Ganoine

The glassy appearance of the scales of this spotted gar is due to ganoine.

Ganoine or ganoin is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some basal ray-finned fishes.^[1] It is composed of rod-like, pseudoprismatic apatite crystallites, with less than 5% of organic matter.^[2] Existing fish groups featuring ganoin are bichirs and gars, but ganoin is also characteristic of several extinct taxa.^[3] It is a characteristic component of ganoid scales.

Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian Cheirolepis.^[3] While often considered a synapomorphic character of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct acanthodii.^[3]

It has been suggested that ganoine is homologous to tooth enamel in vertebrates^[1] or even considered a type of enamel.^[2] Ganoine indeed contains amelogenin-like proteins^[1] and has a mineral content similar to that of tetrapod tooth enamel.^[4]

References

1. Zylberberg, L.; Sire, J. -Y.; Nanci, A. (1997). "Immunodetection of amelogenin-like proteins in the ganoine of experimentally regenerating scales of Calamoichthys calabaricus, a primitive actinopterygian fish". The Anatomical Record. 249 (1): 86–95. doi:10.1002/(SICI)1097-0185(199709)249:1<86::AID-AR11>3.0.CO;2-X. PMID 9294653.
2. Bruet, B. J. F.; Song, J.; Boyce, M. C.; Ortiz, C. (2008). "Materials design principles of ancient fish armour". Nature Materials. 7 (9): 748–756. Bibcode:2008NatMa...7..748B. doi:10.1038/nmat2231. PMID 18660814.
3. Richter, M. (1995). "A microstructural study of the ganoine tissue of selected lower vertebrates". Zoological Journal of the Linnean Society. 114 (2): 173–212. doi:10.1006/zjls.1995.0023.
4. Ørvig, T. (1967). "Phylogeny of tooth tissues: Evolution of some calcified tissues in early vertebrates.". Structural and Chemical Organization of Teeth. New York: Academic Press. pp. 45–110.