‘Henicorhynchus’thaitui, a new species of cavefish from Central Vietnam (Teleostei, Cyprinidae)

Abstract ‘Henicorhynchus’thaituisp. nov. is described from a subterranean stream in a karst cave in Phong Nha Ke Bang National Park, Quang Binh Province, Central Vietnam. It differs from all congeners in having a pale pink body in life, smaller eyes with diameter less than the maxillary barbel length, and two pairs of barbels, the maxillary barbel being much longer than the rostral barbel.


Introduction
Vietnam is rich in karst caves, and there are many magnificent caves throughout the country. The Son Dong Cave is the largest in the world (UNESCO 2016). Many caves, including Vietnam's deepest cave, were discovered in the northern part of the

Material and methods
All measurements were taken point to point with a pair of dial calipers and data were recorded to the nearest 0.1 mm. Counts and measurements were made on the left side of specimens whenever possible, following the methods of Rainboth (1996) and Kottelat (2001). Predorsal, prepectoral, prepelvic and preanal lengths were measured from the snout tip to the dorsal-, pectoral-, pelvic-, and anal-fin origin, respectively. Vertebrae were counted from radiographs following the method outlined by Roberts (1989). Weberian vertebrae and the urostylar complex were included in the counts of vertebrae. The number of specimens used for a given count is indicated in brackets after the count. Values for the holotype are indicated by an asterisk. Measurements of parts of the head are given as proportions of the head length (HL). The head length and measurements of other parts of the body are given as percentages of the standard length (SL Ho and N.T. Hoang, 12 August 2011 (deposited in IEBR).
Diagnosis. 'Henicorhynchus' thaitui sp. nov. can be distinguished from all congeners by having a whitish pink body in life (vs. white or silvery body with a humeral mark or some longitudinal stripes), smaller (vs. larger) eyes (diameter less than vs. greater than maxillary barbel length) and maxillary barbel longer (vs. shorter) than rostral barbel. 'Henicorhynchus' thaitui, along with H. horai (Bănărescu, 1986) and H. inornatus (Roberts, 1997), is further distinct from all other congeners in having 9 (vs. 8) branched dorsal-fin rays. Along with H. horai, it differs from all other congeners in the presence of rostral barbels (vs. absent) and 39-41 (vs. 34-36)  Description. Measurements and meristics of the type series are provided in Table  1. See Figs 1, 2 for general appearance, Fig. 3a for lateral and ventral views of the head, and Fig. 4 for morphology of the oromandibular structures. Body elongate and laterally compressed. Dorsal profile of body from tip of snout to dorsal-fin origin slightly convex. Predorsal profile of body convex, without distinctive hump behind head. Postdorsal profile of body slightly concave. Ventral profile of body from tip of snout to anal-fin origin convex; slightly concave from anal-fin origin to origin of ventral procurrent caudal-fin rays.
Head small, conical, longer than deep, deeper than wide. Eye relatively small, positioned laterodorsally in anterior half of head and considerably behind or above rictus, not visible when head viewed ventrally; diameter less than maxillary-barbel length. Interorbital space slightly convex. Snout slightly pointed in lateral view and broadly rounded in ventral view (Fig. 3a). Nares longitudinal and located closer to orbit than to tip of snout and covered by a flap originating from anterior end. Two pairs of long barbels; maxillary barbel much longer than rostral barbel, extending to posterior margin of orbit or beyond, and rostral barbel extending beyond nostrils but not reaching to anterior margin of orbit. Mouth inferior and slightly arched.
Rostral cap well-developed, overhanging, but covering median part of upper lip base; slightly crenulated, laterally attached to root of maxillary barbel and separated from lower lip. Upper lip well-developed, greatly enlarged, separated from upper jaw, covered with papillae; laterally continuous with lower lip around corners of mouth. Upper jaw bearing a thin, flexible horny sheath on cutting margin. Lower lip anteriorly separated from lower jaw by a deep, transversally arched groove; posteriorly confluent with mental region and covered with papillae. Post-labial groove extended anteromedially, but not meeting its counterpart at midline. Lower jaw bearing a thin horny sheath on cutting margin (Fig. 4) Dorsal fin with 3 unbranched and 9 branched rays, last one split to base; last unbranched ray flexible, without serrations along posterior border; distal margin strongly concave; origin midway between snout tip and caudal-fin base or anterior to pelvicfin insertion. Pectoral fin short, with 1 unbranched and 11 branched rays; tip of adpressed fin not reaching pelvic-fin insertion. Pelvic fin falcate, with 1 unbranched and 8 branched rays; inserted halfway from pectoral-fin insertion to anal-fin origin; tip of adpressed fin extending to vent. Anal fin with 3 unbranched and 5 branched rays; distal margin slightly concave; origin equidistant between pelvic-fin insertion and caudal-fin base. Caudal fin with 9/8 principal rays, deeply forked; upper and lower lobes nearly equal in length.
Coloration. In freshly caught-individuals, body white to pinkish or pale pink with all fins translucent (Fig. 2). In captivity, body pale pink but dorsum turning to pale  (2) brown hue in adults exposed to light, becoming gray dorsally within several hours when exposed to daylight (Fig. 1b). In alcohol-preserved specimens, body uniformly pale yellow, with all fins light gray, particularly in distal portion (Fig. 1a). Etymology. The specific name is a noun in the genitive case, honoring Nguyen Thai Tu, ichthyologist from Vinh University, who has contributed considerably to the taxonomy of freshwater fishes in Vietnam.
