Tachysurus wuyueensis, new species, is described from the Qiantang-Jiang Basin, situated in Suichang County, Zhejiang Province and Xiuning County, Anhui Province, southeast China. The coastal basin drains into the East China Sea. The new species belongs to the T. pratti-T. truncatus group within the genus Tachysurus defined by having a smooth anterior margin of the pectoral-fin spine, short maxillary barbels not extending beyond the base of the pectoral-fin spine, short dorsal spine not exceeding two thirds of head length and an emarginated caudal fin. This new species is distinct from all other species of this group, Tachysurus pratti (Gunther, 1892), T. truncatus (Regan 1913), T. gracilis (Li, Chen & Chan, 2005) and T. brachyrhabdion (Cheng, Ishihara & Zhang, 2008), in having a shorter prepelvic body (length 40.0–46.4% of SL vs. 45.8–54.8%). It further differs from T. pratti, T. truncatus and T. gracilis in having more vertebrae (45–47 vs. 37–44) and more anal fin rays (21–25 vs. 14–20), from T. brachyrhabdion in having a more slender body (depth 10.1–13.5% of HL vs. 13.1–17.6%). Molecular phylogeny, based on the mitochondrial cytochrome b (cyt. b) gene confirms the validity of T. wuyueensis and the T. pratti-T. truncatus group. Furthermore, this study addresses the diagnostic traits distinguishing the T. pratti-T. truncatus group from the T. tenuis-T. crassilabris group which have historically been treated as a single species group due to morphological similarities.

Caudal fin shape, new taxon, stream-dewelling species, taxonomy

The genus Tachysurus Lacépède 1803 is a group of East Asian endemic catfish that is widely distributed throughout most of the East Asian continent (Ku et al. 2007; Watanabe 2010; Shao and Zhang 2023). It is also one of the most diversified catfish genus in China containing more than thirty species with strong differentiation in ecological niches (Shao et al. 2021; Shao and Zhang 2023). South China, which is characterised by high mountains, deep valleys and abundant precipitation (López-Pujol et al. 2011; Wang et al. 2018; Sun et al. 2021), harbours numerous Tachysurus species restricted to montane streams with fast-flowing high-oxygen waters and predominance of rocky substrate, for example, Tachysurus ondon (Shaw 1930), T. adiposalis (Oshima 1919), T. albomarginatus (Rendahl 1928), T. trilineatus (Zheng 1979) and T. tenuis (Günther 1873). Unlike its congeners, species of Tachysurus always possess elongated bodies and rounded, truncated or slightly emarginated (round-tailed) caudal fins regarded herein as key adaptions for the torrent environments (Gosline 1997; Krishnadas et al. 2018). Moreover, the absence of diagnosable characters and ambiguous descriptions have obscured the species boundaries of these species (Ferraris 2007; Shao et al. 2021). For instance, T. albomarginatus has long been synonymised under T. tenuis (Zheng & Dai, 1999) until Cheng et al. (2021) resurrected the former species, based on morphometric and osteological analyses.

In addition, rheophilic (fast-water) fishes are always confined to a narrow distribution which limits their dispersal ability and contributes to their habitat fragmentation (Lima et al. 2017). Previous works have highlighted the underestimated diversity of Tachysurus, v.gr., T. albomarginatus that may be an assemblage of five species (Cheng et al. 2021; Shao et al. 2021). A similar scenario occurs in T. adiposalis, originally described by Oshima (1919) on a single specimen of 170 mm SL (standard length) collected from the Tamusui River in Taiwan. T. adiposalis has been treated as widespread species, with a distribution including Taiwan and river basins from the southern region of Chinese mainland, such as the mid-lower Yangtze, Pearl River and Qiantang River (= Qiantang-Jiang in Chinese) (Zheng and Dai 1999). The key characters useful in differentiating Tachysurus species are absent or obscured in the original description of T. adiposalis, which resulted in the taxonomic confusion. Recent studies have suggested that specimens from the Xi-Jiang (the Pearl River Basin) and the Yuan-Jiang (the middle Yangtze River Basin) belonging to T. adiposalis s.l. were determined as Tachysurus gracilis (Li, Chen & Chan, 2005) and Tachysurus brachyrhabdion (Cheng, Ishihara & Zhang, 2008). However, the taxonomic status of specimens from the Qiantang-Jiang which were also recognised as Tachysurus adiposalis has not been mentioned in previous studies and it potentially represents an undescribed species.

A fish field survey conducted by the authors in the Qiantang-Jiang Basin of Zhejiang and Anhui Provinces yielded 14 specimens initially identified as T. adiposis. Careful morphological examination revealed that these were, in fact, not conspecific with any other known species of Tachysurus and represent a new species. The purpose of the present paper is to provide a formal description of this unnamed species, based on multiple lines of evidence containing morphological and phylogenetic datasets.

