The teeth of the chiton, a sea mollusk, have magnetic iron caps. Discovered by Caltech's late Professor of Paleoecology, Heinz A. Lowenstam^[1], it was the first known instance of ferrous minerals being produced biologically by any living organism. These magnetite-bearing teeth are from the Polyplacophoran mollusk, Cryptochiton stelleri. Each tooth is about 1 mm in size, and capped with a layer of black magnetite (Photo credit: H. A. Lowenstam). Magnetite biomineralization has since been discovered in many living organisms, ranging from bacteria^[2], honey bees^[3], fish^[4-6], birds^[7], and even the human brain^[8].

[1] H. A. Lowenstam, "Magnetite in denticle capping in recent chitons (Polyplacophora)," Bulletin of the Geological Society of America, vol. 73, pp. 435-438, 1962.
[2] R. P. Blakemore, "Magnetotactic bacteria," Science, vol. 190, pp. 377-379, 1975.
[3] J. L. Gould, J. L. Kirschvink, and K. S. Deffeyes, "Bees have magnetic remanence," Science, vol. 201, pp. 1026-1028, 1978.
[4] M. M. Walker, J. L. Kirschvink, S.-B. R. Chang, and A. E. Dizon, "A candidate magnetic sense organ in the Yellowfin Tuna Thunnus albacares," Science, vol. 224, pp. 751-753, 1984.
[5] 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.
[6] 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.
[7] C. Walcott, J. L. Gould, and J. L. Kirschvink, "Pigeons have magnets," Science, vol. 205, pp. 1027-1029, 1979.
[8] J. L. Kirschvink, A. Kobayashi-Kirschvink, and B. J. Woodford, "Magnetite biomineralization in the human brain.," Proceedings of the National Academy of Sciences, vol. 89, pp. 7683-7687, 1992.