Electrical
communication
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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