Pennella exocoeti is a large ectoparasitic copepod, a specialist parasite of flying fish. The adult female copepod clings to the fish's gills or skin and feeds on its body fluids.
Pennella exocoeti was first described by the Danish zoologist Hans Severin Holten in 1802 from a specimen probably found on the mirrorwing flyingfish (Hirundichthys speculiger). He called it a "gill worm" and recognised that it had close affinities with Chondracanthus merluccii, another "gill worm" found on a member of the cod family Gadidae, but he did not realise they were both copepods. Another species was described by the French naturalist Charles Alexandre Lesueur as Pennella blainvilli from the tropical two-wing flyingfish (Exocoetus volitans), but that has since been determined to be a synonym of P. exocoeti.