All living organisms generate electric fields around their bodies, but only some organisms are able to sense them. The Elasmobranchii, which includes sharks, rays, and skates, is one group of animals that possesses electroreceptors enabling them to detect electric fields. These fishes use their ability to perceive electric stimuli to hone in on live prey, after their senses of smell and sight have aided them in the initial search.
An elasmobranch electroreceptor, also called an ampulla of Lorenzini (named after the scientist who first described them), consists of a tubular, insulated canal connecting a pore on the surface of the skin to an internal round sac (ampulla). The canal and ampulla are filled with a gel that readily conducts electric currents from the water outside the pore to receptor cells within the wall of the ampulla. These receptor cells are stimulated by the electric current and send signals via nerves to the brain, which integrates the signals arriving from different activated receptors to generate a whole “picture” of the external electric field.
Ampullae of Lorenzini are found around the head in sharks and on the surfaces of the expanded pectoral fins in skates and rays. The pores are visible to the naked eye on the surface of the skin, appearing as small dots.
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“Even sightless, the shark is nevertheless still being guided toward its victim by its sensory armory — by now the shark’s electrical sense is operating. Beneath the skin in its snout are numerous tiny, electrosensitive organs known as ampullae of Lorenzini, each linked to the outside world by an external pore. They detect the minute electrical fields produced by the shark’s victim and permit the shark to home in on its prey, and to aim with devastating accuracy its first but generally lethal bite.” (Shuker 2001:40)
“[Ampullary electroreceptors] consist of a canal leading from the surface of the skin to an ampulla in whose wall a group of sensory cells is embedded…The apex of each cell bears microvilli and/or a single kinocilium protrudring into the lumen of the ampulla, which is full of low-resistance jelly. Since the canal wall has a high resistance (30-100 times that of the nerve myelin sheath) the canals act as idea ‘submarine cables’ ending in open circuit at the ampullary end of the canal. The ampullary organs are tonic receptors giving a long-lasting respsonse to very low frequency or d.c. [direct current] stimuli.” (Bone et al. 1995:230)
“Ampullary receptors (the ampullae of Lorenzini) occur around the head in sharks and over the upper and lower surface of the ‘wings’ of rays…The receptors are distributed in clusters surrounded by a connective-tissue capsule…The widespread distribution of the ampullae allows the fish to compare electric potentials over a considerable area, no doubt improving their ability to detect the direction of the source of an electric stimulus…It is interesting that the elasmobranchiomorph ampullae are collected into capsules, for it means that while the openings of the canals cover a wide area of the body, the sensory cells are all located close together and as near isopotential as possible, so allowing a greater ability to detect small potential differences over the body surface.” (Bone et al. 1995:231)