Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate . Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such as the electric eel, to stun prey. The capabilities are found almost exclusively in aquatic or amphibious animals, since water is a much better conductor of electricity than air. In passive electrolocation, objects such as prey are detected by sensing the electric fields they create. In active electrolocation, fish generate a weak electric field and sense the different distortions of that field created by objects that conduct or resist electricity. Active electrolocation is practised by two groups of weakly electric fish, the order Gymnotiformes (knifefishes) and family Mormyridae (elephantfishes), and by the monotypic genus Gymnarchus (African knifefish). An electric fish generates an electric field using an electric organ, Evolution from muscles in its tail. The field is called weak if it is only enough to detect prey, and strong if it is powerful enough to stun or kill. The field may be in brief pulses, as in the elephantfishes, or a continuous wave, as in the knifefishes. Some strongly electric fish, such as the electric eel, locate prey by generating a weak electric field, and then discharge their electric organs strongly to stun the prey; other strongly electric fish, such as the electric ray, electrolocate passively. The stargazers are unique in being strongly electric but not using electrolocation.
The electroreceptive ampullae of Lorenzini evolved early in the history of the vertebrates; they are found in both Chondrichthyes such as , and in Osteichthyes such as and , and must therefore be ancient. Most bony fishes have secondarily lost their ampullae of Lorenzini, but other non-homologous electroreceptors have repeatedly evolved, including in two groups of , the (platypus and ) and the (Guiana dolphin).
History
In 1921, the German anatomist Viktor Franz described the (tuberous organs) in the skin of the Mormyridae, again without knowledge of their function as electroreceptors.
In 1949, the Ukrainian-British zoologist Hans Lissmann noticed that the Gymnarchus was able to swim backwards at the same speed and with the same dexterity around obstacles as when it swam forwards, avoiding collisions. He demonstrated in 1950 that the fish was producing a variable electric field, and that the fish reacted to any change in the electric field around it.Lissmann, Hans. " Continuous Electrical Signals from the Tail of a Fish, Gymnarchus Niloticus Cuv", in: Nature, 167, 4240 (1951), pp. 201–202.
Electrolocation
Passive electrolocation
Active electrolocation
Many of these fish, such as Gymnarchus and Apteronotus, keep their body rather rigid, swimming forwards or backwards with equal facility by undulating that extend most of the length of their bodies. Swimming backwards may help them to search for and assess prey using electrosensory cues. Experiments by Lannoo and Lannoo in 1993 support Lissmann's proposal that this style of swimming with a straight back works effectively given the constraints of active electrolocation. Apteronotus can select and catch larger Daphnia water fleas among smaller ones, and they do not discriminate against artificially-darkened water fleas, in both cases with or without light.
These fish create a potential usually smaller than one volt (1 V). Weakly electric fish can discriminate between objects with different resistance and capacitance values, which may help in identifying objects. Active electroreception typically has a range of about one body length, though objects with an electrical impedance similar to that of the surrounding water are nearly undetectable.
Active electrolocation relies upon tuberous electroreceptors which are sensitive to high frequency (20–20,000 Hertz) stimuli. These receptors have a loose plug of epithelial cells which Capacitance couples the sensory receptor cells to the external environment. Mormyridae (Mormyridae) from Africa have tuberous electroreceptors known as Knollenorgans and Mormyromasts in their skin.
Elephantfish emit short pulses to locate their prey. Capacitance and resistive objects affect the electric field differently, enabling the fish to locate objects of different types within a distance of about a body length. Resistive objects increase the amplitude of the pulse; capacitative objects introduce distortions.
The Gymnotiformes, including the glass knifefish (Sternopygidae) and the electric eel (Gymnotidae), differ from the Mormyridae in emitting a continuous wave, approximating a sine wave, from their electric organ. As in the Mormyridae, the generated electric field enables them to discriminate accurately between capacitative and resistive objects.
| Electrolocation of capacitative and resistive objects in glass knifefish. Many gymnotid fish generate a continuous electrical wave, which is distorted differently by objects according to their conductivity. | The electric eel's electric organs occupy much of its body. They can discharge both weakly for electrolocation and strongly to stun prey. |
Electrocommunication
When two glass knifefishes (Sternopygidae) come close together, both individuals shift their discharge frequencies in a jamming avoidance response.
In bluntnose knifefishes, Brachyhypopomus, the electric discharge pattern is similar to the low voltage electrolocative discharge of the electric eel, Electrophorus. This is hypothesized to be Batesian mimicry of the powerfully-protected electric eel. Brachyhypopomus males produce a continuous electric "hum" to attract females; this consumes 11–22% of their total energy budget, whereas female electrocommunication consumes only 3%. Large males produced signals of larger amplitude, and these are preferred by the females. The cost to males is reduced by a circadian rhythm, with more activity coinciding with night-time courtship and spawning, and less at other times.
Fish that prey on electrolocating fish may "eavesdrop" on the discharges of their prey to detect them. The electroreceptive African sharptooth catfish ( Clarias gariepinus) may hunt the weakly electric mormyrid, Marcusenius macrolepidotus in this way. This has driven the prey, in an evolutionary arms race, to develop more complex or higher frequency signals that are harder to detect.
Some shark embryos and pups "freeze" when they detect the characteristic electric signal of their predators.
Evolution and taxonomic distribution
Electric organs have evolved at least eight separate times, each one forming a clade: twice during the evolution of cartilaginous fishes, creating the electric skates and rays, and six times during the evolution of the bony fishes. Passively-electrolocating groups, including those that move their heads to direct their electroreceptors, are shown without symbols. Non-electrolocating species are not shown. Actively electrolocating fish are marked with a small yellow lightning flash and their characteristic discharge waveforms. Electric fish are marked with a red lightning flash .
Cartilaginous fish
Bony fish
Monotremes
The electroreceptive capabilities of the four species of echidna are much simpler. Long-beaked echidnas (genus Zaglossus) have some 2,000 receptors, while short-beaked echidnas ( Tachyglossus aculeatus) have around 400, near the end of the snout. This difference can be attributed to their habitat and feeding methods. Western long-beaked echidnas feed on earthworms in leaf litter in tropical forests, wet enough to conduct electrical signals well. Short-beaked echidnas feeds mainly on and , which live in nests in dry areas; the nest interiors are presumably humid enough for electroreception to work. Experiments have shown that can be trained to respond to weak electric fields in water and moist soil. The electric sense of the echidna is hypothesised to be an evolutionary remnant from a platypus-like ancestor.
Dolphins
Bees
See also
- Active sensory systems
- Feature detection (nervous system)
- Magnetoreception
Further reading
External links
- ReefQuest Centre for Shark Research
- Electrolocation on Scholarpedia
- Video clips of Gnathonemus, Apteronotus, and Ameiurus
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