Communication behavior and sensory mechanisms in weakly electric fishes
Creators: name: Kramer, Bernd
Publisher: name: Universität Regensburg
Year: 1994
Subjects: subject: 570 Biowissenschaften, Biologie
subject: 590 Tiere (Zoologie)

description: Teleost freshwater fishes of the orders Mormyriformes (the elephantfishes plus Gymnarchus from Africa) and Gymnotiformes (the knifefishes from South America) are both electrogenic and electroreceptive. These fishes' electric system has a motor part, the electric organ, and a sensory part, the cutaneous electroreceptors that project to large, specialized brain areas. The electric systems of both groups of fishes, although evolved independently, are adapted to the same two functions, nocturnal electrolocation and communication. Weakly electric fishes discharge their electric organs in a pulselike or in a wavelike fashion ("buzzers" and "hummers," respectively). Whether a species is a hummer or a buzzer does not appear to be correlated with ecology but is strongly linked to phylogeny. There are representatives of both discharge types on both continents where these fishes are found. The elephantfishes (Mormyridae, about 200 species) are, apparently, all pulse fishes, whereas the related, monospecific Gymnarchus (Gymnarchidae) is the only known African wave fish. There are five families of South American knifefishes, with the majority of the 70 or so species being hummers, usually discharging at extremely constant frequencies (about 50- 1800 Hz). The sensory mechanisms of social communication, as studied by behavioral means, are reviewed in this chapter with the question of mechanisms of reproductive isolation in mind. The chapter focuses on the electric organ discharge as the basic communication unit, and on the frequency, repetition rate, or temporal patterns of discharges. In both wave and pulse fishes the frequencies or repetition rates of discharges are not usually species-specific but are species-characteristic, because of more or less broad overlap between two or more species (depending on the local community of species). Electrosensory discrimination thresholds for frequency and intensity are unusually low in a wave fish, lower by far than those for other acoustico-lateral senses of aquatic lower vertebrates, rivaling the discrimination thresholds for audition in the most sensitive mammals (e.g., the human). A similar conclusion applies for the pulse rate sensitivity of a mormyrid. Species specificity becomes apparent when more information about the discharge activity is considered. In the case of pulse fishes, especially mormyrids, this comprises temporal patterns of discharges, which also vary greatly according to behavioral context (like aggression, escape, courtship, feeding, etc.); these fishes have an interdischarge interval code of communication. In the case of wave fishes, various types of frequency modulations and brief, repetitive discharge stops occur. Also, wave fishes may engage in "phase coupling" and "jamming avoidance," maneuvers that involve precise interaction with another fish's discharges. The degree ' of species specificity of a fish's discharge activity is usually enhanced by features of the waveform of a single discharge; this is true in both pulse and wave fishes. Usually there is considerable intraspecific variability of discharge waveforms, and there are also examples of sexual dimorphism. At least a few species can discriminate the individually variable pulse or wave discharge waveforms of their species. In a wave fish, a sensory mechanism based on the temporal analysis of beat patterns can explain the observed results. This new sensory capacity detects the phase modulation within a beat, which always occurs when the wave discharges of two fish mix in the water. In pulse fishes, several hypothetical sensory mechanisms for the discrimination of intraspecific pulse waveforms have been proposed but it is not yet clear which is generally involved. In any case, the sensitivity of weakly electric fishes to the fine detail of their discharges shows that the electrosensory world is much more colorful than could be imagined until recently.