Agonistic behaviour and electric signalling in a mormyrid fish, Gnathonemus petersii

1. Agonistic motor behaviour and concurrent electric signalling were studied in individually held, residential Gnathonemus petersii. Aggressive behaviour was elicited by presenting a specimen of a closely related species, Mormyrus rume, for 3 min a day. 2. The principal agonistic motor patterns are described (Fig. 2). Among them head butt, approach and lateral display were further analysed. 3. The electrical activity displayed during agonistic behaviour was found to differ fundamentally both from isolated resting and swimming conditions. The mean discharge rate recorded during aggressive behaviour (31 Hz, Fig. 3 c) is approximately twice the rate observed in an isolated swimming fish (Fig. 3b) and three times the rate displayed by a resting animal (Fig. 3a). An attacking G. petersii exhibits a much greater range of electric organ discharge (EOD) intervals than isolated swimming or resting individuals. EOD-interval histograms recorded from attacking fish show two sharp modes at high discharge rate; there are no intermediate intervals. 4. During the course of an attack, the initially low and variable discharge rate increases fairly linearly as the distance from the attacked fish diminishes (Fig. 9). The EOD rate associated with physical contact (head butt) comprises between 60 and 80 Hz in 24 of 28 attacks analysed; the dominant mode of the distribution is 61 Hz (Fig. 8). 5. During subsequent lateral display, G. petersii emits a high discharge rate pattern consisting of two types of ldquosteady-staterdquo activities which may last up to a few seconds: the first is a fairly regular alternation of approx. 16 and 8 ms intervals (paired pulses); this pattern gives rise to the two peaks of high discharge rate in the interval histogram (Fig. 3c). The second is a regular sequence of either 16 or 8 ms intervals (Fig. 4A). The only female among the animals used in our study showed the same display but did not exhibit the highest possible discharge rate (i.e. a regular sequence or 8 ms intervals; Fig. 4B). The high discharge rate is terminated by a sudden discharge break (Figs. 4A and 6). 6. It is suggested that the attack-associated EOD rate increase is a remnant of an ordinary locomotory pattern which has changed its function to a ritualised aggressive signal that occurs in a socially significant and well-defined context. The high discharge rate might serve three functions: (i) behavioural isolation of closely related, sympatrically living mormyrids (perhaps by character displacement); (ii) recognition of sexes; (iii)_synchronisation of mates during courtship

Electrifying love: electric fish use species-specific discharge for mate recognition

Mate choice is mediated by a range of sensory cues, and assortative mating based on these cues can drive reproductive isolation among diverging populations. A specific feature of mormyrid fish, the electric organ discharge (EOD), is used for electrolocation and intraspecific communication. We hypothesized that the EOD also facilitates assortative mating and ultimately promotes prezygotic reproductive isolation in African weakly electric fishes. Our behavioural experiments using live males as well as EOD playback demonstrated that female mate recognition is influenced by EOD signals and that females are attracted to EOD characteristics of conspecific males. The dual function of the EOD for both foraging and social communication (including mate recognition leading to assortative mating) underlines the importance of electric signal differentiation for the divergence of African weakly electric fishes. Thus, the EOD provides an intriguing mechanism promoting trophic divergence and reproductive isolation between two closely related Campylomormyrus species occurring in sympatry in the lower Congo rapids

Evidence for Parapatric Speciation in the Mormyrid Fish, Pollimyrus castelnaui (Boulenger, 1911), from the Okavango–Upper Zambezi River Systems: P. marianne sp. nov., Defined by Electric Organ Discharges, Morphology and Genetics

We report on parapatric speciation in the mormyrid fish,Pollimyrus castelnaui (Boulenger, 1911), from the Okavango and the Upper Zambezi River systems. We recognise samples from the Zambezi River as a distinct species, P. marianne, displaying an eastern phenotype of electric organ discharge (EOD) waveform (Type 3) that is distinct from the western EOD phenotype (Type 1) observed in P. castelnaui samples from the neighbouring Okavango. Samples from the geographically intermediate Kwando/Linyanti River (a tributary of the Zambezi that is also intermittently connected to the Okavango) presented a more variable third EOD phenotype (Type 2). In 13 out of 14 morphological characters studied, the Zambezi River samples differed significantly from P. castelnaui. Morphologically and in EOD characters, the Kwando/Linyanti fish are distinct from both P. castelnaui and P. marianne. Sequence analysis of the mitochondrial cytochrome b gene unambiguously reveals that specimens from the Zambezi River System form a well supported taxon which clearly differs from P. castelnaui from the Okavango (1.5–2.5% sequence divergence).Within specimens from theKwando–Zambezi System some geographic differentiation can be detected (nucleotide substitutions up to 0.6%); but groups cannot be resolved with certainty. Significant allozyme differences were found between the Okavango and all other EOD types from the Upper Zambezi System, and, within the Zambezi System, between the Kwando (Type 2) and Zambezi (Type 3) individuals. The low Wright’s fixation index values, the lack of fixed allele differences, and small genetic distances provide little evidence for speciation between groups within the Zambezi System, but moderate to great fixation index values and significant allele frequency differences were observed between the Okavango and the other fishes. It is concluded that within the Zambezi System, differentiation between Kwando/Linyanti and Zambezi populations (as revealed by morphology and EOD waveform comparisons) is so recent that substantial genetic (allozyme and mitochondrial sequence) differences could not have evolved, or were not detected