A new species of Pelodiscus from northeastern Indochina (Testudines, Trionychidae)

Abstract A new, critically endangered species of softshell turtle, Pelodiscusvariegatussp. n. is described from north-central Vietnam and Hainan Island, China, distinguished by a unique set of genetic and morphological traits from all other congeners (P.axenaria, P.maackii, P.parviformis, P.sinensis, and unnamed genetic lineages). Morphologically, P.variegatus is characterized, among others, by its strong ventral ornamentation in all age classes.


Introduction
"Chinese softshell turtles" were considered for decades to represent the morphologically highly variable and geographically widespread species Pelodiscus sinensis (Wiegmann, 1834), distributed from the Russian Far East through the Korean Peninsula, eastern and central China to Vietnam (e.g., Pope 1935;Mertens 1961, 1977;Pritchard 1979;Meylan 1987;Ernst and Barbour 1989). Another species, P. maackii   Gong et al. 2018). Positions refer to the 400-bp-long reference alignment in the Supporting Information. G, H, and I are genetic lineages that represent putatively distinct taxa    In addition to the genetic distinctiveness of P. variegatus, the strong ventral ornamentation clearly sets apart adult individuals from P. maackii, which has a uniform yellowish white or straw yellow plastron devoid of any markings in adults; from P. sinensis, which may retain faint remnants of its juvenile pattern on its snow white to reddish white plastron but the round to oval spots are usually isolated and proportionally much smaller; from P. axenaria, which has a yellowish white plastron with just a single large black central figure enclosed by the hypo-and xiphiplastra throughout its life (Zhou et  al. 1991); and from P. parviformis, which has an unmarked yellowish white plastron at all ages. According to the specimens investigated and data available to us, P. variegatus also reaches a much smaller maximum size (23 cm CL; AMNH 28345) than P. maackii (at least 35 cm CL; Brandt 1857) but grows bigger than P. parviformis (16 cm CL; NMW 30232:6); P. sinensis (23 cm CL; ZMB 9784) and P. axenaria (20 cm CL; Gong et al. 2018) attain dimensions resembling P. variegatus. The diagnostic morphological features of adults of these five Pelodiscus species are summarized in Table 4.
Description of the holotype. Carapace length (CL) 171.0 mm, carapace width (CW) 148.0 mm, plastron length (PL) 134.2 mm, head width (HW) 32.2 mm, eye diameter 9.8 mm, interorbital distance 5.4 mm, snout length (SL) 13.3 mm. Carapace oval, slightly domed but with a medial keel, widest at level of the posterior buttress spurs of the hypoplastra. Marginal ridge low, central tubercle indistinct. Dorsal surface roughened by longitudinal ridging and smaller protuberances spread over the leathery margin. The yellowish gray carapace is adorned with an extremely complex greenish black pattern consisting of reticulations and stellate spots enclosed by incomplete rings of the same color, finely dotted with siskin green on either side of the vertebral keel, with those above the pelvis being more pronounced but with additional ones towards the perimeter of the bony disk. The ridging of the carapace is enhanced by the siskin green color of the vertebral keel and the longitudinal rows of tubercles.
Ventral surfaces yellowish white with distinct greenish gray blotches extending onto the plastron. Two dark patches below the neck along the anterior carapace margin, one oval mark between the epiplastra, one on both sides behind the armpits and continuing towards but not reaching the hyoplastra, as well as on the hyo-and hypoplastra and the xiphiplastra, the latter meeting along the midline but not contacting those covering the hyo-and hypoplastra. Additional blotches at the insertion of the hindlimbs and some vague, bruise-like markings on the bridge and the underside of the leathery margin. Head, extended to posterior level of eyes, terminating in flexible snout. Jaws closed, each covered by fleshy lips except anteriorly where the horny beaks are exposed. Top of head with fine, greenish black specks and streaks. Pre-, sub-and postocular stripes thick (approx. 2 mm wide), locally interrupted and yellowish black in color with thin siskin green outlines. Chin with a contrasting yellowish white pattern on yellowish Table 3. Selection of diagnostic sites of the mtDNA fragment comprising the partial ND4 gene and adjacent DNA coding for tRNAs for Pelodiscus species (91 wild-caught individuals from Gong et al. 2018). Positions refer to the 838-bp-long reference alignment in the Supporting Information. G, H, and I are genetic lineages that represent putatively distinct taxa    Fore-and hindfeet well-webbed, having five digits each, with claws on the first three digits only. Each forelimb with four antebranchial scales, three of them freeedged. These are wide, the upper one stretching across nearly the whole width of the forelimb (approx. 16 mm long) and the lower two overlapping each other (approx. 20 mm together). Each hindlimb with two horny scales, one smooth on the posterodorsal surface while the other, which is free-edged, is located on the posteroventral surface.
