Four specimens of Tachysurus taeniatus were collected from an affluent of Taihu Lake in Huzhou City, Zhejiang Province. They are thus considered topotypical specimens as their collection site is very close (ca 50 km) to its type locality. Fourteen specimens of T. ondon were collected from the Cao’e-Jiang in Xinchang County (type locality), Zhejiang Province. In addition, 64 specimens identified as this species were captured from ten locations (Fig. 1) of coastal rivers in Zhejiang and Fujian Provinces. Captured specimens were stored in 10% formalin liquid after removal of right-side pelvic-fin clips. The extracted fin clips were kept in 95% ethyl alcohol and used for molecular analysis. The voucher specimens are preserved in the ichthyological collection at the Institute of Hydrobiology (IHB), Chinese Academy of Sciences, Wuhan. Careful morphological examination was made in this study on the holotype of T. taeniatus, currently stored in the Lake Biwa Museum, Japan (Fig. 2A, B).

Figure 1. 

Map showing distributions of Tachysurus taeniatus and T. ondon.

Figure 2. 

Tachysurus taeniatus, holotype, BMNH 1873-7-30-73, 150 mm SL, from Shanghai, China A lateral view B dorsal view.

Measurements were conducted using digital calipers, with data recorded to the nearest 0.1 mm. Whenever feasible, measurements were taken on the left side of each individual, following the techniques outlined by Cheng et al. (2008). Head length and measurements of other parts of the body are estimated as percentages of the standard length (SL). Subunits of the head are provided as percentages of the head length (HL). The number of rays in the dorsal and anal fins was determined following the method described by Watanabe (1995). Other fin rays were counted under a binocular dissecting microscope using transmitted light. Vertebral count was taken from X-ray photographs, with the five anteriormost vertebrae, namely the Weberian complex, not counted.

Morphometric data underwent principal component analysis (PCA) to reveal variations and assess relative contribution of specific variables to morphometric differences between the species. PCA was run with SPSS 16 (SPSS, Chicago, IL, USA). Before conducting the analysis, all measurements were standardized according to Reist (1985) to avoid effects of allometry.

Phylogenetic analysis was performed using mtDNA cyt-b gene and the sequences have been uploaded to NCBI GenBank (Table 1). Twenty-seven cyt-b gene sequences amplified from 17 species of Tachysurus were used for molecular phylogenetic analysis. Tachysurus trilineatus was selected as the outgroup due to its identification as the basal lineage within the genus Tachysurus (Ku et al. 2007). The sequences were manually revised and then aligned using ClustalW in MEGA7 (Kumar et al. 2016). Both Bayesian-inference (BI) and maximum-likelihood (ML) methods were used in the phylogenetic analysis. The optimal nucleotide substitution model was selected by ModelFinder (Kalyaanamoorthy et al. 2017) according to the Akaike Information Criterion. ML analysis was performed using 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).

The holotype of Tachysurus taeniatus, currently stored in the British Museum of Natural History (BMNH), had not been examined by any Chinese investigators before this study. Our observation of this holotype (BMNH 1873-7-30-72; Fig. 2) coincides with its original description in the following characters: (1) maxillary barbels extending beyond the insertion of the pectoral fin, (2) nasal barbels extending beyond the posterior edge of the eye, (3) an adipose-fin shorter than the anal-fin in basal length, (4) a dorsal spine shorter than the body and head depth. However, the holotype has a slightly emarginate caudal fin with the upper lobe slightly longer than the lower lobe (Fig. 2B), rather than a rounded one as stated in its original description. Additionally, it also possesses small serrations on the anterior edge of the pectoral spine covered with skin, a character not mentioned in the original description but shared with the topotypical specimens of T. ondon (Fig. 3).

Figure 3. 

Serrations on the anterior edge of pectoral spine A holotype of Tachysurus taeniatus (BMNH 1873-7-30-73, 150 mm SL, from Shanghai, China) B topotype of T. ondon (IHB081570, 81.2 mm SL, from Xinchang County, Zhejiang Province).

