Hippopotamyrus ansorgii species complex in the Upper Zambezi River System with a description of a new species,
H. szaboi (Mormyridae)

Abstract

Specimens referable to Hippopotamyrus ansorgii sampled from the Upper Zambezi River System within Caprivi (Namibia) represent a complex of three species, two of which coexist in the Upper Zambezi River, and a third that inhabits a nearby river, the Kwando, with which the Zambezi has been connected during periods of flooding. All three are indistinguishable in terms of their general appearance, but differ consistently in electric organ discharges (EOD), morphology, and molecular genetic characters. All phenotypes display a monopolar, headpositive EOD pulse with specific post- or prepotentials. For H. ansorgii from the Zambezi
River (HaZ), pulse duration is less than 0.5 ms (down to 0.205 ms; N = 34); for the syntopic H. szaboi sp. n., it is greater than 0.6 ms (up to 1.8 ms at 10% peak amplitude;
N = 19). The parapatric phenotype of H. ansorgii from the Kwando River (HaK) has pulses shorter than 0.215 ms (down to 0.105 ms; N = 36). All three members of the species complex may be distinguished from each other by 7 −
9 anatomical characters, analysed by MANOVA. Based on 22
enzymes and proteins studied, the moderate to high Wright’s fixation index and the significant (P < 0.05) allele differentiation between EOD phenotypes provide additional evidence for incipient speciation. Pairwise analyses of the three different phenotypes showed the two
parapatric species of H. ansorgii grouped together, and distinguishable from individuals of H. szaboi . Analyses of the mitochondrial cytochrome b gene revealed that all specimens which were attributed to H. szaboi form a well-supported monophyletic basal clade (bootstrap support
73% or 82%). The genetic distances (uncorrected p distances) between H. szaboi and the two species of
H. ansorgii are between 0.6% and 1.7%. Within the derived
H. ansorgii clade some phylogeographical differentiation can be seen for fishes from the Zambezi and Kwando
Rivers, but the respective groups are not consistent or supported by significant bootstrap values. The question of which of the two parapatric morphological and EOD phenotypes of H. ansorgii recognized in the present paper represents H. ansorgii (Boulenger, 1905) cannot be
resolved at present because of the paucity and unclear origin of the historical type material.