Multiple colonisations of the Lake Malawi catchment by the genus Opsaridium (Teleostei: Cyprinidae).

It has been proposed that the fish faunas of African rivers assemble through multiple colonisation events, while lake faunas form additionally through intralacustine speciation. While this pattern has been established for many lineages, most notably cichlids, there are opportunities to further investigate the concept using phylogenies of congeneric endemic species within ancient lake catchments. The Lake Malawi catchment contains three river-spawning cyprinids of the genus Opsaridium, two of which are endemic. These species differ in body size, migratory behaviour and habitat use, but it has never previously been tested if these represent a monophyletic radiation, or have instead colonised the lake independently. We placed these species in a broader phylogeny of Opsaridium and the related genus Raiamas, including all known species from the river systems surrounding Lake Malawi. Our results suggest that each of the species has independently colonised the lake catchment, with all three taxa having well-defined sister taxa outside of the lake, and all sharing a common ancestor ∼14.9million years ago, before the Lake Malawi basin started to form ∼8.6million years ago. Additionally, the results strongly support previous observations that Opsaridium is not a monophyletic group, but instead contains Raiamas from the Congo drainage. Together these results are supportive of the concept that river fish faunas within African catchments are primarily assembled through a process of accumulation from independent origins, rather than within-catchment speciation and adaptive radiation. In light of these results we also suggest there is scope for a re-evaluation of systematics of both Opsaridium and Raiamas.