As mentioned earlier, some fishes, such as elasmobranchs, may have such a sensitive electro receptive ability that they can detect the weak electric fields they create as they move through the earth’s magnetic field. This ability would provide these fishes with an indirect way of sensing the earth’s magnetic field and give them directional information with respect to compass headings. Round Stingrays (Urobatis halleri) in the lab learned to orient in induced magnetic fields; the rays switched the location in which they searched for food when the electric field around them was artificially reversed, suggesting that geomagnetic cues might be used in daily activities(Kalmijn 1978).
Some fishes, however, may be able to detect magnetic fields directly. Several species of salmon and trout, eels, Yellowfin Tuna, and sharks and rays can detect magnetic fields (see For Micki et al. 2004), and magnetite has been extracted from the heads of Yellowfin Tuna, Chinook Salmon (Oncorhynchus tshawytscha), and Chum Salmon(O. keta) (Walker et al. 1984; Kirsch Vink et al. 1985; Oguraet al. 1992). The Japanese Eel (Anguilla japonica) can be conditioned to respond to magnetic fields that are similar in magnitude to that of the earth (Nishi et al. 2004), and larval Brown Trout (Salmo trutta) also responded to magnetic Fields (Formicki et al. 2004). The ability to discriminate among different field strengths and inclinations and to orient to the directional polarity of the earth’s magnetic field would aid in magnetic compass orientation and navigation.
The mechanism for direct sensing of magnetic fields remains a mystery. Walker et al. (1997) found crystalline material that they believed to be magnetite within the folds of the olfactory epithelium of Rainbow Trout, and nerve tracts run from these cells to the brain. These observations led Walker et al. to propose that Rainbow Trout have magneto receptivecells in their olfactory capsule. Others, however, have proposed that magneto reception may be related to the other mechanoreceptive sensory systems such as the inner ear and the lateral line system. Harada et al.(2001) studied the chemical composition of the otoliths of several birds and fishes and found significant levels of iron in the lagena of some species. They speculated that, although the two largest otolith organs, the saccule and utricle, responded to movement, the small size and higher iron content of the lagenal otoliths makes this a potential site for geomagnetic sensing. More research is needed to locate the organs of magneto reception in fishes. Among the challenges is that magnetic fields can pass through animal tissues, so magneto reception could take place anywhere in the body. Therefore, receptor cells and their neurons would not have to be concentrated in a particular location – they could be widely dispersed throughout the body. In addition, magnetite particles would be extremely rare and small, making them difficult to identify.
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