Abstract
During a preliminary survey of a brackish mangrove habitat in the Terraba River delta on the Pacific coast of Costa Rica, several adult and subadult specimens of a new anthurid isopod, Cyathura odaliscae n. sp. were discovered. The specimens were collected from decaying wood in low salinity conditions (<5‰). The unique shape and complexity of the male copulatory stylet of C. odaliscae distinguishes it from the other 22 species for which males have been described. A combination of other characters (e.g., morphology of the mouth parts, body setation, shape of telson and first pereopod) is used to distinguish the female from the others within the genus. Illustrations of the known male copulatory stylets for the genus and a table listing diagnostic information, depth ranges, and general distribution for the 32 currently recognized species of Cyathura are provided.