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The Coupling of Atmospheric Electromagnetic Fields

 

 

catfsh to be remarkably sensitive to metallic rod. Tey have a regular response to metallic rod even at

a distance of some centimeters, whereas a glass rod did not elicit a reaction until it actually touched the

skin of the animal (Kalmijn, 1971). Tey demonstrated that nibbling responses were due to the galvanic

currents generated at the interface between metal and aquarium water. Tese responses were elicited by

a current of approximately 1.0 μA between two electrodes about 2.0 cm apart. Te avoidance reactions

were by currents of 1 μA or more. Tese results led to the concept of electroreception.

Electroreception in aquatic animals such as ray fshes is the ability to detect weak static electric felds.

It facilitates the detection of prey. Te electroreception is obtained through direct transmission of stim­

ulus via electrical sensitive organs, called “ampullae of Lorenzini.” Tis name is from Stefano Lorenzini.

He discovered special electric organs of the elasmobranchii (shark and ray). In the 1960s Dutch scientists

Dijkgraaf and Kalmijn established that sharks and rays, which have dermal sense organs, ampullae of

Lorenzini, could sense weak electric currents from their prey organisms such as fatfshes even when

they were buried under sand. Kalmijn (1974) showed that fsh maintains their orientation while swim­

ming in the geomagnetic feld. Sharks and rays have the ampullae of Lorenzini, which are located near

the front of their brains that detect the extremely weak electric feld induced by the geomagnetic feld,

i.e., earth currents. Tese behaviors can be summarized as shown in Figure 3.9 which gives various

mechanisms for detecting electromagnetic felds (Kalmijn, 2000). In Figure 3.9a, the situation when

a shark approaches the vicinity of a dipole feld (0.2–0.5 μV/m) used to simulate prey is shown. As the

shark swims through the geomagnetic feld, in accordance with Faraday’s induction law, a vertical

electromotive force is induced. Tis induced electric feld allows selection of direction relative to the

direction of the geomagnetic feld to be obtained. It has been conjectured that this is used to judge

the direction that the fsh is swimming. In Figure 3.9b, the vector product of the fow of velocity (v) of the

ocean stream (i.e., water current) and the geomagnetic feld’s vertical component (Bv) is equivalent to

the electrical gradient created: current fow (ion fow) occurs, and detection of this current fow allows

perception of the direction (up vs. downstream) of the fowing water; this provides a means of passive

electro-orientation. In a slow ocean current, surface electric felds are 0.05–0.5 μV/cm. In a tidal cur­

rent level, using a cross section of the Gulf Stream an example, total electric felds up to 0.5 μV/cm were

predicted (Rommel and McCleave, 1973).

Michael Faraday discovered a changing magnetic feld induces electric currents in nearby conductive

structures. Tis induction law predicts that the movement of animal in magnetic feld would induce in

electromotive force. Tis is the basis of electroreception in the aquatic animals. In Figure 3.9c, the shark

is moving through the geomagnetic feld: the electric feld resulting from motion of the shark through

the geomagnetic feld gives it a magnetic compass heading; this is active electro-orientation. For a fsh

swimming at velocity (v) through the horizontal geomagnetic feld component (Bh), an electric gradient

is induced by the vector product. For example, a fsh swimming at the speed of 1 m/s through the geo­

magnetic feld horizontal component of 25 μT will induce an electric feld of 0.25 μV/cm. Tis electrical

gradient passes through the ampullae of Lorenzini. Because sharks and rays can detect electric felds of

0.01 μV/cm, they can readily detect this feld. Tus, the aquatic animal might perceive an electric feld

induced by water current or by its own motion in the geomagnetic felds.

Behavior of a shark near a dipole imitating prey buried in the sand is shown in Figure 3.9a. In 1971,

Kalmijn published the key paper in the discovery of electroreception in fshes. Te behavioral experi­

ment is shown in Figure 2 of his paper (Kalmijn, 1971, see p. 377). Te results of these behavioral experi­

ments are suggested as follows: (1) When a shark was eagerly searching for food and passed a plaice at

a distance of 15 cm or less, he generally exhibited a very clear feeding response, although the prey was

almost entirely hidden from view by a thin layer of sand. (2) When the sharks, eagerly swimming about,

passed the agar-screened plaice at a distance of 15 cm or less, they still showed their characteristic, well-

aimed turnings toward the animal. (3) Te plaice in the agar chamber was exchanged for a small bag

of loose Nylon tissue, containing pieces of whiting. In this situation the sharks and rays eagerly tried to

fnd their food at the end of the outlet tube and did not show even the slightest response when swimming

just over the agar roof. (4) Afer the test animals had been motivated with whiting juice, they searched