A linear chain of biogenic magnetite crystals, extracted from tissues in the frontal region of the sockeye salmon^[1] Oncorhynchus nerka, a close relative of the rainbow trout, Oncorhynchus mykiss. These are also single magnetic domains, with crystal alignments similar to those in magnetotactic bacteria. (Photo credit: S. Mann). Subsequent studies have shown that these crystals are indeed a component of the magnetic sensory system of animals^{[2, 3]}, as was predicted over 20 years ago^[4-6].

[1] S. Mann, N. H. C. Sparks, M. M. Walker, and J. L. Kirschvink, "Ultrastructure, morphology and organization of biogenic magnetite from sockeye salmon, Oncorhynchus nerka: Implications for magnetoreception," J. Exp. Biology, vol. 140, pp. 35-49, 1988.
[2] M. M. Walker, C. E. Diebel, C. V. Haugh, P. M. Pankhurst, J. C. Montgomery, and C. R. Green, "Structure and function of the vertebrate magnetic sense," Nature, vol. 390, pp. 371-376, 1997.
[3] J. L. Kirschvink, "Magnetoreception: homing in on vertebrates," Nature, vol. 390, pp. 339-340, 1997.
[4] J. L. Gould, J. L. Kirschvink, and K. S. Deffeyes, "Bees have magnetic remanence," Science, vol. 201, pp. 1026-1028, 1978.
[5] C. Walcott, J. L. Gould, and J. L. Kirschvink, "Pigeons have magnets," Science, vol. 205, pp. 1027-1029, 1979.
[6] J. L. Kirschvink and J. L. Gould, "Biogenic magnetite as a basis for magnetic field sensitivity in animals,," Bio Systems, vol. 13,, pp. 181-201., 1981.