Just like trees with their rings of varying density, the hard parts of animals sometimes contain evidence of passing seasons and even behaviors, such as migrations and reproductive efforts. When scientists are trying to identify how old a particular animal was or how quickly it was growing, this can be invaluable.
Picture of a tree’s rings on display at the Bristol Zoo, England, species unknown (Adrian Pingstone, 2005).
Picture of a sectioned sagittal otolith at magnification, juvenile striped bass Morone saxatilis (Chris Conroy, 2010).
In fish, calcium carbonate (CaCO3) structures within the vestibular system called otoliths are a prime example of such hard parts. The primary function of fish vestibular systems, sometimes referred to as labyrinthine systems, is similar to that of our inner ears; the movement of otoliths within the system help fish to maintain their position and orientation, just like the movement of CaCO3 structures within our utricle and saccule help us to keep balance. Additionally, some fish may use their vestibular systems to sense vibrations, similar to the way we pick up sound waves with our cochlea.
Picture of a sectioned sagittal otolith at magnification, striped bass Morone saxatilis (Chris Conroy, 2010); area of otolith section used for age determination outlined in red; blue arrows indicate direction of count, from the oldest to youngest material (i.e. material laid around the time of hatch and just before capture, respectively); sky blue dots indicate increments that were counted; green lines highlight examples of individual increments; in this fish the number of increments corresponds roughly to the days since hatch (i.e. age in days).
Through observation and laboratory experimentation, scientists have determined that the alternating pattern of continuous (translucent) and discontinuous (largely opaque) zones observed in concentric rings within otoliths results from the consistent formation of layers over time. Depending on taxa and condition, otolith increments (as these zones are often called) can be used as a regular marking of the passage of time throughout a fish’s life. For most aging applications, otolith increments are used to determine the number of years since hatch. However, in some cases otolith increments may be used to estimate the number of days since hatch; as you might suspect, this comes at the cost of precision (i.e. some error is expected). In addition to age, otoliths contain lots of other useful information. When you combine these distances between increments with the overall size of the fish when it was captured, and taxa- and often population-specific estimates of body and otolith size at a very young age, the result is body size in the past. This can be combined with age to estimate growth rate throughout a fish’s life. All of this is very useful information to scientists studying population dynamics. As a bonus, I think their concentric geometry and varied overall shape (depending on the taxa, the population, and even the individual) is also beautiful!
Pictures of sagittal otoliths and otolith sections from a number of species (http://floridasportfishing.com/magazine/ask-the-experts/the-otolith, date unknown)
If you’re interested in learning more about otoliths check out the links below. The chemical composition of otoliths also holds a lot of information that scientists find useful- look for this in a future post.