Genus Parreysia Conrad, 1853

(type species: Unio multidentatus Philippi, 1847 [= Parreysia corrugata (O.F. Müller, 1774)]; by original designation) (Conrad 1853).

(Fig. 6D–I)

Comments: This genus contains eight recent species. Four nominal species belonging to the genus Parreysia were described from Assam: Unio favidens var. marcens Benson, 1862; Unio gowhattensis Theobald, 1873; Parreysia (Parreysia) annandalei Preston, 1912; and Parreysia favidens var. assamensis Preston, 1912 (Subba Rao 1989, Bolotov et al. 2022a). These species were separated on the basis of minor morphological differences in shell shape (Benson 1862, Theobald 1873, Preston 1912). Our phylogenetic data reveal that there is a single Parreysia species in Assam (Supporting Information, Fig. S1), the conchological varieties of which correspond to the four nominal species, listed above. Here, we consider Parreysia marcens (Benson, 1862) stat. rev. as an accepted species based on new DNA sequences and morphological data from Assam, while three other taxa described from this region are considered its synonyms (Table 1). This species has large genetic distance from its congeners (mean uncorrected COI p-distance of 7.3–8.3%) (Supporting Information, Table S1). Parreysia marcens belongs to the keralaensis -group of the genus, being a sister-lineage of Parreysia keralaensis Bolotov et al., 2022a from south-western India. Our occurrence and sequence datasets indicate that Parreysia marcens is endemic to the Brahmaputra River basin and that this species inhabits a broad range of habitats (Supporting Information, Dataset S1). This species was not recorded beyond the Brahmaputra Basin. Published records of other Parreysia species from Assam (Subba Rao 1989, Sonowal et al. 2021, Bolotov et al. 2022a) should be considered as belonging to P. marcens . A published COI sequence of Parreysia from Nagaland (GenBank acc. No. OM074301; Imsong and Murali 2023) that was erroneously identified as Parreysia corrugata (see: Bolotov et al. 2024) also belongs to Parreysia marcens (Table 1). This conclusion is based on the molecular genetic data (Supporting Information, Fig. S1) and published image of the shell (Imsong and Murali 2023: fig. 1G).

Additionally, Parreysia smaragdites (Benson, 1862) is here considered a putative accepted species based on morphological features such as a rather thin shell and prominent, conical umbo (see: Preece et al. 2022: fig. 111A). Comparative analysis of a syntype of P. smaragdites (UMZC I.103360.A) (Preece et al. 2022) and the holotype of Unio corrugatus var. fragilis Hanley and Theobald, 1872 (NHMUK 1968651; unavailable name) from an unknown type locality indicates that these taxa are conspecific (Table 1). It is likely that the latter shell also originated from the Brahmaputra River basin. We were unable to find living specimens of P. smaragdites in Assam, and the identity of this interesting species needs to be checked by means of a DNA sequencing approach in the future.