Cristaria hospes sp. nov.: A new enigmatic cockscomb pearl mussel from the Mekong River basin, Thailand (Bivalvia: Unionidae)

The Mekong River basin is a global hotspot of freshwater biodiversity, but it is currently threatened by invasions of alien species. Here, we describe Cristaria hospes sp. nov. (Bivalvia: Unionidae: Unioninae: Anodontini: Cristariina), which was discovered at two sites within the Chi River basin, in the Mekong River drainage. The shell of this species superficially resembles those of Chinese pond mussels (Sinanodonta spp.), but internal conchological diagnostic features, such as lamellar pseudocardinal and lateral teeth, support its placement within the genus Cristaria. The new species represents a sister lineage to Cristaria bellua (Morelet, 1866), which is endemic to the Mekong Basin. However, the lack of historical records, its prevalence in artificial fish ponds, and the presence of a single COI haplotype suggest that C. hospes sp. nov. is a non-native species in this basin. Given its relatively close relationship to C. bellua, we can infer that its native range encompasses adjacent drainages, such as the headwater tributaries of the Yangtze, Pearl, or Red rivers. Our findings highlight the ongoing homogenisation of freshwater fauna in tropical Asia.