Electric organ discharge divergence promotes ecological speciation in sympatrically occurring african weakly electric fish (Campylomormyrus)

Abstract

While speciation is often discussed mainly with regard to its geographic context, i.e., allopatric versus parapatric versus sympatric, the role of adaptation in speciation has been repeatedly emphasized in the recent past. Such an ecological speciation, in which populations diverge by disruptive selection, can occur in different geographic contexts. Here, we present a combined molecular genetic, electrophysiological, morphometric, and behavioral study on an adaptive radiation of African weakly electric fish (Genus Campylomormyrus). Multilocus genetic data, types of electric organ discharge (EOD), and morphometry identified several reproductively isolated clusters of specimens, i.e., different species, in sympatry. By this analysis, six previously described species could be confirmed, while at least one specimen appeared not to belong to any species described so far. Campylomormyrus is characterized by its elongated trunk-like snout used for preying on insect larvae. Morphometric analysis confirmed that divergence among species is mainly attributed to differences in snout morphology. Though direct evidence is still lacking, it appears reasonable to assume that these differences in the feeding apparatus translate into different diets and/or feeding habits. We could also demonstrate that mate choice occurs with regard to EOD and could hence have triggered speciation by serving as a prezygotic isolation mechanism among species with divergent feeding apparatus. In addition, the extremely divergent EODs (100-fold difference in duration) among closely related species might also have a direct adaptive value, as these differences affect echolocation (the primary function of the EOD) and might hence alter the prey spectrum detected. Under such a scenario, the EOD as a single trait pleiotropically combines natural divergent selection (adaptation to divergent diet and/or feeding habits) and reproductive isolation (assortative mating with regard to EOD). EOD might hence constitute a magic trait which promoted ecological speciation in African weakly electric fish.