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Translated it means 'above normal bone growth'. The word is derived from the Greek hyper - above, osteo - bone, and osis - suffix denoting a condition.

leatherjacket vertebra showing hyperostosis.

Leatherjacket vertebra with hyperostosis

Image: Carl Bento
© Australian Museum

Hyperostosis occurs in about eighty species of fishes across at least six orders and twenty families.Bone sites that are affected and how they grow appear to be fairly consistent within a species. The hump on the head of an adult Snapper develops as a result of hyperostosis.

Pagrus auratus
An x-ray of the head of a Snapper. The supraoccipital and frontal bones are clearly visible. Image: J. King
© J. King

The cause of hyperostosis is unknown. It has been suggested that it may be a response to pollution, or possibly a disease, however evidence suggests that it has a genetic cause.

Hyperostosis doesn't appear to have any effect on the edibility of a fish.

Further reading

Jawad, L.A. 2013. Hyperostosis in three fish species collected from the Sea of Oman. Letters to the Editor. The Anatomical Record. Pp 3

McGrouther, M.A. 1994. Swollen Fish Bones. Australian Natural History. 24(11):79

Smith-Vaniz, W.F., L.S. Kaufman & J. Glowacki. 1995. Species-specific patterns of hyperostosis in marine teleost fishes. Marine Biology 121:573-580