Otoliths, or ear stones, grow inside the inner ear of bony fish. There are over 28,000 species, including such diverse species as clownfish, seahorse, salmon, and tuna. Bony fish have three pairs of otoliths—the sagittae, the lapilli, and the asteriscii. Otoliths are made of calcium carbonate, the same chemical compound that chalk and limestone are made of. Different fish have otoliths that are different in shape and size, so scientists can find the otoliths in the poop of birds and seals to see what kind of fish these animals are eating-yuck! Otoliths allow the fish to keep their balance and to detect sound and water depth. Otoliths grow continuously during a fish's life, a few microns each day from birth until death. One micron is 1,000 times smaller than one millimeter, so otoliths are typically about the size of a grain of sand in juvenile fish and the size of a fingernail in most adult fish.

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