Passive electrolocation in fish

Passive electrolocation is a process where certain species of fish or aquatic amphibians can detect electric fields using specialized electroreceptors to detect and to locate the source of an external electric field in its environment creating the electric field. These external electric fields can be produced from animate sources such as:

or to inanimate sources such as the electric fields induced by movement of a conducting organism through the earth's magnetic field, or from atmospheric electricity. These external fields the fish identifies are low or high frequency, weak electric signals that the fish uses to detect prey, locate other fish, avoid predators, and navigate in the Earth’s magnetic field. Electrorectors probably evolved once or twice in early in vertebrate evolution but the sense was apparently lost in animiotes, and in a large number of the actinopterygians (ray finned fishes) only to reappear independently in two teleost clades. In fish, the ampullary receptor is a specialized receptor that it uses to sense these electric fields and allows the fish to follow electric field lines to their source. Sharks primarily use specialized receptors, called Ampullae of Lorenzini, to detect prey’s low frequency DC fields and may also use their receptors in navigation of the Earth’s magnetic field. Weakly electric fish use their ampullary receptors and tuberous receptors to detect the weakly electric fields produced by other fish, as well as for possible predator avoidance. Passive electrolocation contrasts with active electrolocation, in which the animal emits its own weak self generated electric field and detects nearby objects by detecting the distortion of its produced electric field. In active electrolocation the animal senses its own electromotor discharge or reafference instead of some externally generated electric field or discharge

Electroreception occurred early in evolutionary history with the evolution of an ampullary sensory system that included receptors able to detect weak electric signals in the environment (less than 1 µV/cm or 50 Hz). Sense organs specialized for electroreception have only been found among vertebrates, and around 8.600 species are known to be electroreceptive. The majority of teleosts and amniotes do not have an electroreceptive system, but the distribution of electroreception in terms of evolution involves different classes of fish. First, there is an origin of a common ancestor of current existent vertebrates (close to lampreys and gnathostomes). This ancestor evolved a lateral line that is important in processing sensory information and present in today’s hagfishes. Ampullary receptors are ancestral to jawed fish, for lampreys and agnathans were found to have ampullary receptors 400 million years earlier. Second, there is a loss of electroreception in amniotes. This could be due to the fact that air is a poor medium to effectively conduct the electric fields, unlike water. Third, there is also a loss of electroreception in gars, bowfin, and teleosts (neopterygian fishes). Ampullary receptors are present in all surviving cartilaginous fishes and bony fishes except a few species present in Neopterygii (which include gars, bowfins, and teleosts) Fourth, monotremes and a least three groups of fresh-water teleosts re-evolved electroreception. Fifth, a group of teleosts, Xenomystinae, acquired passive electrolocation with low frequency sensitive ampullary receptors. Sixth, another group of teleosts, Mormyroids, evolved passive electrolocation and active location, as well as a pulse type EOD in active electrolocation. Seventh, Silurifiormes and Gymnoformes evolved passive electrolocation, and Gymnoformes also evolved active electrolocation with a pulse type or wave type EODs.

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