stouter legs in comparison to the larger female. The anterior carapace of the larger female is also proportionately wider than the posterior than in the male. Although the differences in leg proportions follow the pattern of sexual dimorphism observed in other potamids (e.g., Huang et al. 2020b), whether this carapace difference is due to size, sex or general variation remains to be determined.

Taxonomically, the most striking features of Phasmon gen. nov. are its very wide carapace (CW/CL=1.6; Fig. 1) and wide male anterior thoracic sternum (width 2.3 × length; Fig. 2B). These characters combined immediately separate Phasmon gen. nov. from all other potamid genera. Diyutamon cereum occurs in Guizhou, which is relatively close to the type locality of P. typhlops gen. nov. et sp. nov. Phasmon typhlops gen. n. et sp. n. can be separated from D. cereum by its proportionally wider carapace (CW/CL=1.6 vs. 1.3-1.4 in D. cereum; Huang et al. 2017b: fig. 2A); granulate anterolateral carapace margins (Fig. 1) (vs. spinose in D. cereum; Huang et al. 2017b: fig. 2A); proportionally wider male anterior thoracic sternum (width 2.3 × length vs. width 1.7 × length in D. cereum; Huang et al. 2017b: fig. 6C); proportionally wider male pleon (compare Fig. 2C with Huang et al. 2017b: fig. 2C); male thoracic sternite 8 being fully concealed when the pleon is closed (Fig. 2C) (vs. partially exposed in D. cereum; Huang et al. 2017b: fig. 3E, F); and its relatively shorter and stouter walking legs (Fig. 1) (see Huang et al. 2017b: fig. 2A).

Phasmon typhlops gen. nov. et sp. nov. is similar to Cerberusa caeca in general physiognomy and size. However, the new species can immediately be distinguished by its proportionally wider carapace (CW/CL=1.6 vs. 1.3-1.4 in C. caeca; Holthuis 1979: pl. 8); almost indiscernible postorbital cristae (Fig. 1) (vs. low, indicated by a transverse row of granules in C. caeca; Holthuis 1979: fig. 3A); proportionally wider male pleon (compare Fig. 2C with Holthuis 1979: fig. 3C); and its slightly sinuous G1 (Figs 3C, 4A, B) (vs. strongly bent outwards in C. caeca; Holthuis 1979: fig. 3D).

The G1 characteristics of Phasmon gen. nov. are rather unremarkable and particularly similar to those of Chinapotamon and Diyutamon . Chinapotamon is also found in Guangxi and includes two cavernicolous species, C. dashiwei Ng, 2017 and C. clarkei Ng, 2017, of which the latter displays evidence of stygomorphism in reduced body pigmentation and well-developed, albeit proportionally smaller eyes than epigean congeners (Ng 2017). Phasmon gen. nov. is readily distinguished from Chinapotamon in: the proportionally wider carapace (CW/CL=1.6 vs. 1.3-1.4 in Chinapotamon; Ng 2017: figs 2, 6; Zou et al. 2018: fig. 2); the frontal margin being continuous with the supraorbital margin, forming an almost straight transverse margin in dorsal view (Fig. 1) (vs. supraorbital margin distinctly concave in dorsal view in Chinapotamon; Ng 2017: figs 2, 6; Zou et al. 2018: fig. 2); the vestigial, unpigmented eyes (Fig. 2A) (vs. well-developed, pigmented eyes in Chinapotamon; Ng 2017: fig. 6); the almost indiscernible epigastric cristae and postorbital cristae (Fig. 1) (vs. clearly discernible in Chinapotamon; Ng 2017: figs 2, 6; Zou et al. 2018: fig. 2); the proportionally wider male anterior thoracic sternum (width/length 2.3 vs. 1.6-1.7 in Chinapotamon; Ng 2017: figs 3A, 7A; Zou et al. 2018: fig. 3A); and the proportionally wider male pleon (compare Fig. 2C with Ng 2017: figs 3B, 7B; Zou et al. 2018: fig. 3B).