Fourteen specimens of the new species were caught during two field surveys conducted, respectively, in September 2020 and May 2023 into the Qiantang-Jiang flowing into the East China Sea. Amongst them, six specimens were fixed in 10% formalin after removal of right-side pelvic-fin clips. These fin clips were stored in 95% ethyl alcohol and utilised for molecular analysis. The remaining caught specimens were directly preserved in 10% formalin preservative for morphological examination. Their voucher specimens are deposited in the Museum of Aquatic Organisms of the Institute of Hydrobiology (IHB), Chinese Academy of Sciences, Wuhan. Eight species, including T. gracilis, T. truncatus, T. pratti, T. brachyrhabdion, T. crassilabris, T. albomarginatus, T. adiposalis and T. tenuis, were morphologically examined in this study, which came from the following collections: Museum of Aquatic Organisms of Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan (IHB), the Natural History Museum, London (BMNH) and French Museum of Natural History, Paris (FMNH).

Measurements were taken point to point with digital calipers linked to a data recording computer and data were recorded to the nearest 0.1 mm and made on the left side of each individual whenever possible, following the methods for Cheng et al. (2008). The head length and measurements of other parts of the body are estimated as percentages of the standard length (SL). Subunits of head are provided as percentages of the head length (HL). The number of rays from the dorsal- and anal fins were counted using the method by Watanabe (1995). Other fin rays were counted under a binocular dissecting microscope utilising transmitted light. Vertebral count was taken from X-ray photographs, with the anterior five vertebrae, namely the Weberian complex, not counted.

Morphometric measurements were subject to principal component analysis (PCA) in order to examine external morphological differentiation and determine the relative contributions of specific variables to morphological differences in the target species. PCA was run with SPSS 16 (SPSS, Chicago, IL, USA). Prior to the analysis, all morphometric measurements, except standard length, were normalised following the method of Reist (1985) to eliminate the influence of allometry of body parts and sample size.

Phylogenetic analysis was performed on cyt. b and the sequences uploaded to NCBI GenBank (Table 1). The thirty-eight cyt. b gene sequences, here amplified from twenty-five species of Tachysurus, were used for molecular phylogenetic analysis. Tachysurus trilineatus was used as the outgroup which is the basal lineage of Tachysurus genus (Ku et al. 2007). The sequences were revised manually and then aligned using ClustalW in MEGA7 (Kumar et al. 2016). Both Maximum Likelihood (ML) and Bayesian Inference (BI) methods were utilised to reconstruct the phylogenetic relationships. The optimal nucleotide substitution model was selected by ModelFinder (Kalyaanamoorthy et al. 2017) according to the Akaike Information Criterion. Maximum Likelihood analysis was run by IQ-tree (Nguyen et al. 2015), with the selected TIM3+F+I+G4 model and 1,000 non-parametric bootstrap replicates. Bayesian Inference was performed in MrBayes (Ronquist et al. 2012) under the selected GTR+F+I+G4 model. Two independent runs were carried out with four Monte Carlo Markov chains (three hot chains and one cold chain) for 20 million generations to calculate posterior probability. Trees were sampled every 1000 generations. The initial 25% of sampled trees were discarded as burn-in. Convergence of the runs was assessed by the average standard deviation of split frequencies (< 0.01).

The genetic distances, based on cyt. B, were computed in MEGA 7 using the Kimura-2-parameter (K2P) model (Kimura 1980).

Shao and Zhang (2023) erected the Tachysurus pratti group using morphological characters, such as smooth anterior margin of the pectoral spine, short maxillary-barbel not extending to the base of pectoral-fin, fewer than 20 anal-fins and uniform yellow or brown body in adults. Under this taxonomic treatment, the T. pratti group is the largest group within the genus, with more than twenty species. However, the monophyly of the T. pratti group was not supported under the molecular scrutiny in this study (Fig. 1), indicating these species were wrongly clustered due to retention of conservative morphological features, common in catfishes (Zhou et al. 2016). Members in the T. pratti group formed two distinct phylogenetic entities in our analyses: T. wuyueensis clustering with T. truncatus, T. pratti, T. gracilis and T. brachyrhabdion form a well-supported monophyletic clade, namely the T. pratti-T. truncatus group, that is sister to T. nitidus. The remaining species of the “T. pratti group” form another monophyly here defined as the T. tenuis-T. crassilabris group, sister to T. vachelli. In addition, the T. pratti-T. truncatus group plus T. nitidus appears to be sister to a clade with T. vachelli and T. tenuis-T. crassilabris group.

