Electroreceptors are specialized neurons that, in an aquatic or marine environment respond to the presence of an electric eld (or , equivalently, a voltage gradient across them). They are found in the skin of certain sh and in the beak of the duck-billed platypus, the only mammal having this sensory modality. Underwater electric elds can be subdivided into two classes: those that are constant (dc) or slowly varying (up to 50 Hz), and those high-frequency elds arising from special electrogenic organs in so-called weakly electric sh that use self-generated, low-voltage ac elds for navigation.

One type of electroreceptor used to detect dc and low-frequency elds is called the ampulla of Lorenzini; it is found in large numbers on the heads of sharks, skates, and rays. Low-frequency elds can be the result of a physical phenomenon, e.g., the Faraday streaming effect (see Section 2.4.3), or be of biological origin, such as from the contracting muscles of a swimming sh, especially one that has been injured so that its skin is torn.

Tuberous high-frequency eld sensors are called knollenorgans; they are found in weakly electric sh (Mormyrids) and are part of the comple x electric guidance system used by these sh. Mormyrid sh also ha ve smaller, less sensitive electroreceptors called mormyomasts, and some low-frequency ampullary electroreceptors. Knollenorgans respond to frequencies ranging between 300 Hz and 20 kHz. The threshold eld sensiti vity for ampullary receptors is about 0.1 to 0.2 ^V/m (Kalmijn, 1998). Knollenorgans respond to ac elds having a peak value of ~10 mV/m, far less sensitive than ampullary electroreceptors (Carr and Maler, 1986).

Of signi cance is the fact that sh electroreceptors are organized into arrays. Mormyrid sh have several hundred knollenorgans distributed over their bodies, with concentrations on the head and caudal region of the anks. Knollenor gans are integrated into a perceptual system that allows the sh to sense nearby en vironmental features on the basis of how they distort the electric eld surrounding the sh that the sh produces with its electric or gan. Such "blind guidance" is necessary for the survival of these sh, who generally li ve in muddy water where vision is useless. Ampullae of Lorenzini on elasmobranchs are found mostly on their heads and snouts, but are not restricted to these areas. Their distribution is species dependent. On skates, the ampullae are most dense on the anterior surfaces of their pectoral n "wings."

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