A new species of Branchellion Savigny, 1822 (Hirudinida: Piscicolidae), a marine leech parasitic on the giant electric ray Narcine entemedor Jordan & Starks (Batoidea: Narcinidae) off Oaxaca, Mexico.

Branchellion spindolaorum n. sp. (Hirudinida: Piscicolidae) is described based on specimens found parasitising the giant electric ray Narcine entemedor Jordan & Starks off the coast of Oaxaca, Mexico. The new species can be clearly distinguished from the other species of Branchellion Savigny, 1822 by the presence of 30 pairs of lateral branchiae and 10 pairs of pulsatile vesicles. The definition of the genus Branchellion is expanded to include species with either 30, 31 or 33 pairs of foliaceous (plate-like) lateral branchiae in the urosome. In addition, we provide for the first time for the genus, scanning electron micrographs of the secondary suckers located on the ventral surface of the posterior sucker. Additionally, partial DNA sequences of the mitochondrial cytochrome c oxidase subunit 1 (cox1) were generated and compared with homologous sequences of other species of the genus. Branchellion spindolaorum n. sp. represents the fourth species of the genus known in the Eastern Pacific and the first record of a leech parasitising N. entemedor.