Resolving the Pseudorhombus arsius cryptic species complex (Teleostei, Paralichthyidae): phylogenetic evidence and description of Pseudorhombus bahudaensis sp. nov.

The Gangetic Largetooth flounder, Pseudorhombus arsius (Hamilton, 1822), long treated as a single Indo-Pacific species, represents a cryptic species complex. Using a combination of morphological, morphometric, and molecular data, we discovered a distinct lineage that warrants recognition as Pseudorhombus bahudaensis sp. nov. Phylogenetic analyses of COI sequences revealed multiple polyphyletic clades, with two well-supported groups corresponding to P. arsius and the new species.

Divergence dating indicates a separation of approximately 17 million years despite extensive morphological overlap. Diagnosis of the new species is supported by vertebral counts, subtle scale morphology, and differences in the position of the lateral line relative to the eye. These findings highlight the importance of integrating genetic and morphological evidence in resolving cryptic flounder complexes and provide a basis for re-evaluating distribution records attributed to P. arsius across the Indo-Pacific.