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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.
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.
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.
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