The original description (Günther 1873: 240) is based on a specimen from a collection of freshwater fishes from China sent to the British Museum by Robert Swinhoe, H.M. Consul at Shanghai, who collected the fish “at that place” Günther 1873: 240); no specified localities are given. Though, some materials could be collected by Swinhoe during his trip upstream the Yangtze River in 1869 (Heok Hee Ng personal communication), we suppose that Tachysurus taeniatus was found in Shanghai or nearby area because of the following reasons. First, the species hasn’t been collected from Shanghai in recent years due to urbanization; however, it still occurs in areas close to Shanghai, such as Huzhou City in Zhejiang Province. Second, and more important, T. taeniatus shows coloration polymorphism, and the coloration pattern described in the original publication—a continuous black stripe along the mid-body—is only found in Shanghai and some locations in adjacent Zhejiang Province (belonging to coastal rivers of southeast China); this particular coloration is absent in T. taeniatus from other parts of the Yangtze River.

Brief accounts on the coloration pattern of the two species were provided in their original descriptions based only on a single specimen for each species, making it unfeasible to understand the intraspecific variations of coloration. In the T. taeniatus and T. ondon specimens examined in this study, ontogenetic changes in coloration were observed: the lateral blackish band or blotches found in small individuals becomes blurred in adult individuals exceeding 180 mm SL. Moreover, three coloration morphs exist in the juveniles and subadults of the specimens of these two species. One morph includes individuals which possess a yellowish body with three longitudinal blackish bands along the lateral body, of which the median band is continuous but the other two are interrupted to form three rectangular blotches (Fig. 4A). This coloration morph, as stated in the original description of T. taeniatus which reads “a broad blackish band along the side of the body”, is only found in some topotypical specimens (Günther 1873). Despite the shared presence of three blackish bands, another coloration morph develops an uninterrupted median band. This is also present in the topotypical specimens of both T. taeniatus and T. ondon (Fig. 4B, C). Furthermore, one more morph was detected in the specimens identified as T. ondon from Ningde City, Fujian Province with three broad vertical brown blotches on the yellowish background of the body (Fig. 4D).

Figure 4. 

A color in life of adult of topotype of Tachysurus taeniatus B lateral view of T. ondon, topotype, IHB081570, 81.2 mm SL, Xinchang county, Zhejiang Province C lateral view of T. taeniatus, topotype, IHB202406066714, 74.4 mm SL, Huzhou city, Zhejiang Province D lateral view of a live specimen of T. ondon collected from Ningde City, Fujian Province.

The specimens designated for comparative analysis were categorized into three groups: 1. topotypic T. taeniatus, 2. topotypic T. ondon, 3. other specimens primarily identified as T. ondon. The measurements of the examined specimens are summarized in Table 2. No discrete differences between these groups were found by comparing the morphometric data. Meristic counts for the type specimens of T. taeniatus and T. ondon, and other specimens examined of T. ondon are given in Table 3. Three meristic characters have variable counts, namely anal and pelvic fins, and vertebrae. Counts of all three meristic characters are not significantly different with overlapping ranges among the three groups.

In a principal component analysis (PCA) performed on twenty-two morphometric characters, the combinations of PC1 against PC2 and PC3, along with PC2 against PC3, failed to separate the examined specimens of T. ondon and the topotypes of T. taeniatus (Fig. 5).

Figure 5. 

Scatter plots A PC1 against PC2 B PC1 against PC3 C PC2 against PC3 extracted from morphometric data for examined materials of T. taeniatus and T. ondon.

A total of 1066 bps were included in the aligned dataset of the cyt-b gene, with 642 conservative sites, 424 variable sites, 242 parsim-informative sites and 94 singleton sites. The topologies of the phylogenetic trees were found to be similar between ML and BI methods (Fig. 6). The three topotypical specimens of T. taeniatus were found to be together with the specimens examined of T. ondon in a monophyletic group that was recovered with 100% posterior probabilities (pp) and 0.99 bs in ML and BI trees, respectively. This monophyly showed a phylogenetic affinity to the Japan endemic T. aurantiacus, within a clade containing T. koreanus, T. nudiceps, T. brevicorpus, T. tokiensis, T. eupogon, T. sinensis and T. intermedius, all of which were designated as belonging to the T. aurantiacus group by Shao and Zhang (2023).

Figure 6. 

