An organ specialized for detecting electric currents. Such organs are fairly common in marine fishes, which use them for detecting prey or potential attackers (i.e. electrolocation). The best known example is the ampulla of Lorenzini, groups of which are embedded in the head of sharks and rays. Each consists of a jelly-filled cuplike structure connected to the skin surface by a duct, sometimes several centimetres long, which is also filled with jelly. Sensory hair cells in the ampulla detect electric currents in the surrounding water, channelled inwards via the external pore and duct. The apical (outward facing) plasma membrane of each electroreceptor cell has a lower electrical resistance than the basal (inward facing) membrane, so that current moving through the cell causes the basal membrane to become depolarized. This opens voltage-gated calcium channels in the basal membrane, thereby triggering increased release of neurotransmitter by the electroreceptor cell, and hence increasing the frequency of action potentials in the afferent nerves carrying information from the electroreceptor to the brain. The high sensitivity of these organs enables a shark, for example, to sense the very weak electric currents, perhaps just a few microamps, generated by the respiratory muscles of a resting plaice buried in the sand. Sharks and rays also use their ampullae of Lorenzini as magnetoreceptors to detect the earth’s magnetic field.

Similar organs occur in certain teleost fish, for example the marine catfish Plotosus. Some fish generate their own weak electric field as an alarm system or as a means of locating objects or communicating with other individuals of the same species. Disturbances in this field are detected by the fish’s electroreceptors, warning of possible threats from intruders or receiving signals from conspecifics. In the duck-billed platypus (Ornithorhynchus), electrical sensors are distributed in rows over the surface of the bill and used to detect crustaceans, insect larvae, and other prey along the beds of muddy streams and rivers. The sensors—modified mucus glands—are thought to work in conjunction with numerous mechanical sensors, called pushrods, also distributed over the bill surface.