Researchers at the University of California, Berkeley, have provided new insight into how the auditory system of freshwater fish evolved. Millions of years ago, some saltwater fish developed an extraordinary human-like hearing by adapting to life in fresh water.
According to RCO News Agency, By examining a 67-million-year-old fossil, researchers discovered that Otophysan fishes had developed Weber’s sensitive ear system before entering freshwater and in the oceans, not in rivers, as previously thought.
According to ScienceDaily, this discovery shows that two separate stages of the introduction of fish into freshwater occurred, and this could explain why there are so many different species of freshwater fish today.
Human-like ear in fishes
When some marine fish migrated to fresh water, they developed more complex hearing structures, including middle ear bones that resemble those of the human ear.
Today, about two-thirds of freshwater fish species use a special system called the Weberian apparatus, which allows them to hear high-frequency sounds in a range close to human hearing, up to about 15,000 Hz. This group includes more than 10,000 species, including catfish and zebra fish.
Rewriting the evolutionary history of freshwater fish
It was previously thought that autophyses entered freshwater about 180 million years ago, before the breakup of the supercontinent Pangea. But the new research of Dr. Juan Liu shows that this happened about 154 million years ago in the late Jurassic period, when the continents were separated and the modern oceans were forming.
Based on fossils and genetic data, the bones responsible for advanced hearing first evolved during marine life. Then, two separate branches of this lineage migrated to fresh water: one led to the formation of catfish, sawfish, and the other is the origin of today’s carp, zebrafish, and river fish.
“Our analysis showed that the common ancestor of these fishes was marine and returned to freshwater in at least two separate stages,” says Liu. These frequent introductions probably caused the stunning diversity of species in today’s freshwaters.
A small fossil with big information
The new fossil named Acronichthys maccagnoi was discovered in Alberta, Canada and is only five centimeters long. This fossil is the oldest specimen of an autophysean fish in North America and dates back to about 67 million years ago, the late Cretaceous period, just before the extinction of the dinosaurs.
Using 3D scans and computer modeling, Liu’s group showed that this ancient fish had nearly the same hearing as zebrafish, with slightly less sensitivity, but a frequency range closer to 500 to 1,000 Hz.
A fresh look at evolution and diversity
The results of this research show that frequent entry into new habitats, along with innovations such as more sensitive hearing, can cause jumps in species diversity.
“For a long time, we thought that autophyses were of freshwater origin, because most of them live in freshwater,” said co-author Dr. Michael Newberry. But now we know that their origin was marine and this evolutionary picture is new, more logical and complete.
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