An international team of scientists from the Geneva Museum of Natural History (MHNG) and the University of Geneva (UNIGE) reveals that certain coelacanths, fish that are 240 million years old, used their lung to capture sounds underwater.
This discovery, published in
Communications Biology, is based on synchrotron tomography images, a particularly intense form of X-ray radiation, and offers an unprecedented insight into the evolution of sensory systems in vertebrates.
Coelacanths have intrigued biologists since their rediscovery in the 20th century. These fish, now represented by two species of the genus
Latimeria, are closer to terrestrial vertebrates than to most other fish. While current species live in deep water and breathe solely through their gills, their ancestors, which lived approximately 240 million years ago, exhibited a greater diversity of forms and habitats. Some possessed a developed lung, covered with bony plates arranged like roof tiles. Until now, the existence of this organ was mainly interpreted as an adaptation to aerial breathing.
These anatomical remains constitute valuable testimony to the evolutionary history of these fish, and perhaps also that of our own aquatic ancestors.
To explore its other possible functions, a research team led by Lionel Cavin, curator at the MHNG and full professor in the Department of Genetics and Evolution in the Faculty of Science's Biology Section at UNIGE, analyzed fossils of Triassic coelacanths, discovered in Lorraine (France), using the European synchrotron (ESRF) in Grenoble. This particle accelerator made it possible to explore the internal structure of the fossils with micrometric precision.
3D rendering of the skeleton of Graulia branchiodonta. The auditory organ includes the bony wings (red) on the ossified lung (white) which transmitted sound vibrations to the inner ear (not shown) located in the prootic bone within the skull (pink).
© L.Manuelli-MHNG A novel auditory system revealed by imaging
The images revealed an exceptionally well-preserved ossified lung, equipped at its end with wing-shaped bony structures. In parallel, the study of embryos of present-day coelacanths highlighted a canal connecting the hearing and balance organs located on either side of the skull.
By combining these observations, the scientists suggest that these two structures formed a complete sensory system. Sound waves captured by the ossified lung would have been transmitted to the inner ears via this canal, thus allowing the animal to perceive sounds underwater.
"Our hypothesis is based on analogies with modern freshwater fish, such as carp or catfish. In these species, a device called the 'Weberian apparatus' connects the swim bladder to the inner ear. This device allows them to detect underwater waves and thus hear underwater. The air bubble enclosed in the swim bladder is essential for detecting these waves, which, otherwise, would pass through the fish's body undetected," explains Luigi Manuelli, a PhD student in Lionel Cavin's group and first author of the study.
An ability lost during evolution
For now, this anatomical peculiarity has only been observed in two species of Triassic coelacanths. However, it may have been more widespread among ancient coelacanths possessing an ossified lung. "This auditory ability was likely gradually lost when the ancestors of present-day coelacanths adapted to deep marine environments. Their lung then regressed, making this system useless," suggests Lionel Cavin.
Remarkably, however, some structures associated with the inner ear have been conserved. "These anatomical remains now constitute valuable testimony to the evolutionary history of these fish, and perhaps also that of our own aquatic ancestors," concludes the researcher.