Two new Amphipod families recorded in South America shed light on an old biogeographical enigma

Abstract

The known diversity of freshwater amphipods in South America is substantially lower than on other continents. This has puzzled biologists for decades. Two hypotheses have been proposed in attempts to explain this pattern. According to the first one, the majority of amphipod lineages never dispersed across South America. The alternative hypothesis is that the recently diversified hyalellids have outcompeted and depleted the ancestral amphipod fauna. The recently discovered freshwater amphipod species Seborgia potiguar sp. nov. (Seborgidae) and Potiberaba porakuara gen. nov., sp. nov. (Mesogammaridae) from Brazil reveals the existence of two additional families of amphipods in South America. In the light of these discoveries we have analysed the amphipod faunistic structure of South America to test the above two biogeographic hypotheses. First, the number of amphipod families in South America is not as low as was thought. Falklandellididae are limited to the Falkland Islands and Chile. All other families (Ingolfiellidae, Bogidiellidae, Phreatogammaridae, Paraleptamphopidae, Pseudoingolfiellidae, Paracorpohiinae, Mesogammaridae and Seborgidae) but one (Dogielinotidae) share two properties: (1) they have a transoceanic distribution and (2) they are from subterranean waters. Since the dispersal ability of amphipods is limited, trans-oceanic disjunctions are best explained by plate tectonics, which implies their early origin, negating the first biogeographical hypothesis. These ancient families, for unknown reasons, survived only in a stable subterranean environment which can be regarded as a refuge. The only recent colonizer of the continent might be Dogielinotidae with the species-rich genus Hyalella. Although it cannot be determined whether hyallelids truly out-competed ancient amphipods, we suggest that the second hypothesis fits better to the data. Further findings of amphipods are expected in South America, especially from subterranean waters. This habitat is highly endangered in Brazil, and should be more rigorously protected.