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EOD evolution[edit]
A paper on the three different electric organs of Electrophorus electricus, the electric eel, details what each organ’s function is. Overall, all three are used for navigation, communication, hunting, and defense but each purpose happens in different levels of electric organ discharge.[5] E. electricus is the only species that has evolved three separate organs for discharge and one of the only ones that can generate strong discharges. Their goal was to find the differences between the three electric organs and what proteins, phosphosites, and phosphorylation events occurred that may have caused the changes.[5]
The researchers found that the electrolyte cells in the electric organs of E. electricus have a polarized cell arrangement, which is how they are able to generate small voltages that can add up.[5] The cells are triggered by acetylcholine, which causes an action potential-like sequence of events that results in an electric potential difference inside each electrolyte cell, which, combined with every other cell, sums up to create a large voltage.[5] They also found several new phosphorylation sites in the electrogenic proteins, including one that has only been seen elsewhere in the electric ray species Tetronarce californica. These new sites are not seen in nonelectrogenic species which means they may be unique and important to the ability to generate EODs. These sites very likely evolved independent of one another, because the two species are very distantly related.[5] Finally, they also found a consistent and special abundance pattern in a handful of the electrogenic proteins within each of the three electric organs (the main organ, Sach’s organ, and Hunter’s organ). Having different abundances likely reflects the energy each organ requires to emit EOD’s, whether weak or strong.[5]
A recent study from 2020 looked at the evolution of sensorimotor integration in terms of communication signal diversification in Mormyrid fishes, AKA African weakly electric fishes.[10] Corollary discharge is one of the ways motor control can influence sensory processing which is done by filtering out the individual’s own signals from being processed. Because corollary discharge is responsible for cancelling its own signals, it would make evolutionary sense if signal diversification was selected for. To answer this question the researchers looked at 7 different mormyrid species with varying corollary discharge inhibitions (CDI) and EOD durations.[10] The researchers did find a correlation between signal diversification and CDI, as well as between EOD duration and the onset of CDI (but not duration of CDI). With these findings, they were able to conclude that, in response to an individual’s own EODs, SCDIs evolved to shift their time window in order to impede spikes in KO.[10]
Jamming avoidance response[edit]
Main article: Jamming avoidance response
It had been theorized as early as the 1950s that electric fish near each other might experience some type of interference or inability to segregate their own signal from those of neighbors. This issue does not arise, however, because the electric fish adjust to avoid frequency interference. In 1963, two scientists, Akira Watanabe and Kimihisa Takeda, discovered the behavior of the jamming avoidance response in the knifefish Eigenmannia sp. In collaboration with T.H. Bullock and colleagues, the behavior was further developed.[17] Finally, the work of Walter Heiligenberg expanded it into a full neuroethology study by examining the series of neural connections that led to the behavior.[18] Eigenmannia is a weakly electric fish that can self-generate electric discharges through electrocytes in its tail. Furthermore, it has the ability to electrolocate by analyzing the perturbations in its electric field. However, when the frequency of a neighboring fish's current is very close (less than 20 Hz difference) to that of its own, the fish will avoid having their signals interfere through a behavior known as jamming avoidance response. If the neighbor's frequency is higher than the fish's discharge frequency, the fish will lower its frequency, and vice versa. The sign of the frequency difference is determined by analyzing the "beat" pattern of the incoming interference which consists of the combination of the two fish's discharge patterns.[18]
Neuroethologists performed several experiments under Eigenmannia's natural conditions to study how it determined the sign of the frequency difference. They manipulated the fish's discharge by injecting it with curare which prevented its natural electric organ from discharging. Then, an electrode was placed in its mouth and another was placed at the tip of its tail. Likewise, the neighboring fish's electric field was mimicked using another set of electrodes. This experiment allowed neuroethologists to manipulate different discharge frequencies and observe the fish's behavior. From the results, they were able to conclude that the electric field frequency, rather than an internal frequency measure, was used as a reference. This experiment is significant in that not only does it reveal a crucial neural mechanism underlying the behavior but also demonstrates the value neuroethologists place on studying animals in their natural habitats.[18]
Species[edit]
The following is a table of electric fish species listed by family. Most families inhabit fresh water. Two groups of marine fish, the electric rays (Torpediniformes: Narcinidae and Torpedinidae) and the stargazers (Perciformes: Uranoscopidae), are capable of generating strong electric pulses.
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