Thirty-three names currently are recognised as valid for species of Poropuntius (Fricke et al. 2023) with distribution of the genus ranging from the Irrawaddy River basin in Myanmar to the Mekong and Red River basins in Yunnan, China and south through Vietnam, Laos, Cambodia, Thailand, and peninsular Malaysia to Sumatra, Indonesia. The greatest diversity occurs in the Mekong basin of Yunnan and Vietnam, and only one species is known from peninsular Malaysia and one from Sumatra (Fricke et al. 2023).

Poropuntius has been characterised as usually having 8½ branched dorsal rays, large serrae on the posterior edge of the last simple dorsal ray, and tubercles covering the tip of the snout and occurring on the lacrimal bones (Rainboth 1996; Roberts 1998). Tubercles occur on both sexes and, at least in most species, on juveniles as well as adults (Roberts 1998). In addition, most species have rostral and maxillary barbels, a black to dusky submarginal stripe on the upper and lower caudal-fin lobes, and a well-developed keratinised edge on the lower jaw.

Considerable confusion surrounds the taxonomy of Poropuntius, with most species having been diagnosed using morphological characters that tend to be highly plastic and comparisons made to few congenerics. Wu et al. (2013) and Muhammad-Rasul et al. (2018) investigated molecular diversity of Poropuntius and, although they targeted only a few species and small geographic areas, their results suggested some morphological hypotheses concerning species delimitations are incorrect, and larger molecular studies are needed to improve our understanding of species diversity in Poropuntius. Recent fieldwork throughout much of the distribution of Poropuntius, especially in species-rich Vietnam, has provided material for a broader study of molecular variation in the genus. Results of that study, reported herein, suggest that several species names, including nominal species of Acrossocheilus and Hypsibarbus in Vietnam, are synonyms of previously described species. A newly discovered species from Vietnam is described using morphological and molecular characteristics.

The ML trees for the COI and Cytb sequences are shown in Fig. 2. The COI topology is consistent with recognition of 16 species (Fig. 2A), all represented by more than one sequence with ≥ 83% bootstrap support, including one undescribed species.

The Cytb sequences were used to examine the results of the study of P. huangchuchieni in southwest China by Wu et al. (2013) with a greater Cytb dataset (Fig. 2B). The five major lineages identified by Wu et al. (2013) are reassigned. The LX lineage of Wu et al. (2013) from the Song Hong aligned with P. krempfi in our sequences, the SW of the Upper Salween with P. opisthopterus, the RL of the Song Hong with P. alloiopleurus, the MK-A of the Lower Lancang with P. huangchuchieni, and the MK-B of the Lower Lancang with P. laoensis. The greatest similarity in the Cytb sequences was 98% (±0.00), found between P. laoensis, which occurs in the Lower Lancang, Kratie – Stung Treng, northern Annam, Chao Phraya, and Middle & Lower Salween ecoregions, and P. huangchuchieni in the Lower Lancang (Table 1). The COI data for P. huangchuchieni (Fig. 2A) aligned with the reassignments based on the Cytb data and are not discussed further.

Seven of the 16 species recognised with COI data (Fig. 2A) include representatives of other nominal species now to be recognised as synonyms. One, the P. krempfi clade includes 12 specimens of three nominal species currently assigned to two genera: P. krempfi, Hypsibarbus annamensis, and H. macrosquamatus. The newly collected specimens of all three nominal taxa were collected from their type localities in the Song Hong and northern Annam ecoregions and are embedded with P. krempfi specimens from the Song Hong with a mean similarity of 98.8% (± 0.29) (Fig. 2A). This clade received 83% bootstrap support. Hypsibarbus annamensis, and H. macrosquamatus are synonyms of P. krempfi. The P. krempfi lineage in the Song Hong ecoregion has a COI sequence similarity of 97% (± 1.00) with P. alloiopleurus from the Song Hong, P. opisthopterus from the Upper Salween, P. burtoni from the Sittang - Irrawaddy, and P. schanicus from the Inle Lake, Myanmar (Table 2).

