Bathynomus propinquus Richardson, 1910, a valid species of supergiant deep-sea isopod from the Philippines, with notes on B. wilsoni Ahyong, 2025 (Crustacea, Isopoda, Cirolanidae).

The deep-sea isopod Bathynomus propinquus Richardson, 1910, was described based on a single male juvenile from the Verde Island Passage, central-western Philippines, but owing to its immaturity, had long been considered unidentifiable and treated as a nomen dubium. Settling the identity of B. propinquus, however, is important because the name remains available and nomenclaturally precedes all other species of supergiant Bathynomus in the Indo-West Pacific, with its type locality within the reported range of B. jamesi Kou, Chen & Li in Kou et al. 2017, raising the obvious possibility that the two nominal species could be conspecific. Detailed studies of B. jamesi sensu lato in the South China Sea revealed that it comprises two species distinguished by morphology and strong mitochondrial sequence divergence: a northern form (B. jamesi sensu stricto) ranging from Vietnam to off Hainan and the Dongsha Islands, and a southeastern form (here identified as B. propinquus) from the Verde Island Passage, southern Luzon, Philippines. Bathynomus propinquus is here considered a valid species based on re-evaluation of the holotype, additional material, new characters, size-related variation, and molecular data. Bathynomus propinquus belongs to the group of Indo-West Pacific supergiants sharing upturned pleotelson spines, most of which were formerly confused with B. kensleyi Lowry & Dempsey, 2006 from the Coral Sea, herein referred to as the B. kensleyi group. Given the new data on supergiants in the South China Sea, a key to the species of the B. kensleyi group is provided. A new record of B. wilsoni Ahyong, 2025 from the northern Sulu Sea is also reported.