Description of a new Pangasius (Valenciennes, 1840) species, from the Cauvery River extends distribution range of the genus up to South Western Ghats in peninsular India

Introduction

The past few decades have witnessed the discovery of several new catfish species from southeast and south Asia (Pouyaud, Teugels & Legendre, 1999; Roberts, 1999; Pouyaud & Teugels, 2000; Pouyaud, Gustiano & Teugels, 2002; Gustiano, Teugels & Pouyaud, 2003) including some from the peninsular India (Radhakrishnan, Sureshkumar & Ng, 2010; Babu, 2012; Ng, 2013; Lal et al., 2017; Dwivedi et al., 2017). Pangasiid catfishes (Pangasius Valenciennes, 1840) are popular for their commercial value in both aquaculture and wild capture fisheries (Roberts & Vidthayanon, 1991) and are considered to be a delicacy. A total of 22 valid species are distributed in the river basins of southeast and south Asia (Dwivedi et al., 2017; Fricke, Eschmeyer & Fong, 2021). These have also been introduced in many countries for aquaculture. Among the valid species, only two species have been known from the Indian rivers, the most common is Pangasius pangasius (Hamilton, 1822), distributed in the rivers Ganges, Brahmaputra and Godavari, and the other, Pangasius silasi (Dwivedi et al., 2017) is described from the Nagarjuna Sagar Dam, in the Krishna River.

Previously, Hora (1951) had mentioned about the presence of Pangasius in the Stanley Reservoir (also known as Mettur Dam), which is one of the largest fishing reservoirs in South India. However, inadequate data led to consideration of the species as Pangasius pangasius only, though this raised hitherto unresolved query for the biogeographers of the time (Silas, 1952b). They assumed the absence of divergence in Pangasius genus, which probably migrated into peninsular India from Himalayan rivers. This is in contrast to some genera which are common between rivers of the Himalaya and the Indian peninsula, however, are represented by different species such as Silonia (family Schilbidae).

The explorations were conducted in the Cauvery River basin, Tamil Nadu, India and specimens of Pangasius were collected. In the present study, we used mitochondrial gene cytochrome c oxidase (COI) to support the morphological analysis. Morphometrics and molecular marker dataset analysis were used for deciphering phylogenetic relationships and species delimitation. Results explicitly indicated a distinct clade of the specimens from the Cauvery River, which is well differentiated from its congeners and described here as Pangasius icaria, a new species.

Materials & Methods

Specimens described in present study, were collected from the commercial catches from landings along the locations, Mettur Dam, (Tamil Nadu). and upstream of Shivanasamudra falls, Chamarajanagar (Karnataka), on the Cauvery River basin in India. The details of samples and collection localities are presented in Table S1. The right-side pectoral fin was excised and preserved in 95% ethanol for molecular studies form individual specimen. Specimens were tagged with unique code, preserved in formalin (10%) and transported to the laboratory for examination and permanent preservation.

Data for meristic and morphometric measurements were generated following Ng & Dodson (1999). Measurements were taken using a digital caliper (Mitutoyo Digimatic Caliper, Japan) to the nearest 0.1 mm. The body and head subunits were presented as percent (%) of standard length (SL). A total of 31 morphological characters were recorded for comparative assessment with other pangasius species (Roberts & Vidthayanon, 1991; Dwivedi et al., 2017). Individual digital X-ray radiographs were used for vertebrae count following Ng & Kottelat (2013). Morphometric measurements in proportion of standard length (SL) and head length (HL) of the holotype, followed by paratype were documented. The holotype and paratype accessions, examined in the study, were deposited in the notified repository (http://nbaindia.org/uploaded/pdf/notification/1%20designated%20repositories.pdf), National Repository and Fish Museum at ICAR-NBFGR, Lucknow, India.

Results

Pangasius icaria, sp. nov.