The P. alloiopleurus clade (Fig. 2A, B) consists of 13 specimens of six nominal species (plus specimens labelled P. huangchuchieni in Wu et al. (2013) and Zheng et al. (2016)), currently in two genera: P. alloiopleurus, P. baolacensis, P. brevispinus, P. rhomboides, Acrossocheilus macrophthalmus, and A. xamensis collected in the Song Hong and northern Annam ecoregions from their type drainages (Fig. 2A). The newly collected specimens are embedded with P. alloiopleurus specimens from the Song Hong and show a mean similarity of 99.5% (± 0.20). This clade receives 100% bootstrap support. Thus, we conclude that P. baolacensis, P. brevispinus, P. rhomboides, A. macrophthalmus and A. xamensis are synonyms of P. alloiopleurus. The P. alloiopleurus lineage observed in the Song Hong ecoregion has a COI sequence similarity of 97% (±1.00) with P. krempfi from the Song Hong, P. huangchuchieni and P. laoensis from the Lower Lancang, and P. opisthopterus from the Upper Salween (Table 2).

The P. huangchuchieni clade consists of three specimens of two nominal species, P. huangchuchieni and P. angustus collected from their type drainages in the Lower Lancang ecoregion. These specimens have a mean similarity of 100% (± 0.00) (Fig. 2A). Poropuntius angustus is a synonym of P. huangchuchieni. The P. huangchuchieni lineage in the Lower Lancang ecoregion has a COI sequence similarity of 97% (± 1.00) with P. laoensis from the Lower Lancang, P. alloiopleurus from the Song Hong, and P. opisthopterus from the Upper Salween (Table 2).

The P. laoensis clade consists of 17 specimens of three nominal species, P. laoensis, P. aluoiensis, and P. bantamensis collected in the Lower Lancang, Kratie – Stung Treng, northern Annam, Chao Phraya, and Middle & Lower Salween ecoregions including from their type localities. The newly collected specimens are embedded with P. laoensis specimens from the Kratie – Stung Treng and show a mean similarity of 99.1% (± 0.23) (Fig. 2A). This clade receives 97% bootstrap support. Poropuntius aluoiensis and P. bantamensis are synonyms of P. laoensis. The P. laoensis lineage observed in the Lower Lancang, Kratie – Stung Treng, northern Annam, Chao Phraya, and Middle & Lower Salween ecoregions has a COI sequence similarity of 97% (± 1.00) with P. huangchuchieni from the Lower Lancang, P. alloiopleurus from the Song Hong, and P. opisthopterus from the Upper Salween (Table 2).

The P. anlaoensis sp. nov. lineage consists of four specimens of a new species of Poropuntius. This lineage from the Annam ecoregion has its greatest COI sequence similarity of 94% (± 1.00) with P. kontumensis from the Kratie – Stung Treng (Table 2).

The P. kontumensis clade consists of six specimens of two nominal species, P. kontumensis and P. yalyensis, collected in the Kratie – Stung Treng ecoregion from their type localities. The newly collected specimens are embedded with P. kontumensis specimens from the type locality and have a similarity of 100% (± 0.00) (Fig. 2A). Poropuntius yalyensis is a synonym of P. kontumensis. The P. kontumensis lineage from the Kratie – Stung Treng ecoregion is most similar to P. carinatus from the Lower Lancang with a COI sequence similarity of 97% (± 1.00) (Table 2).

The P. deauratus clade consists of 57 specimens of six nominal species, P. normani, P. deauratus, P. bolovenensis, P. lobocheiloides, P. smedleyi, and P. solitus, collected the eastern Gulf of Thailand drainages, Malay Peninsula, Mekong Delta, Kratie – Stung Treng, Khorat Plateau, and Annam ecoregions, including from their type localities. These specimens are embedded with P. deauratus specimens from the Annam ecoregion and have a mean similarity of 98.7% (± 0.25). This clade receives 98% bootstrap support. Poropuntius normani, P. bolovenensis, P. lobocheiloides, P. smedleyi, and P. solitus are synonyms of P. deauratus. The P. deauratus lineage in the Eastern Gulf of Thailand drainages, Malay Peninsula, Mekong Delta, Kratie – Stung Treng, Khorat Plateau, and Annam ecoregions has a COI sequence similarity of 96% (± 1.00) with P. carinatus from the Lower Lancang and P. kontumensis from the Kratie – Stung Treng ecoregion (Table 2).