Although the morphological similarities between these two newly-erected species groups are numerous, there are some distinguishing features that can be used to diagnose them. The T. pratti-T. truncatus group differs from the T. tenuis-T. crassilabris group in having a shorter dorsal spine (length not longer than vs. longer than two-thirds of HL) (Figs 2, 9, 10). Differences in colour patterns were also found to be apparent between the two clades. The body colour of the T. pratti-T. truncatus group is yellowish-brown (Fig. 2), but dark brown in the T. tenuis-T. crassilabris group (Fig. 10). The distinction in colouration of the abdomen is conspicuous with numerous dark spots in the T. tenuis-T. crassilabris group, but is absent in the T. pratti-T. truncatus group (Fig. 11). In addition, the caudal-fin colouration of round-tailed species can be used as another diagnostic character between these two clades. Except for T. tenuis, the white or yellowish caudal-fin margin is broad in the T. tenuis-T. crassilabris group (Fig. 10), but which is narrow or absent in the T. pratti-T. truncatus group (Fig. 2). The T. pratti-T. truncatus group can also be distinguished from other species groups of Tachysurus in the external morphology. It differs from the T. aurantiacus group, the T. vachelli group and the T. trilineatus group in having short maxillary barbels not extending to (vs. extending beyond or reaching) the base of pectoral fin, from the T. nitidus group, the T. vachelli group and T. virgatus group in having a dorsal fin spine shorter (vs. longer) than two-thirds of HL.

Figure 9. 

Relationship between dorsal spine length and HL for the T. pratti-T. truncatus group (including T. wuyueensis, T. pratti, T. truncatus, T. brachyrhabdion, T. gracilis) and the T. tenuis-T. crassilabris group (including T. tenuis, T. albomarginatus, T. analis, T. lani, T. zhangfei, T. ussuriensis, T. crassilabris).

Figure 10. 

Lateral view of T. albomarginatus (A) for IHB 202110025885, 133.5 mm SL, Macheng County, Hubei Province; T. analis (B) for IHB201707014311, 124.2 mm SL, Yudu County, Jiangxi Province, in Gan-Jiang; T. adiposalis (C) for FMNH 59079, holotype, 172 mm SL, Tamusui River, Taiwan Island; (D) Colour in life of adult of T. adiposalis, cited from Zhou and Gao (2011).

The topotypes of T. adiposalis were not available in this study, but our photographic examination on the type (FMNH59079) confirmed that T. adiposalis has a long dorsal-fin spine (longer than two-thirds of HL) and 19 anal-fin rays (Fig. 10C), which is distinct from T. wuyueensis. Based on data from Taiwan, the dark brown body and wide white margin in the caudal fin are present in a living specimen of T. adiposalis (Fig. 10D) (Zhou and Gao 2011), suggesting this species may belong to the T. tenuis-T. crassilabris group. Moreover, a phylogenetic analysis, based on a short segment of mtDNA, showed the specimens of T. adiposalis collected from Taiwan are closely related to T. ussuriensis of the T. tenuis-T. crassilabris group. These data confirm our conclusion (Watanabe et al. 2007).

Figure 11. 

Ventral view of T. zhangfei (A) and T. gracilis (B) that belong to the T. tenuis-T. crassilabris group and the T. pratti-T. truncatus group, respectively.

High levels of inter-specific variations in the shape of the caudal fin occur in the T. pratti-T. truncatus group; for example, T. pratti has a deeply forked caudal fin, but the remaining species of the group have round-tailed caudal fins. A similar scenario also occurs in the T. tenuis-T. crassilabris group. It suggests the convergent evolution of caudal fin morphology in Tachysurus. There is a tight link between the shape of the caudal fin and habitat preference: round-tailed species present habitat affinities to montane streams, while fork-tailed species are restricted to rivers’ main stream and lakes. The observation describes the effects of ecological niches by shaping the caudal fin in Tachysurus.

The authors declare that they have no conflict of interest.

This work was partially funded by a grant from the National Science & Technology Fundamental Resources Investigation Program of Mount Dabie (Grant No.2019FY101800).

Jia-Jun Zhou designed the study and revised the manuscript. Yuan Le-Yang extracted the genomic DNA and performed the molecular analysis. Wei-Han Shao examined the specimens and prepared the manuscript. All authors read and approved the final version of the manuscript.

We are grateful to Huan-Shan Wang (Museum of Aquatic Organisms at the Institute of Hydrobiology, IHB) for providing comparative specimens; to Zhan-sheng Tang (Zhejiang Jiulongshan National Natural Reserve) for assistance in field surveys.