Fossil otoliths show patterns of growth

Fossilized fish ear stones — known as otoliths — may reveal more than previously thought. In a recent study, a team of pathologists from the University of Vienna demonstrated that an improved electron microscopy technique can make even the finest growth rings visible. These microscopic structures can reveal a fish’s life story in just a few hours. This is an important advance for understanding fish development, biomineralization, and climate change over millennia. The study was published recently Limnology and Oceanography: Methods.

All fish have tiny mineral structures called otoliths in their inner ears.

“These structures preserve a fish’s entire life story in the form of growth rings – like the annual rings of a tree. They can tell us about age, growth stages and even environmental conditions,” explains lead author Isabella Leonhard from the Department of Paleontology.

In modern marine biology and fisheries research, otoliths have long been essential tools for studying fish growth, migration, and population dynamics. However, in paleontology, they were often neglected.

“That’s starting to change, as new technologies—from high-resolution imaging to chemical analysis—lead the way. As a result, otoliths are gaining attention in paleontology and offering unique insights into ancient fish populations,” says Leonard.

‘Diary Entries’ of Fishes

Of particular interest are the ultrafine growth rings in the otoliths, which can be read like “diary entries” of the fish. In fossil specimens, these rings were previously difficult to detect because the preservation of the material varied and because conventional light and electron microscopy had reached their limits.

Leonard and his team adapted a well-established technique from geologists to study these growth patterns: backscatter electron imaging (BSE). This method exploits the fact that electrons are reflected differently by different structures within the material, revealing even the most delicate internal patterns.

By fine-tuning the imaging settings, the researchers were able to visualize very finely banded growth rings in fossil otoliths from black goby (Gobius niger) found in the northern Adriatic Sea, buried in the seafloor for more than 7,600 years. With this improved method, they detected up to 275% more growth rings than standard imaging techniques allowed.

“With the electron microscope, we were able to make even the smallest growth spurts visible,” explained Leonhardt. Generally, these circles form in a daily rhythm. In addition, there are microincrements that form independently of the daily cycle. All of these daily patterns reflect feeding, movement, environmental changes or stressors the fish were exposed to.

“We discovered very fine, regularly spaced structures that appear at intervals much shorter than a day. Their pattern also follows a biological rhythm – but we still don’t know what causes them,” says expert Amelia Jaruchowska from the University of Utrecht. “Further investigation will require controlled growth experiments.”

Using fossil otoliths to understand modern change

This improved BSE method enables scientists to compare fossil and modern fish populations in extraordinary detail, providing a wider timescale for current changes. “In times of climate change and overfishing, it is very important to understand how fish populations evolve over long periods of time,” emphasizes Martin Zuschen, head of the Department of Paleontology and co-author of the study.

“Our findings show that fossil otoliths have tremendous potential, and can help us better understand the changes we’re seeing today.”