Electric Organ Discharge

A species-specific spatiotemporal pattern denotes the specific wavelength and frequency of signal emitted by the electric organ of varying types of gymnotiformes. This signal is specific to the gymnotiform type and thus is hard to intercept. The adaptive value of the spatiotemporal pattern is that it is useful for intra-species communications.  Alejo Rodriguez-Cattaneo and his team introduces the idea that the species-specificity of the pattern of electric discharge is important for each species to differentiate between foe and friend, as well as ‘tone out the excess nois and to become a more streamlined form of communication.” Besides communication, electric signals bounce off of objects and back to the fish's electroreceptors to orient the fish in space in a similar manner to that of echolocation. Electric signals become an elaborate mating ritual that relates to the male size of electric organ and the female's receptivity.

Rodriguez-Cattaneo et al( 2008) performed an experiment that compared the electromotive forces expressed by G. carapo, G. coropinae, and G. omari, three species of the phylum Gymnotus. They found that each of these species share a similar pattern of electromotive force coming from the rostral portion of their bodies. Most of the differentiation between the electric discharge made by each species was generated in the main body and in the tail regions, adding to the variation in amplitude and duration of the impulses.

Rodriguez-Cattaneo and his team show that G. omari electric discharge waves follow a substantially different wave path compared to those of G. carapo and G. coropinae which originate in a similar location. The G. omari  has a very weak negative discharge, with pulse duration averaging at 3.17ms indicating that its’ main electric generator resides near the anal fin.  G carapo and G. coropinae have much weaker electric discharges with shorter duration that is generated very rapidly from head to tail, indicating that these fish rely on the electric organs that are dispersed among different sites of their bodies to send out electric pulses.
Rodriguez-Cattaneo define the electric organ discharge of gymnotiformes as: “the result from the transformation of a single neural impulse originating from a synchronous pacemaker into a pattern of electromotive force which results from the sum of action currents generated by electrogenic cells.” (p.7)

Electric organ signals are generated from the neural source in the fish brain medulla, the dorsal torus semicircularis. This neural bundle controls incoming responses to signalsin the ear cannals from other fish and the efferent signals emitted from the fish's electric organ. The dorsal torus semicircularis in the fish midbrain controls beat rate and is sensitive to a 2-6 Hz range of emission, though it can filter incoming signals of up to 64 Hz. The torus semicircularis lies in pacemaker neurons in the fish medulla, which connects to the midbrain on one end and runs into a cholinergic synapse near the spinal nerve on the other. (Markham 2009). Two types of temporally selective neurons in the semicircularis exist; baseline and burst-selective neurons. Baseline selective neurons show higher postsynaptic potential amplitude in response to non-signaling electromotor activity, whereas burst selective neurons respond with higher postsynaptic potential amplitude to high pulse activity. (Pluta and Kawasaki, 2010). The length of electric current discharge can differ between electric fish by nearly two orders of magnitude, and males usually express a longer duration than females (Katz, Paul S., 2006). Recent research suggests that testosterone or dihydrotestosterone can effect the duration of discharge so that when females and immature males are injected with testosterone, they can produce EODs twice as long as they would be able to normally. Estradiol has little effect on EOD production, suggesting that electric organ function increases with the presence of androgens. (Zakon, 1993).
          
 A video of electric fish organ discharge recorded by Professor Carl Hopkins at Cornell University