When I collect and research fish specimens for the Canadian Museum of Nature’s collection, I’m often particularly interested in the rocks in their heads.
These tiny rocky structures, just millimetres in size, are called otoliths, or ear stones. For fishes, otoliths are used for balance and hearing (humans have smaller ear stones used for similar purposes). For scientists, these little otoliths are scientific gold, providing an encyclopedia of information. And, most intriguing of all, fish otoliths hold a few unsolved mysteries.
Otoliths come in pairs, one per ear, and are found near a fish’s brain. The ear stones are composed of calcium carbonate in one of three mineral forms: primarily aragonite; sometimes calcite; and rarely vaterite.
The Museum has 2258 otoliths representing 477 species as part of the fish osteological (bone) collection. Researchers use ear stones to investigate biological questions about the fish they came from, ecological questions about the fishes’ environment, and anthropological questions about how people used fish in the past.
Each species of fish has otoliths with a distinctive shape. This means that even when isolated otoliths are found–whether in the gut contents of predators, in feces deposited by animals that eat fish, or preserved on archaeological sites–we can identify the fish they came from.
Most importantly, each otolith tells a detailed story about a fish’s life. Otolith size provides an estimate of its body length and the otolith growth rings record a fish’s age and rate of growth in a way analogous to tree rings. The shape of the annual growth rings also captures climatic data, even recording the season of the year when the fish died.
Chemical analysis of otoliths reveals the average water temperature during a fish’s life and the strontium, calcium and zinc ratios in them reveals patterns of migration when fish move from marine to fresh waters, and even within different habitats in large rivers such as the Amazon.
Fish otoliths even help us understand ourselves, in particular our history. For example, a study of heaps of cod otoliths collected from a sixteenth-century Basque fishing settlement in Labrador provided extensive information about Atlantic Cod populations prior to increasingly intensive commercial fishing. Otoliths can be radiocarbon dated to identify when a site was used, and changes in the chemistry and structure of otoliths can even provide information about how a fish was cooked.
One of the outstanding fish ear stone mysteries also relates to humans and fish. Farmed fish of all species have ten times the amount of vaterite in their otoliths as compared to their wild cousins, but as yet we don’t know why.
The good thing is that fish have rocks in their heads that help them hear—and help scientists like me tune into amazing natural history stories.