Nerve impulses are electric impulses, and water, especially if it contains dissolved minerals (called electrolytes), is a good electrical conductor. This means that every time an animal that lives in the water uses a muscle it gives off a minute electrical signal. It should come as not surprise to learn that many species of fish have learned to detect these signals.
With electrical sensing a fish can detect and locate prey that is hidden from sight and against the direction of the flow of the water, perhaps buried in the mud or sand even if their prey item is not moving. Rays in particular use electrical sensing to detect buried prey. Electrically sensitive fish can also detect the approach of both conspecifics and predators.
In most electrically sensitive species the sensors, called 'external pit organs' in teleosts and 'ampullae of lorenzini' in elasmobranchs are located on the animals head, however they may also be scattered over the body, or along the lateral line. The sensors consist of canals of electrically conductive gel that open to the water at certain points called pits.
It is known that fish can detect the weak electrical field generated by their own movement through the Earth's magnetic field, an ability which would obviously be useful for migratory species. Species which can sense electrical impulses of other animals but not generate their own special fields include Sharks, Rays, Skates, Catfish and Paddlefish.
The next step in evolution is to generate an electrical field specifically for the purpose of electrolocation. Fish that can purposefully generate an electrical discharge in excess of the normal electrical patterns given off by all living things can perceive, not only living organisms, but all the world around them electrically. This has happened in 2 families of Rays and in 10 families of Teleost fish. Although the most famous electrical fish are the Electric Catfish of Africa (Gymnarchus niloticus) and Electric Eels of South America (Electrophorus electricus) many other interesting, albeit less powerful, species exist.
|Family||Common Name||No. Species||Region|
|Torpedinidae||Electric Rays||14||Marine, various|
|Mormyridae||Elephant Fish||198||F.W., Africa|
|Malopteridae||Electric Catfish||2||F.W., Africa|
|Gymnotidae||Naked-backed Knifefishes||9||F.W., S. America|
|Apteronotidae||Ghost Knifefishes||64+||F.W., S. America|
|Hypopomidae||Electric Knifefishes||35+||F.W., S. America|
|Sternopygidae||Glass Knifefishes||41+||F.W., S. America|
|Rhamphictthydae||Sand Knifefishes||6||F.W., S. America|
|1. The three families with + contain many undescribed species.|
|2. The Uranoscopidae contains about 50 species but only the four are electrically sensitive.|
These fish have electricity generating cells that constantly give off an electrical pulse that in tern creates an electrical field around them. Everything that touches this field changes the feedback image the fish receives from its field. thus the fish can build up an image of its world that is not dependant on sight, sound, smell or touch. Such an ability is very useful if you live in muddy water, as most electrical teleosts do. Also most electric fish are nocturnal. Furthermore these fish can control and change their own electrical field to improve their image of the world, or to change another electrically sensitive fish's perception of them.
Apart from generating an electrical field for the purposes of perception many electrical fish can release an electric discharge into the water around them. By doing so they can both deter predators and stun their own prey.
As all muscular movement generates electrical impulses, movement can distort a fish's electrical image of the world. For this reason most electric fish (fish that generate a special electric field for perceptive purposes) tend to move slowly and to hold their body straight, using only their elongated dorsal fin to swim with.
Most fish, especially the electrically stronger species, produce their electrical field as a result of timed pulses of electrical discharge, but a few of the weakly electrical fish use a continual wave to produce their field. Most species, even the most powerful, normally only produce relatively weak discharges to maintain their field, saving the power bursts for special occasions.
The cells producing the electricity are called 'electrocytes'. Each electrocyte produces only a very little electricity, to produce a large voltage a fish must have many such cells arranged in a batteries that are able to discharge simultaneously. These electrocytes are normally located in the head in marine species and in the tail in fresh water species.
Fish produce their electrical fields at individual frequencies. When two fish of th same species meet who are using similar frequencies they change their frequencies to make them less similar in order to avoid having their perception jammed by the other fish's output. In other species males and females use different frequency ranges to help them recognise potential mates and to avoid jamming each other. In Apteronotus leptorhynchus the females use frequencies between 600 hz and 800 hz while the males use frequencies of between 800 hz and 1050 hz.
Not all fish can produce electrical discharges of equal potential difference, it is ecologically expensive to carry around so much generating equipment. The champion is without doubt the Electric Eel (Electrophorus electricus) which can generate a potential difference of up to 600 volts. The Electric Catfish (Gymnarchus niloticus) can generate about 300-350 volts, a Torpedo Ray about 220 volts and some Stargazers can produce 50 volts, however most electric fish are limited to between 8 and 40 volts.
The two largest electric fish, The Electric Eel of South America and the Electric Catfish of Africa prey specifically on the smaller and weaker species of electric fish. They can turn off their own electrical field to help keep their presence unknown to their prey and then when a smaller species comes close they suddenly release a large killing discharge. The smaller, weakly electric fish have been observed trying to hide from such predators by also turning their electrical field off, or by turning it of and on in a irregular way.
The voltage of an electrical discharge rapidly diminishes as it travels away from its source. Electrolocation is ultimately a close range sense with a working sphere for most species of between 1 and 2 metres.
Finally, as I said earlier, anything within a fish's electrical field with a different conductivity will distort the the electrical flow pattern set up by the fish and thus change the field potential around the fish. Detecting these continual changes allows the fish to map the physical world around it. Of course a fish's ability to make sense of this flow of information is dependant on its brain size an complexity.
Mormyrids are well known for their large brain to body weight ratios which are far higher than in any other fish, equal to a human beings in fact. Does this equate with higher intelligence? The answer seems to be yes. Aquarists (people who keep fish) have known for a long time that the Nile River basin Mormyrids, which use more complicated signals than the Amazon River basin Gymnotids, are playful. They are well known for making toys out of objects in their aquaria, carrying them around with their noses. Also scientist have long considered the Mormyrids to be the easiest of fish to teach tricks in the laboratory. Both playfulness and learning are signs of intelligence.