Introduction
The African weakly electric fish superfamily Mormyroidea forms a natural group within the Osteoglossomorpha (bony-tongue fishes) with two families: the monotypic Gymnarchidae (Gymnarchus niloticus) and the speciose Mormyridae. The family Mormyridae is, in turn, divided into two subfamilies: the Mormyrinae with 19 genera and more than 160 valid species and the Petrocephalinae, comprising only one genus, Petrocephalus Marcusen 1864, in which more than 40 species are currently considered valid.
Petrocephalus species form a natural group that is the sister group of the remaining mormyrids (i.e., the subfamily Mormyrinae) from which it can easily be distinguished by several unique morphological characteristics. The most salient are: 1) the presence of an orbitosphenoid, 2) the presence of a basisphenoid, 3) two single unsegmented and unbranched rays at the origin of the dorsal fin and 4) two nostrils closely apposed with the posterior one very close to the eye. Furthermore, the electric organ discharge waveforms of Petrocephalus are always of short duration (only few milliseconds long).
Petrocephalus are small fishes (typically <11cm in standard length; the largest species are Petrocephalus bane and Petrocephalus sauvagii, they reach 20cm in standard length) that can form large schools, and are mostly active at night. Petrocephalus have a widespread distribution in the freshwaters of subtropical and tropical regions of Africa. They may be locally abundant and diverse, as in several localities of the Congo basin with more than 10 sympatric species. In some fish communities, Petrocephalus species are the most abundant electric fishes, or perhaps even the most abundant of all fish species present.
Like other mormyrids, Petrocephalus species continuously produce weak electric pulses from an organ in the caudal peduncle for the purposes of orientation and communication. While in other mormyrid genera electric organ discharge (EOD) waveforms often differ substantially between species and are useful taxonomic markers, electric organ discharge waveforms of Petrocephalus are relatively conserved across the genus. In most cases, the electric organ discharge waveform of a Petrocephalus species exhibits two or three main phases of alternating polarities, often with the presence of one or two additional phases of much smaller relative amplitude. The first main phase is positive and the total duration of the EOD waveform is short. All Petrocephalus species examined to date have electric organs with the same simple type of electrocytes (i.e., type "NPp": electrocytes posteriorly innervated with non-penetrating stalks).
Identification of Petrocephalus species is often difficult due to the slight amount of morphological differentiation between many species. Despite the apparent conservatism in morphology and EOD waveforms among Petrocephalus, a high degree of genetic diversity previously noted within the group suggested that this genus is more speciose than currently considered.
The African weakly electric fish superfamily Mormyroidea forms a natural group within the Osteoglossomorpha (bony-tongue fishes) with two families: the monotypic Gymnarchidae (Gymnarchus niloticus) and the speciose Mormyridae. The family Mormyridae is, in turn, divided into two subfamilies: the Mormyrinae with 19 genera and more than 160 valid species and the Petrocephalinae, comprising only one genus, Petrocephalus Marcusen 1864, in which more than 40 species are currently considered valid.
Petrocephalus species form a natural group that is the sister group of the remaining mormyrids (i.e., the subfamily Mormyrinae) from which it can easily be distinguished by several unique morphological characteristics. The most salient are: 1) the presence of an orbitosphenoid, 2) the presence of a basisphenoid, 3) two single unsegmented and unbranched rays at the origin of the dorsal fin and 4) two nostrils closely apposed with the posterior one very close to the eye. Furthermore, the electric organ discharge waveforms of Petrocephalus are always of short duration (only few milliseconds long).
Petrocephalus are small fishes (typically <11cm in standard length; the largest species are Petrocephalus bane and Petrocephalus sauvagii, they reach 20cm in standard length) that can form large schools, and are mostly active at night. Petrocephalus have a widespread distribution in the freshwaters of subtropical and tropical regions of Africa. They may be locally abundant and diverse, as in several localities of the Congo basin with more than 10 sympatric species. In some fish communities, Petrocephalus species are the most abundant electric fishes, or perhaps even the most abundant of all fish species present.
Like other mormyrids, Petrocephalus species continuously produce weak electric pulses from an organ in the caudal peduncle for the purposes of orientation and communication. While in other mormyrid genera electric organ discharge (EOD) waveforms often differ substantially between species and are useful taxonomic markers, electric organ discharge waveforms of Petrocephalus are relatively conserved across the genus. In most cases, the electric organ discharge waveform of a Petrocephalus species exhibits two or three main phases of alternating polarities, often with the presence of one or two additional phases of much smaller relative amplitude. The first main phase is positive and the total duration of the EOD waveform is short. All Petrocephalus species examined to date have electric organs with the same simple type of electrocytes (i.e., type "NPp": electrocytes posteriorly innervated with non-penetrating stalks).
Identification of Petrocephalus species is often difficult due to the slight amount of morphological differentiation between many species. Despite the apparent conservatism in morphology and EOD waveforms among Petrocephalus, a high degree of genetic diversity previously noted within the group suggested that this genus is more speciose than currently considered.