Figs. 1–8; Table 1 and Table S4

Holotype. NBFGR/PANPTAM, 211.6 mm SL, India, Tamil Nadu, Salem district, Mettur Dam village, Mettur Dam, 11°48′47.1′N; 77°48′08.2′E, 30 June 2021, Kathirvelpandian and Kodeeswaran.

Paratypes. NBFGR/PANPTAM.1–11, 11 ex., 130.6–257.6 mm SL, same data as holotype. NBFGR/PANPTAM.12–15, 4 ex., 313–420 mm SL, upstream of Shivanasamudra falls, Cauvery river basin, Chamarajanagar, 12°16′11.6′N; 77°10′08.0′E, Karnataka, Charan, Basheer and Rahul.

Diagnosis. Pangasius icaria differs from all the sister species of south Asia by by the following combination of characters: a set of widely placed, small and rounded vomerine and palatine tooth plates, moderately rounded snout on dorsally viewed, maxillary barbel reaching beyond the base of the pectoral spine, eye diameter 2.4–4.4% SL, smaller-interorbital distance 10.1–12.4% SL, caudal peduncle depth 6.5–8.2% SL, filamentous first dorsal- and pectoral-fin ray, gill rakers 16–19, 50 vertebrae and a reddish dorsal-, anal- and pectoral-fin base.

Description. Morphometric data of the P. icaria as in Table 1. Head depressed and snout rounded. Mouth sub-terminal, upper jaw tooth band faintly exposed when mouth closed (Figs. 1 and 2). Relatively shorter median longitudinal groove, reaching the base of occipital process, occipital process terminates before the dorsal-spine plate (Fig. 3). Lower jaw slightly curved or rounded. Mouth moderately wide vomerine and palatine tooth bands are small and rounded and widely placed from each other (Fig. 4). Barbels well developed, maxillary barbels long extending beyond the pectoral-spine base; mandibular barbels long extending beyond gill opening. All the fin-rays branched, dorsal-fin rays seven, originating vertical through half the length of the pectoral spine. Dorsal spine long, sturdy with blunt tip, ventral margin serrated dorsal margin weakly serrated. Adipose fin short, its base 2.2% SL. Pectoral-fin rays 11, with filamentous extension reaching the base of the pelvic fin; pelvic-fin rays 6, its origin vertical through the posterior end of the dorsal-fin base, not reaching anal fin; anal fin with 27–29 rays, terminates well-before caudal-fin base. Dorsal procurrent caudal-fin rays 16–17; ventral procurrent caudal-fin rays 13–15; principal caudal-fin rays 17 (8+9), deeply forked.

Coloration. Live specimens: Dorsal surface bronze green to dark-green, ventral silvery white; dorsal-fin base reddish to bright-orange with black margin; caudal fin reddish-black with creamy white margin; pectoral fin dark-red with white margin; pelvic fin creamy-red white; anal-fin base brick-red becoming translucently white towards the margin; adipose fin dark-grey base with white margin (Figs. 2A, 2B). Eye pupil black with golden margin. Maxillary barbel dark-greyish in colour, mandibular barbel creamy-white. In preserved condition; dorsal surface dark, ventral region creamy-white or pale-grey; all the fins beige to tan with pale yellowish base.

Feeding. Gut content analysis of three samples revealed the presence of the molluscs, Bellamya bengalensis (Fig. 5A); X-ray radiographs also revealed the presence of numerous molluscs in the gut (Fig. 5B).

Habitat and distribution. Presently known only from Cauvery River basin collected from two locations, at Mettur Dam, and in the upstream of Shivanasamudra Falls, Chamarajanagar, Karnataka. The species was collected using a gill net at a depth of 5–15 m during the discharge of water from the main dam (Fig. 6).

Etymology. The species is named after the Indian Council of Agricultural Research (ICAR) and used its abbreviated form. ICAR is the parent organization for NBFGR, which has conducted this research.

