Ganoine or ganoin is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some non-teleost ray-finned fishes,[1] such as gars and bichirs, as well as lobe-finned coelacanths.[2] It is composed of rod-like, pseudoprismatic apatite crystallites, with less than 5% of organic matter.[3] Existing fish groups featuring ganoin are bichirs and gars, but ganoin is also characteristic of several extinct taxa.[4] It is a characteristic component of ganoid scales.
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Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian Cheirolepis.[4] While often considered a synapomorphic character of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct acanthodii.[4]
It has been suggested that ganoine is homologous to tooth enamel in vertebrates[1] or even considered a type of enamel.[3] Ganoine indeed contains amelogenin-like proteins[1] and has a mineral content similar to that of tetrapod tooth enamel.[5]