Freshwater fish learned to hear millions of years before rivers formed

Fish don’t have ears that stick out of their heads like we do. But many of them hear just fine – especially freshwater fish like catfish, minnows, and tetras.

What’s even more surprising is that they might actually hear better than most saltwater fish. Scientists now think they know how that happened – and it’s not the story they once believed.

For a long time, researchers believed that a certain group of freshwater fish developed their super-hearing abilities after settling into rivers and lakes when the ancient supercontinent Pangea was still in one piece.

But new fossil evidence just flipped that theory on its head. Turns out, these fish started evolving their fancy ear bones before they even left the ocean.

Most fish have poor hearing

The majority of fish species can’t hear very well. That’s partly because sound travels through water and fish bodies at about the same speed.

That makes it hard for sound waves to vibrate the parts of a fish’s inner ear enough to send signals to the brain.

But certain fish developed a clever workaround. Some species have an internal, air-filled bladder that helps them stay afloat. In a few of them, this bladder also acts as a sound catcher.

It picks up vibrations from sounds in the water and passes them along. However, in most saltwater fish, this connection is fairly simple – it only lets them hear deep, low-pitched sounds, nothing above about 200 hertz.

Then there are the otophysan fish. They took this system and seriously upgraded it. By adding a small chain of bones – called ossicles – between the air bladder and the inner ear, these fish can hear sounds up to 15,000 hertz. That’s close to the 20,000-hertz that is the upper limit of human hearing.

This specialized system is called the Weberian apparatus. About two-thirds of all freshwater fish species today hear using this system.

Rewriting the evolution of fish hearing

Until recently, scientists believed that otophysan fish evolved their Weberian hearing system only after moving into freshwater environments, more than 180 million years ago.

But new research led by paleontologist Juan Liu changes that timeline. The discovery of a tiny fossil fish – just two inches (5 centimeters) long – named Acronichthys maccagnoi tells a different story. This fish lived about 67 million years ago in what is now Alberta, Canada.

Thanks to exceptionally well-preserved fossils, researchers could clearly see its ear bones.

Liu and her team used 3D X-ray scans from the Canadian Light Source and McGill University to model the fish’s middle ear structure. They then ran simulations to test how well those bones might have picked up sounds.

“It turns out the simulation worked,” said Liu, an assistant adjunct professor of integrative biology and an assistant curator in the UC Museum of Paleontology.

“The Weberian apparatus has just a little bit lower output power, which means lower sensitivity, compared to a zebrafish.”

The fossil’s hearing peak ranged between 500 and 1,000 hertz, similar to zebrafish, suggesting early otophysans already detected sounds at high frequencies.

Two paths to freshwater life

Liu’s analysis of fossil and genetic data suggests that the ancestors of these freshwater fish actually began evolving the pieces of their hearing system while they were still living in the ocean.

Later, as landmasses split apart and formed today’s continents, two different branches of these fish moved into freshwater environments.

One group eventually gave rise to catfish, knifefish, and South American tetras, while the other led to carps, minnows, and zebrafish.

“For a long time, the consensus was that these bony fish had a single freshwater origin on the supercontinent Pangea and later dispersed as the continents separated,” Liu said.

A burst of new species

The team’s analysis of multiple fossils revealed unexpected results. The most recent common ancestor of otophysan fish came from a marine lineage – with at least two separate transitions into freshwater after that lineage diverged.

According to the researchers, each move into a new environment may have opened the door to a burst of new species.

“These repeated incursions into freshwater during the early divergence stage likely accelerated speciation and are key to explaining the extraordinary hyper-diversity of otophysans in modern freshwater faunas,” Liu said.

A ripple through evolution

“For a long time, we presumed that the Otophysi probably had a freshwater origin because this group consists almost entirely of freshwater fishes,” said ichthyologist Michael Newbrey, a co-author of the study.

“The new species provides crucial information for reinterpreting the evolutionary pathways of the Otophysi with a marine origin. It just makes so much more sense.”

These findings not only push the timeline forward – they also offer a different view of how evolutionary innovation can unfold.

Liu believes the discovery highlights a pattern seen across many species: repeated entries into new habitats, combined with useful new traits, often lead to rapid bursts of biodiversity.

In this case, fish took a curious little setup of bones and a bubble of air – and turned it into a tool for hearing the world in a whole new way.

The full study was published in the journal Science.

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