My partner recently brought home a large snapper that was caught off the coast of Adelaide, South Australia. When we had finished devouring the delicious fish I noticed that the skeleton had some unusual deformities on the backbone in the form of three hard, bony spheres (see photo). My partner claims not to have seen anything like it before, and my rudimentary internet search produced no pictures of fish skeletons with similar features.
Can someone tell me whether these are a normal part of snapper morphology or an abnormality?
鈥he apparent deformities on the backbone of the snapper are known as . They are not uncommon in older specimens of the snapper Sparus auratus and are also reported in 92 other species of marine bony fish in 22 families. Hyperostoses take the form of regular, cellular swellings in otherwise thin, acellular bone, such as vertebral spines or the thin bones of the skull. In each species they generally occur in the same specific places.
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The Australian old-man snapper that the questioner enjoyed so much is characterised by a bony knob or casque on the back of the skull, which is another example of hyperostosis. Fish hyperostoses also occur as fossils (sometimes called fossil brains by collectors), and have been the subject of research for a long time. They are often known as 鈥淭illy bones鈥� by palaeontologists, after the palaeoneurologist Tilly Edinger (1897-1967), who made them a special interest.
Hyperostoses do not seem to inconvenience the fish and are apparently not pathological, but their purpose is uncertain. However, because they occur only in tropical and subtropical marine fish (plus a few temperate species such as the snapper), and a couple of species of freshwater fish in hypersaline environments, it is thought that they may play some part in calcium regulation.
鈥淭he bony spheres do not seem to inconvenience the fish, and could play a role in calcium regulation鈥�
Neville Pledge, South Australian Museum, Adelaide, Australia