Phasmon typhlops, Huang & Ahyong & Shih, 2020
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publication ID |
https://dx.doi.org/10.3897/zookeys.1008.58854 |
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publication LSID |
lsid:zoobank.org:pub:6BF494BE-A31B-42D7-88AF-2AB04EEB4725 |
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persistent identifier |
https://treatment.plazi.org/id/FEE83A41-2669-4B3F-B3A0-5E1F15F1DEB1 |
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taxon LSID |
lsid:zoobank.org:act:FEE83A41-2669-4B3F-B3A0-5E1F15F1DEB1 |
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treatment provided by |
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scientific name |
Phasmon typhlops |
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sp. nov. |
Phasmon typhlops sp. nov. Figs 1 View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 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 View Figure 1 ). Frontal margin weakly sinuous, continuous with supraorbital margin, forming almost straight transverse margin in dorsal view (Figs 1A, B View Figure 1 ). Epigastric cristae and postorbital cristae almost indiscernible (Figs 1A View Figure 1 , 2A View Figure 2 ). Branchial regions slightly swollen (Figs 1A View Figure 1 , 2A View Figure 2 ). Cervical groove shallow (Fig. 1A View Figure 1 ). Mesogastric region slightly convex (Fig. 1A View Figure 1 ). External orbital angle obsolete, outer margin convex, almost indistinguishable from anterolateral margin (Figs 1A View Figure 1 , 2A View Figure 2 ). Epibranchial tooth granular, inconspicuous (Fig. 1A View Figure 1 ). Anterolateral margin lined with 15-20 small, single or partially fused granules. Posterolateral margin posteriorly convergent (Fig. 1A View Figure 1 ); posterolateral surface generally smooth (Fig. 1A View Figure 1 ). Orbits shallow; supraorbital margins weakly cristate, infraorbital margins lined with granules (Fig. 2A View Figure 2 ). Eyes almost immobile, greatly reduced, tapering, length about half orbital width; peduncle short, stout; cornea vestigial, surface without facets, unpigmented (Figs 2A View Figure 2 , 3F View Figure 3 ). Sub-orbital, pterygostomial and sub-hepatic regions generally smooth, pitted (Fig. 2A View Figure 2 ). Antennules large, folded within broad fossae; antennae very short (Fig. 2A View Figure 2 ). Median lobe of epistome posterior margin broadly triangular, lateral margins sinuous (Fig. 2A View Figure 2 ).
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 View Figure 3 ).
Chelipeds (pereiopod 1) subequal (Figs 1 View Figure 1 , 3D, E View Figure 3 ). Merus trigonal in cross section; margins slightly crenulated, surface generally smooth (Figs 1A View Figure 1 , 2A View Figure 2 ). Carpus with sharp spine at inner-distal angle (Fig. 1 A View Figure 1 ). Major cheliped palm length about 1.5 × height; dactylus 0.9 × palm length (male) (Fig. 3D, E View Figure 3 ), as long as palm (female). Palm surface pitted (Fig. 3D, E View Figure 3 ). Dactylus as long as pollex (Fig. 3D, E View Figure 3 ). Occlusal margin of fingers with 18-20 irregular blunt teeth, without gape when closed (Fig. 3D, E View Figure 3 ).
Ambulatory legs (pereiopods 2-5) slender with very sparse short setae (Fig. 1 View Figure 1 ). Pereiopod 3 merus 0.9 × CL (male) (Fig. 1A View Figure 1 ), 0.8 × CL (female) (Fig. 1B View Figure 1 ). Pereiopod 5 propodus length 2.8 × height (male) (Fig. 1A View Figure 1 ), 3.4 height (female) (Fig. 1B View Figure 1 ), shorter than dactylus; dactylus length 6.1 × height (male) (Fig. 1A View Figure 1 ), 6.2 × height (female) (Fig. 1B View Figure 1 ).
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 View Figure 2 ). Male sterno-pleonal cavity reaching anteriorly slightly beyond level of cheliped coxa articular condyle (Fig. 2B View Figure 2 ); deep median longitudinal groove between sternites 7, 8 (Fig. 2D View Figure 2 ). Male pleonal locking tubercle positioned at mid-length of sternite 5 (Fig. 2D View Figure 2 ). 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 View Figure 2 ).
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 View Figure 2 ). Female pleon subovate (Fig. 2E View Figure 2 ).
G1 tapering, slightly sinuous, tip exceeding pleonal locking tubercle but not reaching suture between thoracic sternites 4/5 in situ (Fig. 2D View Figure 2 ); proximal segment length about 2.3 × length of distal segment (Figs 3C View Figure 3 , 4A, B View Figure 4 ). Distal segment slender, tapering anteriorly, slightly inclined towards midline; tip pointed upwards in dissected view (Figs 3C View Figure 3 , 4A, B View Figure 4 ). G2 slender, almost straight, proximal portion with distal two-thirds subcylindrical, length about 2.4 × length of distal portion (Figs 3B View Figure 3 , 4C, D View Figure 4 ); 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 View Figure 3 ).
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 View Figure 1 ) and wide male anterior thoracic sternum (width 2.3 × length; Fig. 2B View Figure 2 ). 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 View Figure 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 View Figure 2 with Huang et al. 2017b: fig. 2C); male thoracic sternite 8 being fully concealed when the pleon is closed (Fig. 2C View Figure 2 ) (vs. partially exposed in D. cereum ; Huang et al. 2017b: fig. 3E, F); and its relatively shorter and stouter walking legs (Fig. 1 View Figure 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 View Figure 1 ) (vs. low, indicated by a transverse row of granules in C. caeca ; Holthuis 1979: fig. 3A); proportionally wider male pleon (compare Fig. 2C View Figure 2 with Holthuis 1979: fig. 3C); and its slightly sinuous G1 (Figs 3C View Figure 3 , 4A, B View Figure 4 ) (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 View Figure 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 View Figure 2 ) (vs. well-developed, pigmented eyes in Chinapotamon ; Ng 2017: fig. 6); the almost indiscernible epigastric cristae and postorbital cristae (Fig. 1 View Figure 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 View Figure 2 with Ng 2017: figs 3B, 7B; Zou et al. 2018: fig. 3B).
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