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Electrophysiology and behavior
The American knifefishes (Teleostei: Gymnotiformes) are animportant component of the nocturnal ichthyofauna of Middle and South American freshwaters. Over 100 gymnotoid species have beendiscovered in the neotropics, ranging from Río de La Plata of Argentina (35 degrees S) to the Río San Nicolas of southwestern Chiapas,Mexico (18 degrees N). In addition, many undescribed species are recognized in museum collections, and more forms are discovered each year.
An outstanding feature of gymnotoid biology, observed in allknown species, is the capacity to produce and detect weak electricfields. Gymnotoid fishes continually emit electric discharges,which they use in object location and communication by detecting changes in the shape, amplitude, or frequency of the electricfield. This electrosensory system has been the subject of numerous physiological, anatomical, and behavioral studies. Several review volumes have been published in recent years (Bullock andHeiligenberg, 1986; Heiligenberg, 1991) summarizing the major discoveries of this research program.
The sensory components of this system include the integumentalhair cell receptors, as well as their peripheral and centralnervous system connections. These structures are very similar tothose described in other electroreceptive vertebrate groups despitethe fact that electroreception appears to have evolved severaltimes independently. Many of the similarities shared byelectroreceptive lateral line systems are in fact also observed inthe mechanosensory lateral line system of craniates, indicatingtheir derivation from a common developmental pathway. Gymnotoids are also characterized by the presence of anelongate anal fin, with more than 200 anal-fin rays, which givesthem a knife-like appearance. This feature is functionally coupledwith electroreception and is the source of their common names inmany languages, including "knifefish" in English, and "cuchillo"(knife) or "anguilla" (eel) in Spanish. Gymnotoid fishes locomote by undulating this elongate anal fin, rather than the lateral body-wall musculature as do other fishes. Since the entire surface of gymnotoids is covered by electroreceptor organs, maintaining a somewhat rigid body axis facilitates the use of the integument asan undistorted sensory surface. The shimmering appearance of the undulating anal fin, and the nocturnal habits of knifefishes, arethe origin of the name "sarapo" (ghost) used by the Tupi-Guarani indigenous Amazonian peoples.
The motor effectors of the electrosensory system are specialized muscle or nerve cells called electrocytes which generate rhythmic electric organ discharges (EOD). Certain aspects of the frequency spectrum and wave form of the EOD appear to beunique to each species while other aspects may be used to characterize entire gymnotoid linages. Electric organ discharges are also used as social signals, in both territorial and sexual behaviors.
Electric fishes have been informally classified as either"wave" or "pulse" species depending on the EOD repetition rate. The relative ease with which these EOD signals and electroreceptor responses may be monitored has facilitated the use of gymnotoids in behavioral studies. Data collected in the field and laboratory settings confirm the role of electroreception in both trophic and social interactions. Investigations into the neural basis of electroreception has also provoked interest into methods of breeding and raising gymnotoids in captivity. As a result, comparative data are now available on what would be otherwise inaccessible character systems, including reproductive behavior,the behavior of larvae and juveniles and patterns in the development of the electric organs. The ability to breed knifefish in captivity has also permitted the application of experimental methods to the study gymnotoid regeneration. These advances facilitated the recognition of a fossil gymnotoid, Ellisella kirschbaumi, from the Upper Miocene (c. 10-12 mya) of Bolivia.