Nilssonia formosa

Nilssonia formosa View in CoL

is known from numerous locations across the border in Myanmar, whereas its supposed presence in Yunnan, China ( Liebing et al. 2012; Horne et al. 2021) is now thought to have been caused by human introduction ( TTWG 2025). The closest documented point of occurrence to those in Manipur lies at approximately 190 km air distance on the Yat Nar Sandbank ( 25°36.78’N, 95°26.70’E) of the upper Chindwin River in the Chin State of Myanmar, 143 m a.s.l. ( Kuchling et al. 2012). The Manipur River   GoogleMaps (called Myittha   GoogleMaps on the Myanmar side of the border) is in direct connection with the Chindwin River   GoogleMaps system, so this species’ occupation of Indian   GoogleMaps territory hardly comes as a surprise. The Wikipedia   GoogleMaps page on N. formosa (Anon. 2025) states that it has been recorded from the Karbi Anglong district   GoogleMaps of Assam but does not cite any references, and our internet search has not produced any specifics. However   GoogleMaps , as the major rivers, such as Dhansiri   GoogleMaps and Kopili   GoogleMaps , in Karbi Anglong   GoogleMaps are part of the Brahmaputra   GoogleMaps system, we believe this claim to be erroneous. With   GoogleMaps the present record, the number of Nilssonia species known from India increases to five, encompassing all recognised species of the genus ( N. formosa , N. gangetica , N. hurum , N. leithii , and N. nigricans ).

Although N. formosa and N. hurum exhibit similar head and carapace patterns as juveniles, N. formosa lacks a light-coloured blotch across the snout typical for N. hurum , and has bar-like yellow patches on the temples that extend from ear level up to the crown. As growth proceeds, these pale head markings are gradually replaced by black spots and vermiculations on a cinnamon ground. The temporal blotches, ranging from yellow to orange or bluish grey in colour depending on population, do not reach the top of the head in N. hurum and remain distinct throughout life in all except very old or melanistic individuals, allowing for the easy distinction of the two species ( Figs. 1b, c View FIGURE 1 ). The same diagnostic traits differentiating N. hurum from N. formosa refer also to N. nigricans , another species recorded from nearby regions ( Assam, Tripura, and West Bengal in India, Bangladesh, Nepal; cf. Praschag et al. 2021; TTWG 2025).

Nilssonia formosa is known to co-occur with Amyda ornata phayrei throughout its range ( Kuchling et al. 2004), so also in Manipur. The Indian records presented here not only extend its geographical range towards the west but also shift its upper elevation limit from 200 m a.s.l. ( Horne et al. 2021) to 780 m a.s.l.