Troglomorphic characters. 'Henicorhynchus' thaitui sp. nov. presents a mixture of characters characterizing hypogean and epigean fish species. The absence of pigmentation, reduced eye size, and well-developed barbels are troglomorphic characters observed in 'H.' thaitui. The pale pink or white to pinkish body is shared with hypogean  fish species. The eyes are smaller than in congeneric epigean species, but not vestigial or absent as is common in hypogean fish species. The barbels are longer and thicker compared with all congeneric epigean species, but in this regard similar to hypogean fish species.
Distribution and habitat. 'Henicorhynchus' thaitui sp. nov. is known only from the Khe Lanh Cave where it inhabits shallow to deep (0.2-0.8 m) cave streams and pools about 800-1000 m from the cave entrance (Figs 5, 6). This cave is located approximately 25 km south of Phong Nha village in the Son Trach commune. It has a length of 1-2 km, completely without light, with a mixed substrate of mud and sands. The type series of 'H.' thaitui was collected in August 2011, roughly 1 km from the cave entrance. At least 50 individuals of various sizes were observed in streams and pools, 14 of which were caught by hand net (Fig. 7). The fishes were swimming slowly and haphazardly, rather close to the water surface; they swam deeper when disturbed. A new shrimp species (Do and Nguyen 2014)
The Labeonini are characterized by a high degree of morphological modification of their oromandibular structures, variation in which is the basis for the diagnosis of the majority of included genera. The well-developed, greatly enlarged upper lip separated from the upper jaw in 'H.' thaitui is shared only with some species of the labeonine genera Osteochilus Günther, Labiobarbus van Hasselt, Labeo Cuvier, Cirrhinus and Henicorhynchus (Wu et al. 1977;Zhang et al. 2000;Zhang and Chen 2006). It differs from Osteochilus (sensu Roberts, 1989) in the absence of elongate folds or plicae on the upper lip; and from Labiobarbus (sensu Rainboth, 1996) in having 9 rather than 18-30 branched rays of the dorsal fin. Labeo, as traditionally defined, is widespread in the tropical regions of Asia and Africa (Reid 1985). Its non-monophyly was shown in Yang et al.'s (2012) molecular phylogenetic analysis of the Labeonini; these authors therefore restricted Labeo s. str. to the clade sister to 'Cirrhinus' microlepis Sauvage, 1878, but upholding this clade may result in multiple generic lineages. In this context, Asian Labeo is here confined to species like Labeo dyocheilus McClelland, 1839, following Rainboth (1996) and Kottelat (2013). 'Henicorhynchus' thaitui is not congeneric with Asian species of Labeo, lacking a thickened lower lip with a deep postlabial groove narrowly interrupted at the isthmus with its counterpart. It is highly likely that 'H.' thaitui is a member of Henicorhynchus or Cirrhinus, or even of a distinct genus.
'Henicorhynchus' thaitui sp. nov. can be distinguished by its troglomorphic characters from all other species of Henicorhynchus and closely related genera like Cirrhinus and Gymnostomus. Except for the two species of Speolabeo (Nguyen et al. 2018) and some species of Garra Hamilton in the Middle East (Khalaf-Sakerfalke von Jaffa 2009; Hamidan et al. 2014), no species with troglomorphic characters like this are found in the Labeonini.
The generic placement of 'H.' thaitui is not straightforward, however. Compared with species of Henicorhynchus or closely related genera which have relatively simple oromandibular structures (see Roberts 1997 for details), 'H.' thaitui has unique modifications in these structures, including rostral cap pendulous with a slightly crenulated distal margin, laterally attached to the root of the maxillary barbel and discontinuous with the lower lip, and papillated lips (Fig. 4). These characters might justify placing 'H.' thaitui in a new genus. Such act will need a more thorough analysis, involving all labeonine genera. For the moment, the recognition of 'H.' thaitui as a distinct species, regardless of genus, is justified on account of its restricted distribution and specialized ecology. Anthropogenic activities on both the global and local scales pose a potential threat to survival of all cavefishes, and their identification by a scientific name will facilitate conservation actions.
Taxonomic confusion regarding Henicorhynchus, Gymnostomus, and Cirrhinus was clarified in recent molecular phylogenetic analyses of the Labeonini Zheng et al. 2012Zheng et al. , 2016. It was demonstrated that sampled species of Henicorhynchus nested into an independent lineage, and so did that of Gymnostomus; Cirrhinus was non-monophyletic, and C. microlepis and C. molitorella were distantly related to each other or to the rest of the analyzed congeneric species. On the basis of these findings, it can be concluded that Gymnostomus [type species: Cyprinus ariza Hamilton, 1807, a species designated by Roberts (1997) to Cirrhinus] is a valid genus; Henicorhynchus possibly includes species with 8 branched dorsal-fin rays; and both C. molitorella and C. microlepis should be removed from Cirrhinus. However, insufficient sampling made it unlikely to reach a decisive conclusion about the generic placement of these three genera. Two species, namely H. inornatus and H. horai, with 9 branched dorsal-fin rays, were not sampled in Yang et al.'s (2012) analysis, rendering their generic status untested. Fricke et al. (2020) nevertheless referred these two species to Henicorhynchus.