Tail short, barely extending beyond the rear margin of the carapace. Undersurface of soft parts of body yellowish white embellished with large yellowish gray markings, encroaching on soles and palms, and on either side of the tail.
Variations. There is considerable, in part also age-dependent, variation in pattern intensity among Pelodiscus variegatus. For instance, our male paratype (HNHM 2018.111.1; 109.7 mm PL; Fig. 4A) has somewhat smaller but much more conspicuous stellate spots on its carapace and very large dark (greenish black) markings on its undersurfaces. Those on the hyo-, hypo-and xiphiplastra are fused into a single mushroom-shaped figure, while the "bruises" on the bridge and the ventral surface of the leathery margin also manifest themselves as true blotches.
In one of our female paratypes (MTD 44045; 75.2 mm PL; Fig. 2A, C) the dark blotches on either side of the tail are connected with those at the insertion of the hindlimbs, the leathery margin and the bridge, whereas the ones in front of the entoplastron are fused with those in the armpits, extend towards the central patch between the epiplastra and actually reach the marks on the anterior edge of the plastron, at the base of the neck.
In a battered, presumably very old male examined by us (ZFMK 44212; 116.4 mm PL; Fig. 4B) comparable in size to our male paratype, the ventral ornamentation has lost definition but is still perceptible.
Hatchlings (Fig. 4C, D) have similar markings to adults but the overall effect is even more striking. The parts colored yellowish white in preservative are orpiment to reddish orange in life (Farkas and McCormack 2010) and fade remarkably slowly as age advances (Figs 2-3). In some individuals the light blotches framed with black on the sides of the neck intermingle to form wide bands on a yellowish gray (in life yellowish brown) ground ( Fig. 2A). The subocular stripes are occasionally reduced to short streaks and vary between two and three in number. Although the dark longitudinal striation on the nape of juveniles dissolves in adults, a central spot typically remains discernible just in front of the marginal ridge.
Our modest sample size does not allow us to draw definite conclusions about ontogenetic variation and it is presently unknown at what CL sexual maturity is reached in this species. Individuals are sexually clearly dimorphic, with males having much longer and thicker tails, at a PL of 98.8 mm (ZFMK 44213) but a slight variation in TL can be noticed even among hatchlings (NMW 30221:1-6). Anyhow, smaller (younger) specimens appear to possess proportionally wider heads and rounder shells than larger (older) ones. PL/HW 3.50-4.44, mean 3.925; CL/CW 1.02-1.23, mean 1.149; HW/SL 1.98-2.60, mean 2.213, CL/PL 1.18-1.43, mean 1.283.
Etymology. The specific epithet variegatus (spotted) is a Latin adjective in masculine gender alluding to the highly contrasting markings, especially the large plastral blotches, of the new species.
Remarks. In addition to the characters used here for diagnosing P. variegatus, Gong et al. (2018) described some further genetic differences to other Pelodiscus species. Fritz et al. (2010) suggested that the taxon now named Pelodiscus variegatus resembles P. parviformis, prompting the TTWG (2011,2012,2014,2017) to identify the Pelodis-  Currently known presence points of Pelodiscus species based on our own data as well as distribution maps published by the TTWG (2017) and Gong et al. (2018). Earlier records of P. sinensis from Hainan Island are referable to P. parviformis or P. variegatus sp. n. (see Remarks).
cus records from Vietnam with the latter species. However, as explained in Gong et al. (2018), this is no longer tenable in the face of the genetic distinctness of the two species.
Conservation implications. While Pelodiscus sinensis is listed as "Vulnerable (VU)" by the IUCN Red List of Threatened Species (Asian Turtle Trade Working Group 2000), the conservation status of P. axenaria, P. maackii, P. parviformis, and now P. variegatus, remains unassessed, in spite of their proven genetic distinctness (Fritz et al. 2010;Yang et al. 2011;Gong et al. 2018). Given their restricted distributional ranges and the intense exploitation to which they are subjected, all these species would certainly classify for a higher category rating. In this vein, the most recent red list of Chinese vertebrates compiled by Jiang et al. (2016) proposed the conservation status of P. axenaria, P. parviformis and P. sinensis be upgraded to "Endangered (EN)" and indicated P. maackii to be "Data Deficient (DD)." Rhodin et al. (2018) suggested for P. parviformis "Critically Endangered (CR)" and for P. sinensis "Endangered (EN)," whereas P. axenaria and P. maackii were identified as "Data Deficient (DD)." Consequently, also P. variegatus, which was included in P. parviformis by Rhodin et al. (2018), should be classified as "Critically Endangered (CR)."