Phylogenetic tree of Tachysurus species inferred from cyt-b using Bayesian-inference and maximum-likelihood methods. Bayesian posterior probabilities (>0.6) and maximum-likelihood bootstrap values (>60%) are shown, respectively. All members of the T. aurantiacus group (as defined by Shao and Zhang 2023), with the exception of T. brevianalis, were included in the phylogenetic tree.

The two closely related species of T. taeniatus and T. aurantiacus have a genetic distance of 1.5% and the distances between T. taeniatus and other members of the T. aurantiacus group are in the range of 6.4–10.0% (Table 4).

Main distinguishing characters of Tachysurus taeniatus, as herein diagnosed, include: (1) a serrated anterior edge of the pectoral spine; (2) a slightly emarginate caudal fin with the upper lobe slightly longer than the lower lobe; (3) longer maxillary barbels extending beyond the base of the pectoral spine; and (4) a shorter dorsal-fin spine than body depth. The first character was not mentioned in the original description. Our examination of the holotype confirmed observation by Watanabe and Kitabayashi (2001) that T. taeniatus has a serrated anterior edge of the pectoral spine (Fig. 3A). The statement of the second character (caudal-fin shape) in the original description of this species is inaccurate. According to our examination of the holotype, T. taeniatus has a slightly emarginate caudal fin with an upper lobe slightly longer than the lower lobe, in contrast with its original description that reads: “Caudal rounded”. As far as is known, rounded caudal fins are only present in T. tenuis, T. trilineatus, T. analis, and T. lani (Cheng et al. 2021; Shao et al. 2021). Clearly, misled by the vagueness and inaccuracy of its original description and no accessibility to type materials, subsequent Chinese researchers (Zheng and Dai 1999) had a misconception that T. taeniatus, thus erroneously identify it as a species with a smooth anterior margin of the pectoral spine and a rounded caudal fin. This can reasonably explain why no additional specimens of T. taeniatus have been found from its type locality and nearby river systems since the original description.

Both T. taeniatus and T. ondon are currently assigned to the T. aurantiacus group which, thus, included eleven species (Shao and Zhang 2023). Four species, namely T. nudiceps, T. sinensis, T. eupogon, and T. intermedius, develop deeply forked caudal fins. A slightly emarginate caudal fin is found in the remaining seven species, viz., T. aurantiacus, T. koreanus, T. tokiensis, T. brevianalis, T. brevicorpus, T. taeniatus, and T. ondon. The last two species are known only from the mainland China, with the other species occurring in Korea Peninsula, Japan Archipelago, and Taiwan Island. However, our molecular phylogenetic analysis based on the cyt-b gene showed that all samples identified as these two species from Zhejiang and Fujian provinces of China, including those from the type locality of T. ondon and an affluent of Lake Taihu in proximity to the type locality of T. taeniatus, clustered together into a lineage sister to T. aurantiacus, a species endemic to western Kyushu of Japan (Watanabe and Maeda 1995).

Despite a 1.5% genetic distance between T. taeniatus (including T. ondon) and T. aurantiacus, they are clearly distinguishable by the snout length, 33–41% HL vs 23–33% (data for T. aurantiacus from Watanabe and Maeda (1995) and serration on the anterior edge of the pectoral spine which is better developed in T. aurantiacus incomparison with T. taeniatus. Although, the genetic distances between the two taxa are comparatively low, it lies within the range used for delineating currently described species of Tachysurus, for example, the genetic distance between T. pratti and T. truncatus is 1.4% and between T. longispinalis and T. kyphus is 1.6% (Ku et al. 2007). The relatively low genetic distance between closely related species within Tachysurus may be attributed to the significantly low substitution rate of the mitochondrial genes in the East Asian bagrids and the recent estimated divergence time as supposed by Peng et al. (2002), Campbell and Piller (2017) and Shao et al. (2021). Besides, the two species have discontinuous distribution patterns. So, we treat them as two distinct species.

The genetic divergence among samples collected from coastal rivers of Zhejiang and Fujian provinces was 0.3–0.4%. No significant morphological variations were found among different geographic populations. All these findings suggest that the samples from mainland China, with a serrated anterior edge of the pectoral spine and a slightly emarginate caudal fin, represent a single species. According to International Code for Zoological Nomenclature, Art. 23.3 (International Commission on Zoological Nomenclature 1999), T. taeniatus should be considered a senior subjective synonym of T. ondon.

