How did ancient fish perceive their environment in the deep-sea? An international team led by scientists from the Natural History Museum of Geneva (MHNG) and the University of Geneva (UNIGE) reveals that some coelacanths – fish living 240 million years ago – used their lung to detect sounds underwater. These findings, published in the journal Communications Biology, were obtained using synchrotron imaging, an especially powerful X-ray technique. They shed new light on the evolution of sensory systems in vertebrates.
Coelacanths have fascinated biologists since their rediscovery in the 20th century. These fish, now represented by two species of the genus Latimeria, are more closely related to terrestrial vertebrates than to other fishes. While modern species live at great depths and breathe exclusively through gills, their ancestors from around 240 million years ago displayed a much wider diversity of forms and habitats. Some had a well-developed lung covered with bony plates arranged like roof tiles. Until now, this organ has mainly been interpreted as an adaptation for air breathing.
To explore its potential additional functions, a research team led by Lionel Cavin, curator at the Natural History Museum of Geneva and adjunct professor in the Department of Genetics and Evolution at the Faculty of Science of the University of Geneva, analyzed Triassic coelacanth fossils discovered in Lorraine (France). The fossils were examined using the European Synchrotron Radiation Facility (ESRF) in Grenoble. This particle accelerator made it possible to investigate the internal structure of the fossils with micrometric precision.
A new auditory system revealed by imaging
The images revealed an exceptionally well-preserved ossified lung, featuring wing-like bony structures at its extremity. At the same time, the study of embryos of modern coelacanths highlighted a canal connecting the organs of hearing and balance 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, enabling 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 structure known as the Weberian apparatus connects the swim bladder to the inner ear. This system allows them to detect underwater waves and therefore hear underwater. The air bubble contained in the swim bladder is essential for detecting these waves, which would otherwise pass through the fish’s body undetected,” explains Luigi Manuelli, a doctoral student in Lionel Cavin’s group and first author of the study.
A capacity lost over the course of evolution
For now, this anatomical feature 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 as the ancestors of modern coelacanths adapted to deep marine environments. Their lung regressed, making this system unnecessary,” suggests Lionel Cavin.
Remarkably, some structures associated with the inner ear have nonetheless been preserved. “These anatomical remnants now provide valuable insight into the evolutionary history of these fish – and perhaps also into that of our own aquatic ancestors,” the researcher concludes.