The objective of this study was to test, using molecular data, the species diversity of Poropuntius, especially that of Vietnam in which 15 species names have been recognised recently as valid. As noted above, most species of Poropuntius have been described using only morphological data, and recent investigations using molecular data suggested that some morphological hypotheses were incorrect. Our data indicate that 17 names assigned to species are synonyms.

Several factors led to the inflation of species names for Poropuntius. These include limited efforts overall in collecting and classifying cyprinid fishes, especially in northern Vietnam, limited access to type specimens in European museums, limited availability of publications (often in foreign languages – although now this is being alleviated by online access), inadequate descriptions containing limited information, and failure to appreciate morphological variation related to trophic diversification within species. Finally, a substantial portion of the cyprinid diversity of northern Vietnam is shared with that of southern China, but researchers in the two countries have not had access to specimens from both countries or to original descriptions in other languages. Consequently, studies were limited to in-country fauna, and species in each country were treated as endemic to that country.

Ecomorphological variation in Poropuntius was described and figured by Roberts (1998: figs 6, 8), and Kang et al. (2016) interpreted P. bolovenensis, P. lobocheiloides, and P. solitus as ecomorphs of P. bolovenensis. The often-extreme intraspecific phenotypic variation in the shape of the mouth and lips, and in the development of the horny sheath on the lower jaw, has led to taxonomic confusion in Poropuntius. Ecophenotypic variation is expressed especially clearly in species occupying pools isolated by barriers such as waterfalls in streams on plateaus at high elevations (e.g., on the Bolaven, Kontum, and Langbiang Plateaus) and in drainages leading to the Gulf of Tonkin (in the Song Hong and Annam ecoregions). Species not exhibiting such extreme polymorphism may nevertheless carry information in their genomes necessary to generate an array of potentially taxonomically confusing, continuous variation as well as more disjunct phenotypes. For example, the position of the dorsal fin, often used in taxonomic diagnoses, varies from more anteriorly to more posteriorly positioned in Poropuntius, as seen in P. alloiopleurus, P. huangchuchieni, and P. laoensis (Wu et al. 2013; this study). Specimens of Poropuntius from flowing water habitats are characterised by a more elongated body and a relatively long caudal peduncle, while those from pools have a deeper body and larger unpaired fins (Figs 3A, B, D, E, 4G, H). Similar patterns have been described in other cyprinids, e.g., Rasbora paviana and Lobocheilos rhabdoura, with populations inhabiting fast-flowing streams having more slender bodies than those inhabiting slower-flowing habitats (Stolbunov et al. 2011; Ciccotto and Page 2016).

During a decade of field work on Southeast Asian freshwater fishes, the first author and colleagues have collected samples of P. deauratus, P. krempfi, P. alloiopleurus, and P. laoensis revealing marked intraspecific phenotypic variation. These samples include those from river basins in which few or no fish collections have been made or reported upon previously, and which fill in gaps in the distributions of these species. Pronounced variation among adult individuals in these species has been observed in body depth, dorsal-fin spine length, ossification and serration of the last simple dorsal-fin ray, body colour, and caudal fin colour. For example, in the wide-ranging P. deauratus, the typical colour in life is a silvery to light green gold body with bright yellow on the posterior half of the body and a bright lemon-yellow caudal fin with bold black submarginal stripes. However, colour in this species may be much more subtle with the body uniformly dusky to dark and the fins dusky (Fig. 4B, H).

This study has revealed that scientific names have been applied erroneously to populations of P. krempfi, P. alloiopleurus, P. huangchuchieni, P. laoensis, P. kontumensis, and P. deauratus. Errors in the taxonomy of Poropuntius have resulted primarily from inadequate sampling and reliance on characters that vary intraspecifically. Additional samples from unsampled or poorly sampled populations allowed the use of molecular data to test previous species hypotheses. More sampling and use of molecular as well as morphological data on species of Poropuntius that could not be included in this study are likely to require additional taxonomic changes.