In Chinese literature, a continuous black longitudinal stripe on the lateral body is viewed as the main diagnostic character for T. taeniatus (Zheng and Dai 1999). This body coloration pattern was utilized in this study to identify four specimens as T. taeniatus collected from an affluent of Lake Taihu, very close to the type locality. Ontogenetic changes in body coloration, though, were detected in T. taeniatus; there are conspicuous variations found between juveniles/sub-adults and adults. The blackish stripes or blotches on the flank become less distinct or disappear entirely in individuals of more than 180 mm SL, thus giving a uniformly brown body coloration. A similar ontogenetic change in body coloration has also been documented for closely related species, including T. aurantiacus, T. tokiensis, and T. koreanus (Lee and Kim 1990; Watanabe and Maeda 1995), all of them being members of the T. aurantiacus group. Recent studies of catfishes have inferred that ontogenetic coloration transformations are closely related to ontogenetic changes in daily activity period (Zanata and Prmitivo 2013; Vilardo et al. 2020; Costa et al. 2023). Juveniles and subadults exhibit heightened activity during the day, whereas adults display a preference for nocturnal behavior, necessitating a darker coloration phenotype. This explanation aligns with the field evidence presented in this study. Coloration polymorphism was also observed in the juveniles and sub-adults of T. taeniatus with three coloration morphs detected (Fig. 2). This implies that the body coloration of this species can vary not only due to ontogenetic changes but also in response to local environmental conditions and geographical distributions.

It is apparent that the existence of ontogenetic coloration changes and coloration polymorphism in T. taeniatus and closely related species argue against solely relying on body coloration for distinguishing species of Tachysurus. However, it is also incorrect to totally dismiss the taxonomic value of body coloration in Tachysurus. For example, T. trilineatus has three longitudinal brownish narrow bands running along its flank with the median band featuring a row of yellow spots along the lateral line, a unique body coloration separating it from all congeneric species (Shao and Zhang 2023). This unique body coloration has led to the designation of this species as a monotypic group, a classification supported by a molecular phylogenetic analysis (Ku et al. 2007). Body coloration should not be discarded a priori as evidence of species boundaries within Tachysurus. It is essential to reinforce species delineation hypotheses using additional morphological evidence such as morphometric characters or molecular data rather than relying solely on coloration.

The smoothness of the anterior edge of the pectoral-fin spine has been considered a significant diagnostic character in Tachysurus (Ku et al. 2007; Shao and Zhang 2023). The T. aurantiacus group, containing nine species (Fig. 6), can be distinguished from the congeners by the presence of a rough pectoral-fin spine (Shao and Zhang 2023). With the exception of T. brevianalis, which is endemic to Taiwan Island, all members of this group were included in the phylogenetic analysis in the present study that revealed its monophyly. The group is further subdivided into two clades (Fig. 6) different in some morphological features and ecological niches. Clade 1 includes T. sinensis, T. intermedius, and T. eupogon, distributed in mainland China and northern Vietnam. This clade is characterized by a forked caudal fin and a preference to inhabit the main stream of rivers. The remaining species of T. aurantiacus group form clade 2 (T. taeniatus, T. aurantiacus, T. koreanus, T. nudiceps, T. brevicorpus, T. tokiensis). Except for T. nudiceps, the members of this clade have round-tailed caudal fins and are adapted to fast-flowing montane streams (Krishnadas et al. 2018). All members of this clade, with the exception of T. taeniatus, are distributed in Japan or the Korean Peninsula. Since T. taeniatus appears to be the youngest lineage in clade 2, it is likely that the ancestor of clade 2 likely originated in Japan and/or the Korean Peninsula, subsequently dispersed to mainland China during interglacial periods (Zahiri et al. 2019) followed by subsequent isolation resulted in separation of T. taeniatus at the species level.

The authors have declared that no competing interests exist.

No ethical statement was reported.

The project is funded by the Natural Science Foundation of China (no. 32460919).

Conceptualization: WHS. Data curation: JLC, WHS. Formal analysis: EZ, WHS. Funding acquisition: JLC, WHS. Supervision: EZ. Validation: EZ. Visualization: EZ. Writing - review and editing: JLC.

E. Zhang https://orcid.org/0000-0002-6971-7160

All of the data that support the findings of this study are available in the main text.