LAUSR

A new species of the genus Acanthocyclops is described from temporal freshwater bodies in the Sian Ka’an and Calakmul Biosphere Reserves in Southeast Mexico. Acanthocyclops fiersi n. sp. represents the third species of the genus described from the Neotropics, only after A. rebecae and A. smithae. Acanthocyclops fiersi n. sp., is recognized as a member of the A. vernalis-robustus species-complex, and its clear divergence with respect to its congeners was supported using comparative morphology and molecular methods. The mtCOI marker revealed a 0.54% K2P divergence within populations of A. fiersi n. sp. and 17.6-22.41% with respect to other members of the A. vernalis-robustus species-complex. Acanthocyclops fiersi n. sp. might be restricted to the Yucatán Peninsula. Comments about the likely underestimated regional diversity of Acanthocyclops and the need of further studies in the Neotropics are provided. An identification key to the species of Acanthocyclops distributed in Mexico is included.