Assamnaia Bolotov, Sonowal, Kardong, Pasupuleti and Subba Rao, 2024

Genus Assamnaia Bolotov, Sonowal, Kardong, Pasupuleti and Subba Rao View in CoL gen. nov.

(type species: Unio involutus Hanley, 1856 View in CoL ).

( Figs 6A–C View Figure 6 , 7A View Figure 7 )

LSID: https://zoobank.org/ urn:lsid:zoobank.org:act:B1F90D7F-3153-484D-8FDC-54CCFD0E3955

Differential diagnosis: From a phylogenetic perspective, the new genus is the sister-lineage of Parreysia but it could be distinguished from the latter genus by having a combination of the following characters: thin, fragile shell (vs. thick, massive), thin, lamella-like pseudocardinal teeth (vs. thick, massive, strongly indented), almost invisible adductor muscle scars (vs. deep, well visible). Among the Asian representatives of the subfamily, the new genus is more similar to Lamellidens but differs from it by having a much broader and larger umbo, longer and thinner pseudocardinal teeth, and the central position of umbo. Conchologically, the East African species Coelatura cridlandi Mandahl-Barth, 1954 ( Coelaturini ) that is endemic to Lake Victoria looks similar to the new genus but this external appearance most likely reflects convergent evolution.

Description: Shell thin, very inflated; anterior margin broadly rounded; posterior margin triangular, with narrowly rounded apex; dorsal margin convex, dominated by massive, broad, very prominent umbo; ventral margin broadly convex. Umbo situated in the middle of the shell. Periostracum glossy, light green, usually with dirty yellow patches and bands of long growth lines. Pseudocardinal teeth elongated, narrow, thin, lamella-like, two teeth in right valve and one tooth in left valve. Lateral teeth elongated, lamella-like, one tooth in right valve and two teeth in left valve. Anterior and posterior adductor muscle scars almost invisible.

Etymology: The name of this genus is derived from two words: ‘Assam’ (Indian state, in which it ranges) and ‘ naia ’ (a nymph in Latin, reference to the genus Indonaia , in which it was placed traditionally).

Distribution: Endemic to the Brahmaputra River basin in India and to the Surma River in Bangladesh.

Habitats: Topotypes of Assamnaia involuta ( Hanley, 1856) gen. et comb. nov. (N = 3 sequenced specimens and one dead shell) were collected from an oxbow lake of the Dhansiri River, close to the Kaziranga National Park in Assam ( Fig. 7A View Figure 7 ). This water body is connected with the main river channel during the rainy season. The mussels were sampled from silt and clay bottom substrate .

Comments: Here, we transfer the nominal species Unio involutus Hanley from Indonaia (the involuta -group) to Assamnaia gen. nov. and propose A. involuta gen. et comb. nov. The other member of the involuta -group, Indonaia olivaria (Lea, 1831) from the Ganges River, needs future study. In the absence of available DNA sequences, we prefer to consider it as an unassigned-togroup (incertae sedis) representative of the genus Indonaia .