The calanoid copepod family Ridgewayiidae, with both demersal and cave-dwelling species, has representatives worldwide, but most have been known from the northwestern tropical Atlantic. Biological collections from anchialine cave environments of the Yucatán Peninsula, Mexico
contained ridgewayiid copepods. These specimens were found to belong to an undescribed species of the genus Exumella Fosshagen. The new species of Exumella is distinguished by a combination of characters, including a relatively large anal somite, the presence of 24 antennular
free segments in the female and 19 in the male right antennule, and by peculiar details of the structure of the male and female fifth legs. Although other ridgewayiids are present in the western Caribbean, this one represents the first record of the family in Mexico. The general distribution
of the genus Exumella suggests a Tethyan pattern. The presence of ridgewayiids in the Yucatán anchialine systems probably results from an old biogeographic pattern derived of the presence and colonization of coastal marine ancestors. A key for the identification of the known
species of Exumella is also included herein.
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