This answer has been selected and edited by New Scientist staffThe apparent deformities on the backbone of the snapper are known as
fish hyperostoses. 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.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 "Tilly bones" 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.Neville Pledge, South Australian Museum, Adelaide, Australia
