This November is Wikipedia Asian Month

Organ evolution[edit]

The electric organ evolved in fish six times in evolutionary history.^[11] Most organs evolved from myogenic tissue, however one group of gymnotiformes, the Apteronotidae, derived their electric organ from neurogenic tissue.^[2] In the electric fish Gymnarchus niloticus (AKA the African knifefish), the tail, trunk, hypobranchial, and eye muscles have been found to be incorporated into the organ, most likely to provide rigid fixation for the electrodes while swimming.^[6] This provides evidence for a convergent evolution. In some other species, complete loss or considerable reduction of the tail fin has occurred, also indicating a convergence. This evolution is hypothesized to provide support against lateral bending while swimming and to maintain symmetry in the electric field for object detection.^[6] If an electric fish lives in an environment with little to no obstructions, such as some bottom-living fish, their electric organ has been seen to have less prominent evolutionized convergences between the trunk and the organ.^[6]

Strongly electric fish[edit]

Strongly electric fish are fish with an electric organ discharge that is powerful enough to stun prey or be used for defense. Typical examples are the electric eel, the electric catfishes, and electric rays. The amplitude of the signal can range from 10 to 860 volts with a current of up to 1 ampere, according to the surroundings, for example different conductances of salt and fresh water.^[12] To maximize the power delivered to the surroundings, the impedances of the electric organ and the water must be matched:

• 
  Strongly electric marine fish give low voltage, high current electric discharges. In salt water, a small voltage can drive a large current limited by the internal resistance of the electric organ. Hence, the electric organ consists of many electrocytes in parallel.
  
• 
  Freshwater fish have high voltage, low current discharges. In freshwater, the power is limited by the voltage needed to drive the current through the large resistance of the medium. Hence, these fish have numerous cells in series.^[13]
  

Weakly electric fish[edit]

The elephantnose fish is a weakly electric fish which generates an electric field with its electric organ and then processes the returns from its electroreceptors to locate nearby objects.^[14]

Weakly electric fish generate a discharge that is typically less than one volt. These are too weak to stun prey and instead are used for navigation, object detection (electrolocation) and communication with other electric fish (electrocommunication). Two of the best-known and most-studied examples are Peters's elephantnose fish (Gnathonemus petersii) and the black ghost knifefish (Apteronotus albifrons). The males of the nocturnal Brachyhypopomus pinnicaudatus, a toothless knifefish native to the Amazon basin, give off big, long electric hums to attract a mate.^[15]

The electric organ discharge waveform takes two general forms depending on the species. In some species the waveform is continuous and almost sinusoidal (for example the genera Apteronotus, Eigenmannia and Gymnarchus) and these are said to have a wave-type electric organ discharge. In other species, the electric organ discharge waveform consists of brief pulses separated by longer gaps (for example Gnathonemus, Gymnotus, Leucoraja) and these are said to have a pulse-type electric organ discharge.

EOD patterns[edit]

Electric organ discharges are generated from the animal’s electric organ.^[9] It emits pulse-like electric signals for a multitude of reasons, depending on the species. Many species use it for communication, while others use it for electrolocation, hunting, or defense.^[9] Their electric signals are often very simple as well as stereotyped, ie. always the same.^[8] A study from 2018 on two species of weakly electric African fish (Campylomormyrus compressirostris and the blunt jawed elephant nose, Campylomormyrus tamandua) looked at the communication aspect of their signals, specifically what information they are sending and receiving and how they are sending and receiving it.

Previous research has found that the two components of electrocommunication are EODs and sequence pulse interval, AKA SPI (ie. the temporal pattern EODs are released in).^[9] Using this as a starting point, the researchers conducted playback experiments to find the differences between EOD waveform and SPI between the two species, specifically how they relate to species recognition and discrimination, and what cues each species use to do this. They found that for SPI, C. compressirostris showed a tendency to burst when resting while C. tamandua presented a discharge pattern that was more heterogeneous.^[9] They also saw that the average EOD frequency and the average duration of SPI serial correlations were species specific which suggests that SPI may convey information to the receiver. In addition, the results showed evidence to support the idea that males mediate species recognition and discrimination in C. compressirostris as well as other mormyrid species.^[9] The researchers also noticed a significant relationship between EOD waveforms when they were paired with a natural SPI recording in C. compressirostris, however, this preference was not seen in all conditions. The males did not respond to artificial SPI recordings which researchers think suggests that there is some important information within the normal SPIs.^[9]

Another group of researchers studied the genetics of three species of the family Gymnotus (the naked back knifefishes G. arapaima, G. mamiraua, and G. jonasi of the Central Amazon Floodplain) and the diversity of their chromosomal and electrical signals.^[16] As of 2012, Gymnotus is the most diverse group out of the gymnotiform and mormyriform genera.^[8] They looked at their chromosomes and genes to find similarities, differences, and patterns to see how exactly they all evolved and are related to each other. The researchers used these species specifically because they are a model species for studying how postzygotic and prezygotic reproductive isolation events could lead to speciation and diversification.^[16] This paper is one of the first karyotypic analyses for these three species that also looks at EOD variation. They made a significant discovery for one species. They found that G. arapaima has a karyotypic formula that has never been seen before for the genus.^[16] They decided to place it within a small clade with a few other species, which all have more rows of scales and a larger body size. Their findings suggest G. arapaima to be similar to other species within this clade but also distinct because it has a smaller number of bi-armed chromosomes.^[16]