Photographic record of the butterfly ray Gymnura cf. poecilura (Myliobatiformes: Gymnuridae) from the Bhagirathi-Hooghly River in West Bengal, eastern India

This brief note presents the first photographic record of the butterfly ray, Gymnura cf. poecilura, from the Bhagirathi-Hooghly River in West Bengal, eastern India. The photographs were discovered during a pilot survey of riverine elasmobranchs in West Bengal. This record is an important contribution to the knowledge of elasmobranch biodiversity in Indian rivers. The author suggests that further research is needed to better understand the ecology, biology and conservation status of riverine elasmobranchs in India.

Butterfly rays, family Gymnuridae, comprise a single genus Gymnura van Hasselt, 1823 and are generally marine, although sometimes found in brackish water areas (McEachran & Carvalho 2002). It includes 12 valid species found in the Indo-Pacific and the Atlantic Oceans (Yokota et al. 2016). Globally, three Gymnura species are marginal, and one is brackish marginal (Martin 2005).
Records of elasmobranchs from the freshwater reaches of the Ganges go back centuries. Hamilton (1822) described rays (called "skates") occurring far away from the tidal reaches of the river. At present, rays are a rare bycatch in parts of the freshwater reaches of the Ganges in West Bengal (author pers. obs. 14.vi.2012; 7.i.2018; 22.i.2022).
During a pilot survey of riverine elasmobranchs in West Bengal, eastern India, a fisher showed photographs of a "Shankar Maach" (local name for stingray) that he had caught in a set net. He said he had caught the fish in April 2019 from the Bhagirathi-Hooghly River, a tributary of the Ganga (22.311 0 N, 88.094 0 E) near Barkolia village in Howrah district of West Bengal (Figure 1). He stated that the fish weighed about 8.5 kg and had a disc width of about 90 cm (Image 1). The fish was subsequently identified from the photographs as a gymnurid ray based on its unique body shape with a strongly depressed body and pectoral fins extending into a "lozenge-shaped" disc, and as Gymnura cf. poecilura (Shaw, 1804) based on its long tail with 13 black bands and lack of a dorsal fin (Yokota et al. 2016). The fisher stated that he and his family had eaten the fish. As the species was only recognised from photographs, the specific name is preceded by the qualifier 'cf'.
The nearest marine environment from which gymnurids have been reported is over 100 km away in Digha (Yennawar et al. 2017), and they have also been listed in the mangrove-lined brackish waters of the Sundarbans in southeast West Bengal (Mishra et al. 2019), which is also a considerable distance from the capture site of this study. Therefore, this study reports the first occurrence of Gymnuridae, i.e. Gymnura cf. poecilura, from the Bhagirathi-Hooghly River in West

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Bengal and India's first Gymnuridae so far inland. A separate species of Gymnura, also referred to as G. cf. poecilura, occurs in the northern Arabian Sea (Muktha et al. 2016). However, it is unlikely that the specimen in this study is G. cf. poecilura (sensu Muktha et al. 2016), as it was captured from a tributary of the Ganga that flows into the western Bay of Bengal.
Although the identification of the species is based on only two photographs, this evidence is crucial as G. poecilura is listed as Vulnerable by the International Union for the Conservation of Nature (IUCN) Red List, and its current population trend is reported to be in decline (Sherman et al. 2021). This study shows how fishers' knowledge can help fill information gaps about rare riverine elasmobranchs.
Thus, this study helps direct future research to document the diversity of elasmobranchs in Indian rivers and to understand how they use their non-marine habitats. It is also important to study the impact of fishing on elasmobranch populations in rivers. This will help to detect any population declines.