Research Article |
Corresponding author: Hsi-Te Shih ( htshih@dragon.nchu.edu.tw ) Academic editor: Ingo S. Wehrtmann
© 2020 Chao Huang, Shane T. Ahyong, Hsi-Te Shih.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Huang C, Ahyong ST, Shih H-T (2020) The second known stygomorphic freshwater crab from China, Phasmon typhlops gen. nov. et sp. nov. (Crustacea, Decapoda, Potamidae), diverged at the beginning of the Late Miocene. ZooKeys 1008: 1-15. https://doi.org/10.3897/zookeys.1008.58854
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A new genus and new species of blind freshwater cave crab are described from Chongzuo City, Guangxi Zhuang Autonomous Region, China based on morphology and mitochondrial 16S rDNA sequences. The new genus, Phasmon gen. nov., is established for P. typhlops sp. nov., which is only the second blind cave crab known from China and East Asia. The combination of a very wide carapace, overall depigmentation, reduced orbits and vestigial unpigmented eyes of Phasmon immediately separates it from all known potamid genera. Molecular divergence estimates based on 16S rDNA suggest that the lineage to which the new genus belongs diverged from other potamids at the beginning of the Late Miocene (10.8 million years ago), much earlier than other Chinese cave crabs.
16S rDNA, blind crab, cave crab, new genus, new species, Guangxi, subterranean
The Guangxi Zhuang Autonomous Region is located in southern China within the Huanan freshwater zoogeographic province (
Stygomorphic potamid crabs are rare, and only a few species were previously known. Chinapotamon clarkei Ng, 2017 and Diyutamon cereum Huang, Shih & Ng, 2017b, from China; Cerberusa tipula Holthuis, 1979 and Cerberusa caeca Holthuis, 1979, from Borneo; Erebusa calobates Yeo & Ng, 1999, from Laos; and Teretamon spelaeum Absar, Mitra & Kharkongor, 2017, from India, all – exhibit varying degrees of stygomorphism. Of these, only D. cereum and C. caeca exhibit strong eye reduction and depigmentation–both appear to be blind and can be considered the most highly stygomorphic of known subterranean potamids.
In 2018, a local collector from Chongzuo City, Guangxi, China, alerted us to an unusual crab trapped from a karst spring. This crab, a female, was apparently a stygobite, lacking body pigmentation and having vestigial, unpigmented eyes. Many further attempts were made to collect more specimens of this unusual species, but they remained elusive. The collector only succeeded once in trapping a second specimen from the karst spring, but only half of the carcass remained when he checked the trap; the other half was apparently consumed by a specimen of the epigean crab Lacunipotamon cymatile Huang, Shih & Ahyong, 2020 (
Specimens were collected by hand, preserved in 75% ethanol, and deposited in the collections of the Sun Yat-sen Museum of Biology, Sun Yat-sen University, Guangzhou, China (SYSBM) and the Australian Museum, Sydney, Australia (
A 16S sequence was obtained from the paratype (
The best-fitting model for sequence evolution of 16S was determined by PartitionFinder (vers. 2.1.1;
Subfamily Potamiscinae Bott, 1970
Phasmon typhlops, by present designation.
Small sized (carapace width less than 30 mm). Carapace 1.6× wider than long; fronto-orbital width about twice width of posterior margin; dorsal surface weakly convex (Figs
The genus name is an arbitrary combination of the Latin word “phasma”, meaning ghost, which refers to the type species’ pale appearance and dark habitat, and the genus name Potamon, which is the type genus of the family. Gender neuter.
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:
Phasmon typhlops gen. nov. et sp. nov., male holotype (14.4 × 9.0 mm), SYSBM 001982 (A–D) female paratype (22.1 × 13.7 mm),
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.
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.
Chelipeds (pereiopod 1) subequal (Figs
Ambulatory legs (pereiopods 2–5) slender with very sparse short setae (Fig.
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.
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.
G1 tapering, slightly sinuous, tip exceeding pleonal locking tubercle but not reaching suture between thoracic sternites 4/5 in situ (Fig.
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.
Pale yellowish-white all over (Fig.
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. (
Chongzuo City, Guangxi Province, China.
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 (
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.,
Taxonomically, the most striking features of Phasmon gen. nov. are its very wide carapace (CW/CL=1.6; Fig.
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;
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 (
A 512-basepair segment of the 16S rDNA gene, excluding the variable regions, was amplified and aligned from 25 potamid genera. The phylogenetic tree of the 16S sequences reconstructed using BI analysis is shown with support values from ML analysis (Fig.
The divergence time of 10.8 mya estimated for Phasmon gen. nov. is much older than the 5.7 mya estimated for another Chinese cave crab genus, Diyutamon (
This study was partially supported by a grant from the Ministry of Science and Technology (MOST 108-2313-B-005-007-MY3), Executive Yuan, Taiwan, to HTS. We also acknowledge the reviewer Célio Magalhães, an anonymous reviewer, and the editor Ingo S. Wehrtmann, who all helped to improve the manuscript.