Several species of sharks have demonstrated the ability to sense magnetic fields (Kalmijn, 1978; Ryan, 1980; Klimley, 1993; 2002). The Ampullae of Lorenzini organ within sharks is used to detect weak electrical fields at short ranges. The detection range of this organ is effective only within inches, as sharks sense bioelectrical fields in the final stages of prey capture. SharkDefense has found that flux per unit area of certain permanent magnets, particularly Neodymium-Iron-Boride and Barium-Ferrite magnets, corresponds closely with the detection range of the Ampullae of Lorenzini. A permanent magnet with the correct specifications is hypotheiszed to over-stimulate the Ampullae of Lorenzini, and may therefore be used as selective shark repellent.

The fields generated by these permanent magnets decreases at the inverse cube of the distance from the magnet. Therefore, at distances of a few meters from the magnet, the field exerted is less than the Earth's magnetic field. Animals which lack that Ampullae of Lorenzini organ do not display aversive behavior in close proximity to the magnetic field, making this technology selective to sharks and rays (elasmobranchs).

The ampullae of Lorenzini are small vesicles and pores that form part of a subcutaneous sensory network of sharks. These vesicles and pores are found around the head of the shark and are visible to the naked eye. They appear as dark spots in this photograph of a porbeagle shark head. (Photo: Dr. Steven Campana, Bedford Institute of Oceanography)

There are several theories on the sensory mechanisms responsible for magnetoreception, including magnetite based magnetoreception and indirect magnetoreception by electroreception during electromagnetic induction.

Currently, the most commonly accepted theory on magnetite based magnetoreception involves thousands of small magnetic crystals (magnetite, Fe₂O₃) linked to the phospholipid bilayer of neurons via glycoproteins (Figure 1).

The glycoproteins act as small stoppers on ion channels when the ambient magnetic field orients the magnetite in such a way as to block the ion channel. When a migratory animal moves through the earth's magnetic field, the magnetite reorients allowing free flow of ions across the phospholipids bilayer generating an action potential which transmits geolocation information to the brain for processing. Magnetite based magnetoreception has been reported for many migratory marine species including yellowfin tuna (Walker et al. 1982), rainbow trout (Diebel et al. 2000), sea turtles (Lohmann et al. 2001) and spiny lobster (Boles and Lohmann 2003).

Elasmobranch fishes (sharks, skates and rays) have demonstrated the ability detect the earth's magnetic field, although the mechanism remains undescribed (Lohmann and Johnsen 2000). Indirect magnetoreception via electromagnetic induction is currently the most widely accepted mechanism, although magnetite based magnetoreception and chemical magnetoreception remain potential explanations.

Figure 1 - Illustration of a potential mechanism for magnetite-based magnetoreception involving changes in magnetite orientation in the presence of magnetic flux. The magnetic torque on the magnetite crystal removes the glycoprotein from the ion channel allowing ion exchange across the nervous membrane and generating an action potential (i.e., nervous signal) carrying the information to the brain for processing.

Figure 2 -Illustration of electromagnetic induction from a shark swimming through the Earth's magnetic field. Electrosensory organs known as the Ampullae of Lorenzini around the shark's mouth and nose detect the voltage drop induced by electrical current allowing navigation information to he processed by the brain as the fish's head moves back and forth during swimming. (Redrawn from Montgomery and Walker 2001).

 When a shark swims through the earth's magnetic field, electromagnetic induction – phenomena which generates an electric field as charged particles move through a magnetic field – creates an electric field around the shark (Figure 2).

Minute differences in the earth's magnetic field at different locations result in minute differences in the induced electric field which may be detected by the shark's sensitive electroreceptors, especially as the head region moves back and forth during swimming (Lohmann and Johnsen 2000).

Faraday's law of electromagnetic induction states (see right):

where:

¥ is the electromotive force (EMF) in volts

¥_B is the magnetic flux in webers.