Fishes are unique in that some species both produce and receive electrical information based on very weak electrical output (see Electrical communication). The electric organ discharge (EOD) is species and often sex specific in South American gymnotiform knifefishes and African mormyriform elephantfishes (the German common name, tapirfi sche, likens the latter family more to tapirs and is actually more descriptive). A fish can modify amplitude, frequency, pulse length, or interpulse length of its discharge, or alter parts of its EOD such as the fundamental frequency or peak power frequency. Fish can thus exchange information about species, sex, size, maturational and motivational state, location, distance, and individual identification. Electric discharges are used commonly during agonistic interactions (Bullock et al. 1972; Westby 1979; Hagedorn 1986; Hopkins 1986).
Much research has been conducted on the social context and function of EODs during courtship and territorial encounters in both groups (Møller 2006). Most but not all species have sexually dimorphic EODs. In apteronotid knifefishes, the male emits at a higher frequency in some species but in others it is the female that has a higher frequency discharge (Zhou & Smith 2006). Isolation of male hypopomid knifefish, Brachyhypopomus pinnicaudatus, leads to a gradual decrease in the sexually dimorphic component of the duration and amplitude of its waveform. The differences are restored when a second fish is introduced to the test animal, suggesting that maintaining sexual differences in EOD comes at some cost, perhaps explaining why sexual dimorphism is not universal (Franchina et al. 2001). If two knifefish are emitting at the same frequency, the overlap can cause interference (=jamming). A jamming avoidance response is well known in gymnotids, whereby fish avoid jamming by shifting their EOD frequency away from that of nearby conspecifics. What has been shown more recently, however, is that the Brown Ghost Knifefish, Apteronotus leptorhynchus, actively jams the output of others during competitive interactions (Tallarovic & Zakon 2005). Both male and female Brown Ghosts presented with actual or simulated (via electrical playback) intruders with a higher EOD frequency than their own raise their EOD frequencies to within potential jamming range.
In mormyriform fishes, shifts in EOD duration and phase amplitudes occur during agonistic encounters in juvenile as well as adult fishes, regardless of gender. EODs are used during interactions in combination with other display modes, utilizing multisensory communication systems that enhance signal transmission and reception (Schuster 2006). Interacting fish will head butt one another and also swim parallel and in place, which could push water and sound waves at the other fish as well as providing visual and tactile cues (Terlaph & Møller 2003; Terlaph 2004).
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