The taxonomy of the Asian Pit Viper genus Trimeresurus have proved difficult to unravel, as species tend to be both similar to one-another and morphologically variable. The Mangrove Pit Vipers, Trimeresurus purpureomaculatus and Trimeresurus erythrurus are considered to form a species complex, with Trimeresurus purpureomaculatus being an obligate Mangrove-inhabitant with variable colouration and dark blotches on its back, found in southern Thailand, Peninsula Malaysia, and on the island of Sumatra, while Trimeresurus erythrurus is a green Snake lacking markings, found in but not restricted to Mangroves, found in northern Myanmar, and eastern Bangladesh and India. Between these, in southern Myanmar, is a population of Snakes which show a mixture of traits seen in the other two groups, which have been assumed to represent a zone of hybridization. However, a recent study of genetic structures within the group has suggested that this southern Myanmar population is actually a separate species, which forms a sister taxon to Trimeresurus purpureomaculatus, although this species does show signs of hybridizing with Trimeresurus erythrurus.
In a paper published in the journal ZooKeys on 13 December 2023, Kin Onn Chan of the Lee Kong Chian Natural History Museum at the National University of Singapore, and the School of Biological Sciences at the Universiti Sains Malaysia, Shahrul Anuar, also of the School of Biological Sciences at the Universiti Sains Malaysia, Ananthanarayanan Sankar of the Herpetological Society of Singapore, and the Department of Biological Sciences at the National University of Singapore, Ingg Thong Law and Ing Sind Law, also of the Herpetological Society of Singapore, Rasu Shivaram, also of the Lee Kong Chian Natural History Museum at the National University of Singapore, and of the Herpetological Society of Singapore, Ching Christian, again of the Herpetological Society of Singapore, and of the Department of Life Sciences at the Natural History Museum, Daniel Mulcahy of the Museum für Naturkunde at the Leibniz Institute for Evolution and Biodiversity Science, and Anita Malhotra of the School of Natural Sciences at Bangor University, formally describe the southern Myanmar Trimeresurus population as a new species.
The new species is named Trimeresurus ayeyarwadyensis, meaning 'from Ayeyarwady'; the species is known from the Ayeyarwady and Yangon regions on the Ayeyarwady delta in southern Myanmar. The species has partially fused first infralabial and nasal scales, a condition not seen in any other member of the genus Trimeresurus, but otherwise shows a mosaic of traits seen in other species.
Unhybridized populations of Trimeresurus ayeyarwadyensis were found in Mangrove forests in the Pyapon and Myaungmya districts of the Ayeyarwady Region, as well as in a forest surrounding a lake unconnected to any Mangrove system in the Hlawga Park in the Yangon Region. In the Pathein District of the Ayeyarwady Region a population of Trimeresurus ayeyarwadyensis hybridized with Trimeresurus erythrurus was found living alongside an unhybridized population of Trimeresurus erythrurus. Trimeresurus purpureomaculatus is known from the Dawei District in the Tanintharyi Region of Myanmar, and could potentially have a contact zone with Trimeresurus ayeyarwadyensis in Mon State, where no members of the genus Trimeresurus have been recorded, but this is thought likely to be due to a lack of searching rather than an absence of Snakes.
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