Comparison. Pangasius icaria differs from all other sister species by having widely placed, small and rounded vomerine and palatine tooth plates (vs. single curved and joined palatine and vomerine tooth plates in Pangaisus larnaudii Bocourt 1866; uninterrupted broad, strongly curved, palatal tooth band in Pangasius krempfi Fang & Chaux 1949 and Pangasius sanitwongsei Smith 1931; squared vomerine tooth plate with smaller palatine tooth bands in Pangasius myanmar (Roberts & Vidthayanon, 1991) and Pangasius polyuranodon Bleeker 1852; wider vomerine tooth plate with large palatine plates in Pangasius bocourti Sauvage 1880 and Pangasius djambal Bleeker 1846; single large median palatal tooth plate in Pangasius humeralis Roberts 1989, Pangasius kinabatanganensis (Roberts & Vidthayanon, 1991), Pangasius lithostoma Roberts 1989 and Pangasius nieuwenhuisii (Popta 1904).

Further, P. icaria differs from sister species from south Asia, P. pangasius, by having more vertebral count 50 (vs. 44 in P. pangasius); lesser gill rakers 16–19 (vs. 23); four small and round tooth plates which were widely separated from each (vs. elongated and curved toothplates placed close to each other); lesser caudal peduncle depth 6.5–8.2% SL (vs. 9.8–13.1% SL); smaller-interorbital distance 10.1–12.4% SL (vs. 13.8–14.7% SL); larger maxillary barbel length, 11.1–23.9% SL (vs. 10.56–10.87% SL). Pangasius icaria differs from its sister species P. silasi by having a widely separated and rounded small tooth plates (vs. uninterrupted combined tooth plates in P. silasi); more vertebral counts 50 (vs. 48); lesser gill rakers 16–19 (vs. 21); longer pectoral- and dorsal-spine length, 15.1–19.7% SL; 14.4–18.6% SL (vs. 4.02–9.42% SL; 8.46–11.62% SL); longer caudal peduncle length, 14.6–19.2% SL (vs. 11.04–14.88% SL); lesser caudal peduncle depth 6.5–8.2% SL (vs. 10.40–12.24% SL); shorter adipose-fin base length, 1.5–2.9% SL (vs. 3.96–5.87% SL); larger maxillary barbel length, reaching pectoral-fin base, 11.1–23.9% SL (vs. relatively shorter in length, 10.94–13.38% SL); smaller-interorbital distance 10.1–12.4% SL (vs. 12.66–15.11% SL). Pangasius icaria shares a similar vertebral count and have a filamentous extension on the dorsal and pectoral fin with Pangasius santiwongsei Smith, 1931 but readily differs by having a smaller mouth width gape 6.2–9.0% SL (vs. 17% SL). Further, the new species shares a similar vomerine palatine tooth band with Pangasius macronema Bleeker, 1850 but readily differs by having more vertebrae 50 (vs. 42–45 in Thailand species and 41–43 in Borneo species); gill rakers 16–19 (vs. 36–45); having smaller eye, 2.4–4.4% SL (vs. 5.2–9.6% SL); larger adipose-fin base, 1.5–2.9% SL (vs. 0.1–1.2% SL).

Discussion

In South Asia, the genus Pangasius is represented by two known species, to date. P. pangasius has a wide distribution in the rivers of Ganga, the Brahmaputra in India, Bangladesh and the Irrawaddy basin in Myanmar. The results from species delimitation approaches indicate consensus support for the presence of groups within genus Pangasius and provide clear and distinct monophyly of P. icaria. The new species reported here from a peninsular Cauvery River is clearly distinct from P. silasi, which appears to be its nearest neighbour. The species, P. silasi which is endemic to peninsular India and known only from the Krishna River system, so far (Dwivedi et al., 2017). It is interesting to note that feeding habit of P. icaria is similar to P. silasi (Ajith Kumar, Santosh & Lal, 2020) and both are molluscivorous.

