New research is shedding light on the complex anatomy of the coelacanth, an ancient fish often called a “living fossil,” revealing that its lung wasn’t just for respiration, but also likely played a role in hearing. A study published in in Nature, combined with decades of anatomical investigation, suggests that the lungs of extinct coelacanth species were capable of transmitting sound pressure to the inner ear, effectively functioning as an auditory organ alongside its respiratory role.
The coelacanth, a lobe-finned fish, has captivated scientists since its rediscovery in . Its anatomy provides a window into the evolutionary transition of vertebrates from water to land. While the extant (living) coelacanth, Latimeria chalumnae, possesses a vestigial lung – meaning it’s present but non-functional for breathing – its fossil relatives had much larger, more developed lungs. The purpose of these large, bony chambers in fossil coelacanths has been a long-standing mystery. Previous hypotheses centered on either respiratory function or a role in buoyancy control.
The recent research, utilizing synchrotron phase-contrast microCT scans of 240-million-year-old latimerioid coelacanth fossils, provides compelling evidence for the dual function. Synchrotron imaging, a high-resolution X-ray technique, allowed researchers to visualize the internal structures of the fossils in unprecedented detail. These scans, alongside developmental studies of the modern L. Chalumnae and reconstructions of a Devonian coelacanth, revealed a connection between the lung and the inner ear via a perilymphatic system – the fluid-filled space within the inner ear responsible for transmitting sound vibrations.
“We propose that the lung of extinct coelacanths supported both respiratory and auditory functions,” the researchers state. The ossified (bony) structure of the ancient coelacanth lung appears to have been specifically adapted to transmit sound pressure waves. This suggests that these fish may have been able to detect underwater vibrations more effectively, potentially aiding in prey detection or predator avoidance. The research builds on earlier work establishing the homology – shared ancestry – between the bony plates within the fossil lung and smaller plates surrounding the lung in the extant species.
The discovery isn’t entirely surprising given what’s known about the evolution of hearing in other fish. Many fish species utilize various mechanisms to detect sound, including the swim bladder, an air-filled sac used for buoyancy control, which can also amplify sound vibrations. The coelacanth lung, represents an early evolutionary step towards more sophisticated auditory systems.
Camila Cupello, a researcher involved in the study of coelacanth lung anatomy, has extensively documented the allometric growth of the lung in the extant species. Allometry refers to changes in body proportions during growth. Her work, published in in R Soc Open Sci, demonstrates that the vestigial lung in modern coelacanths is a result of adaptation to deep-water environments. As coelacanths transitioned to deeper waters, the need for air breathing diminished, leading to a reduction in lung size and function. However, the underlying anatomical structures remained, hinting at their importance in the fish’s evolutionary history.
Further supporting this connection between lung structure and hearing is research on the inner ear of the coelacanth. Studies have shown similarities between the coelacanth inner ear and those of tetrapods (four-limbed vertebrates), suggesting a shared evolutionary heritage. The perilymphatic system, crucial for sound transmission, is a key feature of both coelacanth and tetrapod inner ears.
The findings also have implications for understanding the evolution of air breathing in vertebrates. The lung, initially evolving for respiration, may have been “co-opted” for auditory purposes in early fish. This highlights the plasticity of biological structures and how they can be repurposed over evolutionary time. The research aligns with broader theories about the water-to-land transition, suggesting that the development of air-breathing organs may have been linked to the emergence of terrestrial hearing.
Researchers are continuing to investigate the coelacanth’s anatomy, utilizing advanced imaging techniques and comparative studies with other fish species. Detailed reconstructions of Mesozoic coelacanths, like Graulia branchiodonta, are providing further insights into the evolution of these fascinating creatures. The availability of high-resolution data from synchrotron scans, accessible through the ESRF Paleontology Database, is facilitating collaborative research and accelerating our understanding of coelacanth evolution. The ongoing work promises to reveal even more about the evolutionary history of this “living fossil” and its place in the tree of life.
