A new species of Aphyosemion is described from Gabon, based on ten specimens collected in a small stream belonging to the hydrographic system of the Ikoy River on the northwestern edge of the Massif du Chaillu. Aphyosemion grelli is distinguished from all congeners by possessing a unique colour pattern of the unpaired and pelvic fins in the female consisting of the combination of a yellow basal portion and greyish to dark grey broad margin. Males of the new species share the black margins of the unpaired fins with A. congicum, A. labarrei, A. ocellatum, A. passaroi, and A. teugelsi, but differ from these by the combination of colouration characters and morphology. Phylogenetic relationships of the new species are still unclear; a possible close relationship with the A. coeleste group is tentatively excluded because, although geographically close, there are differences on flank colour pattern, head length and number of anal-fin rays.

Killifish, Africa, Ikoy River, Ikobey, systematics, taxonomy, biogeography