Ergasilus lyraephorus n. sp. (Copepoda: Cyclopoida: Ergasilidae) parasitic on the Longtail Knifefish Sternopygus macrurus (Bloch & Schneider, 1801) (Actinopterygii: Sternopygidae) from Northeast Brazil.

The cyclopoid family Ergasilidae Burmeister, 1835, is the most common group of parasitic copepods infesting fish in Brazil, and the type-genus Ergasilus von Nordmann, 1832 comprises the highest number of species. During a survey of freshwater fish in Northeast Brazil, a new species of Ergasilus was found on the gills of the Longtail Knifefish Sternopygus macrurus (Bloch & Schneider) (Actinopterygii: Sternopygidae) in the Viana lake system, State of Maranhão. Ergasilus lyraephorus n. sp. can be distinguished from its closest congeners mainly because it has a lyre-shaped ornamentation on the ventral surface of first pedigerous somite, a feature that has never been reported in the family. In addition, the new species differs from closely related congeners by having a maxillule bearing three elements, by the large spinules on the interpodal plates of legs 1, 2 and 3, and by having leg 5 reduced to a single seta of moderate size. The present study is the first report of an ergasilid parasitizing S. macrurus, as well as the first parasitic copepod found on a host belonging to the family Sternopygidae Cope.