Right running head: Analysis of a riverine species flock DISCOVERY AND PHYLOGENETIC ANALYSIS OF A RIVERINE SPECIES FLOCK OF AFRICAN ELECTRIC FISHES

Analysis of a riverine species flock 1 ABSTRACT We have discovered surprising undescribed diversity within a clade of mormyrid electric fish in our recent collections from the Ogooué and Ntem River basins of Gabon, Africa. We recognize 38 distinct forms within this group on the basis of unique morphologies and electric organ discharge (EOD) characteristics of which only four clearly correspond to described species. The remaining 34, treated here as operational taxonomic units (OTUs), may represent as many new species. We sequenced the complete cytochrome b gene from 86 specimens sampled from these OTUs and recovered 65 distinct cytochrome b haplotypes. Trees obtained by both maximum parsimony and maximum likelihood analyses, rooted with sequence data from outgroup taxa, provide strong support for the monophyly of all 38 of these OTUs. The small genetic distances between many distinct forms suggests an ongoing and rapid radiation. However, in many instances, the cytochrome b tree topology fails to support the monophyly of individual OTUs or close relationships between OTUs which are similar in morphology and EOD characteristics. In other cases, individuals from distinct Sullivan, Lavoué and Hopkins Analysis of a riverine species flock 2 OTUs share identical, or nearly identical, haplotypes. We infer from these results a disjunction between the mitochondrial gene tree and the overall species phylogeny of these organisms. The presence of divergent haplotypes within single populations of some forms suggests incomplete mitochondrial lineage sorting, while the grouping of phenotypically distinct, but geographically proximate forms in other cases suggests local hybridization and introgression of mitochondrial genomes among forms. Both incomplete lineage sorting and introgression may contribute to the lack of congruence between the mitochondrial gene tree and the natural groups supported by morphological and EOD data. Diversification in this clade of riverine electric fishes and the problems associated with recovering a meaningful species-level phylogeny from mitochondrial data suggest a parallel to the well-known lacustrine fish " species flocks. " Social selection on EOD waveforms may be involved in the rapid radiation of these fishes.