The discovery of P. icaria increases the number of Pangasiid catfishes in south Asia to three. Pangasius species can be easily distinguished by the dorsal view of the head and vomerine, palatine tooth plates as suggested by Roberts & Vidthayanon (1991). The new species is easily distinguishable from its known sister species, in south Asia, P. pangasius, and P. silasi by possessing a small, round and widely separated pair of tooth plates and filamentous fin extension. A common and widely cultivated Pangasiid catfish Pangasianodon hypothalamus (Sauvage, 1878) in India was also collected from the cultured pond of the same locality of the new species. The new species differs from the latter by having longer maxillary and mandibular barbels (vs. very short in P. hypothalamus). Genetic analysis also reveals the valid species status of the new species described in this study. Locally, the species is well known among the fisherfolks as ‘Aie Keluthi’ in the vernacular Tamil language. It is likely that Talwar & Jhingran (1991) have documented this new described species as P. pangasius from the Cauvery River, bearing the same vernacular name of ‘Aie Keluthi’. Similarly, Silas (1952a) also documented P. pangasius from the Cauvery river system which might be the species described here. In the study of the evolutionary status of fishes of Himalayan and peninsular rivers, Silas (1952b) documented that fishes found that peninsular rivers of India harbour the genera common to Himalayan rivers but are represented by the alternate species. This was attributed to the probable migration of Malayan elements, during the Pleistocene period from Assam Himalayas to peninsular India (Hora, 1951; Menon, 1980) and named as Satpura hypothesis by Hora (1949). The geological changes and vicariant alteration of river courses causing separation of migrating populations consequently resulted in the observed speciation in the peninsular rivers. The separation of these species might have occurred during middle Pleistocene (around 0.78–0.126 Mya) and led to the evolution of endemic fish species distributed in the peninsular rivers (Briggs, 2003). The present results did not reveal presence of any specimen which is similar to P. pangasius, the most common species found in India. Dwivedi et al. (2017) validated the presence of P. pangasius only upto the rivers Mahanadi and Godavari (with minor morphological difference), however, discovered an undescribed species in the Krishna River, named as P.silasi. Therfore, the earlier reports of P. pangasius in the Krishna River (Ramakrishnaya, 1986) were not confirmed by the later studies (Dwivedi et al., 2017; Ajith Kumar, Santosh & Lal, 2020). The Mahanadi, Godavari, Krishna and Cauvery rivers are the large independent basins in peninsular India, which flow eastward to drain into Bay of Bengal. The Krishna River has the Mahanadi and Godavari rivers on its northern side and the Cauvery River on its south. The discovery of this new species, P. icaria from the Cauvery River confirms that the genus Pangasius has extended distribution upto the southern division of the Western Ghats and has undergone speciation, like other Himalayan species that have same genus but are represented by alternate species in the peninsular India (Silas, 1952b) for example Silonia silondia (Ganges and Mahanadi) were represented as S. childreni in peninsular rivers. This addresses the earlier paleobiogeographic conflict that the genus Pangasius might be a late migrant and has not undergone the species divergence (Silas, 1952b; Menon, 1980).

In the present study, the P. icaria, is described in collections from two different and distant locations in the Cauvery River. The rivers of peninsular India flows through the Western Ghat region, which is part of the designated mega biodiversity hotspots and is known for endemism. The future programs on fine-scale explorations of these rivers will be significant in delimiting the distribution ranges of the newly discovered pangasius species (P.silasi and P. icaria) for documentation of within species genetic diversity, their conservation strategies and also with the possibility of new undescribed species.

Conclusions

A new species of the genus Pangasius is described that was collected from the Cauvery River, Tamil Nadu, India. This new discovery of P. icaria clearly highlights the native presence of genus Pangasius in peninsular India and is represented by two recorded divergent species. Future research and explorations are needed to ascertain the distributional range of this endemic species for devising conservation and management of the species and also to evaluate for its aquaculture utilization potential.

Supplemental Information