The Bone that Logs a Lifetime

a red snapper's ear stone in the palm of someone's hand
Scientists use the ear stones as a way to age the fish. Also called otoliths, they are bones found right behind the skulls of bony fishes. (Florida Fish and Wildlife)

Slice through the trunk of a tree and you reveal a pattern of rings that grow out from the center of the trunk. Count them, and you have just determined the age of the tree. With every passing year, the trunk grows outward, laying down new woody tissue and simultaneously a physical record of time gone by. If only all plants and animals had such a simple way of showing their exact age.

If you’re a fish biologist, you’re in luck. In the inner ears of fish, there is a series of small bones called otoliths that grow just like rings in a tree. Created when the fish is just a larvae, the three otoliths serve as a record of the fish’s entire life. In some species, the otolith is the size of a small coin, and in others, it is as small as a grain of rice. 

“We call them the flight data recorders for fish,” said Steve Murawski, a fisheries biologist at the University of South Florida.

Just as humans have an inner ear for balance, an otolith helps the fish understand its orientation in space, an important task when moving up and down throughout the water. These “ear stones,” as they are often called, sit in a sack lined with hair cells within the ear, and as the fish moves up and down through space, so does the otolith. When the bone knocks into the side and collides with the hairs, a signal is sent to the fish’s brain which helps the fish maintain balance. 

ear bones of a grouper fish
These are some of the inner ear bones of a red grouper fish. Every year the earbone grows, leaving behind a visible ring. (Florida Fish and Wildlife)

But for scientists, an otolith’s worth comes from how it grows. Every day a new layer of calcium carbonate is added to the outside of the otolith. Slicing through the center of the bone reveals a series of alternating light and dark rings, each set representing a year of the fish’s life. The layers are distinguishable because of seasonal changes in growth. During the spring and summer, when food is plentiful, the fish grows rapidly. The otolith is also growing quickly and, as it does, it leaves behind a thick, opaque ring. But as the season changes to fall and the fish’s growth begins to slow, so does the otolith’s growth and the ring darkens. For this reason, fish that live in highly seasonal locations have the most distinguishable rings.

It’s easy to see an otolith’s rings with a microscope and count them to find a fish’s age (if you know how). That in and of itself is an incredible tool to study fish. But hidden within the chemical structure of the otolith is a wealth of information about the environment that the fish lives in. As the otolith grows it incorporates metals from the environment into its structure. Some of these, like magnesium, strontium, and barium, naturally occur in the ocean, while other metals, like cadmium, nickel, vanadium, zinc, and copper, can indicate exposure to pollutants. Pair the chemical information with the sequential rings, and a carefully laid out timeline of the fish’s environmental history falls into place.

After removing the otolith from the back of the fish’s skull, scientists can then decipher the chemical clues within the bone. A special drill bores a line of up to 60 microscopic holes from the center of the otolith out to the edge. From each hole comes a fine powder that can then be chemically analyzed for specific metals. Since each hole corresponds to a specific year of the fish’s life, it represents a snapshot of the world the fish lived in at that time.

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Dispatches from the Gulf

Some scientists use this chemical timeline to figure out where a fish has lived. A specific location in the ocean will have specific levels of magnesium, strontium, and barium. Match the chemical profile of the fish’s otolith to the profile of a location, and you can plot their movement through time.  

Murawski is more interested in the chemicals that indicate trouble. In 2010, the Deepwater Horizon oil spill spewed over 4 million barrels of oil into the Gulf of Mexico. Oil is a nasty mixture of not only hydrocarbons (molecules made of hydrogen and carbon), but also several metals including cadmium, zinc, and copper. Animals throughout the Gulf, including fish, began swimming in the threatening mixture leading scientists, governmental officials, and the public to worry about the health of offshore populations. Murawski and his colleague Ernst Peebles (also at the University of South Florida) wondered, can otoliths be used to measure the long-term effects of oil exposure?

After laboriously collecting fish of various species across the Gulf, Murawski and his team began to uncover several trends. First, fish in the Gulf of Mexico are continuously exposed to oil and had been even before the Deepwater Horizon spill. Backtracking from the last ring, Murawski’s colleague Jennifer Granneman was able to determine which ring in the otolith was formed during 2010, the year of the spill. She found that several fish species that were consistently exposed to oil throughout their life (specifically zinc and nickel metals) were more likely to have unhealthy skin lesions, an indication of poor health. In this case, the oil spill itself did not seem to affect the health of the fish, but consistent oil exposure from other oil-related compounds did. 

In a separate study, Elisabeth Herdter, also from the University of South Florida, found that after 2011 the rings in red snapper otoliths significantly decreased in width. This meant that the red snappers were not growing as fast as in previous years—exposure to Deepwater Horizon’s oil could be hampering their growth in some way (though other environmental changes like cooler temperature and food availability can also cause the same response).

While there’s no smoking gun that definitively connects Deepwater Horizon oil exposure to long-term fish health, the evidence locked away in the otoliths are critical pieces to understanding the oil spill puzzle. Combined with evidence from the scales, fins, gills, and liver of various fishes, the otoliths support the evidence that oil exposure significantly damages the health of fish. A tiny bone the size of a nickel can carry a lifetime of knowledge. 


The Ocean Portal receives support from the Gulf of Mexico Research Initiative (GoMRI) to develop and share stories about GoMRI and oil spill science. The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization, and remediation technologies.

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December 2019