Fossilized fish ear stones – known as otoliths – can reveal far more than previously thought. In a recent study, a team of palaeontologists from the University of Vienna demonstrated that a refined electron microscopy technique can make even the finest growth rings visible. These microscopic structures may reflect a fish's life story down to just a few hours – an important breakthrough for understanding fish growth, biomineralization, and environmental change across millennia. The study was recently published in Limnology and Oceanography: Methods.

All fish have tiny mineral structures in their inner ears called otoliths. "These structures store 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 phases, and even environmental conditions," explains the leading author Isabella Leonhard from the Department of Palaeontology.

In modern marine biology and fisheries research, otoliths have long been essential tools for studying fish growth, migration, and population dynamics. In palaeontology, however, they were often overlooked.

"That is beginning to change," says Leonhard, "because new technologies – ranging from high-resolution imaging to chemical analysis – are making it possible to apply biological methods to fossils that are thousands or even millions of years old." As a result, otoliths are gaining attention in the palaeontological community and offer unique insights into ancient fish populations.

"Diary Entries" of fishes

Of particular interest are the ultra-fine growth rings in otoliths, which can be read like fish’s "diary entries." In fossil specimens, these rings were previously hard to detect – both because the material’s preservation varies and because traditional light and electron microscopy had reached their limits.

Leonhard and her team adapted a well-established technique from geology 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 extremely finely banded growth rings in fossil otoliths from the black goby (Gobius niger), found in the northern Adriatic Sea and buried in the seafloor for over 7,600 years. With this optimized method, they detected up to 275% more growth rings than standard imaging techniques would allow.

"With the electron microscope, we were able to make even the smallest growth increments visible," Leonhard explains. Typically, these rings form in a daily rhythm. In addition, micro-increments exist that form independently of the daily cycle – these sub daily patterns reflect feeding, movement, environmental changes, or stressors the fish was exposed to.

"We discovered extremely fine, regularly spaced structures that appear in much shorter intervals than a day. Their pattern suggests they also follow a biological rhythm – but we still do not know exactly what causes them," says Emilia Jarochowska, palaeontologist and study collaborator from Utrecht University. "Controlled growth experiments will be needed to investigate further."

Using Fossil Otoliths to Understand Modern Change

This optimised BSE method enables scientists to compare fossil and modern fish populations in unprecedented detail, providing a broader timescale for current changes. "In times of climate change and overfishing, it is crucial to understand how fish populations have developed over long periods," emphasizes Martin Zuschin, head of the Department of Palaeontology and a co-author of the study.
"Our results show that fossil otoliths have enormous untapped potential, and can help us to better understand the changes we are seeing today."

The study is part of the doctorate project of Isabella (Survival oft the smallest? The long-term impact of climate change on non-commercial fishes in the Adriatic Sea; Supervision: Martin Zuschin), funded by the Austrian Academy of Sciences. Isabella is a doctorate candidate at the VDSEE (Vienna Doctoral School of Ecology and Evolution).