Isonebula clavuslongus n. sp. (Isopoda: Cymothoidae) parasitizing Myloplus ternetzi (Norman, 1929), a fish of the family Serrasalmidae, in the Brazilian Amazon region.

Crustaceans of the family Cymothoidae are important components of the invertebrate fauna of marine and freshwater environments. The genus Isonebula Taberner, 1977, has only two described species (Isonebula maculatus (Taberner, 1977) and Isonebula acanthopleon Taberner 1998), which are both parasites of freshwater fish in South America. During a field survey of fish parasites in the Araguari River, state of Amapá, Brazilian Amazon region, we found a new species of the genus Isonebula infesting the mouth (on the host's tongue) of Myloplus ternetzi (Norman, 1929), a characiform of the family Serrasalmidae, for which we proposed the name Isonebula clavuslongus n. sp. Isonebula clavuslongus n. sp. differs from its congeneric species through the larger size of the dactyl of its 6th pereopod, with an inner measurement of 4.50 mm. Additionally, it possesses 7 joints in the second antenna, whereas 9 joints were reported for I. acanthopleon and I. maculatus. The present study thus expands the knowledge of the diversity of Cymothoidae in the Amazon region to 26 species, and of the genus Isonebula, with 3 species, which stands out as one of the genera with the lowest number of known species among Neotropical Cymothoidae that parasitize freshwater fish.