A new species of parasitic copepod, Colobomatus deborae n. sp., belonging to the cyclopoid family Philichthyidae Vogt, 1877, is proposed based on adult female specimens collected from the interorbital canals of the Caitipa mojarra Diapterus rhombeus (Cuvier) (Gerreidae) from Sepetiba Bay, off the State of Rio de Janeiro, southeastern Brazil. The new species closely resembles six species of Colobomatus Hesse, 1873, but differs from these close congeners based on the combination of the following characters: lateral cephalic processes forked basally with sharp tips, midventral cephalic process representing about one third of the length of the laterals, thoracic processes forked and caudal rami with a pair of unequal processes. Together with the new species described in the present study, the philichthyid fauna of Brazil rises to 10 species reported from 14 Brazilian marine fish species. This is also the third report of Colobomatus in a gerreid fish, but the first species found parasitizing the genus Diapterus.
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