Coelacanths are giant fish with large, fleshy fins, and they were thought to have gone extinct millions of years ago until they were discovered swimming in deep waters off the coast of South Africa in the Indian Ocean in 1938. These so-called “living fossils” breathe using gills, but according to a new Nature Communications study, today’s coelacanths also have a well-developed lung. While this lung is likely not functional, it hints at how ancient lobe-finned fish lived 410 million years ago. 

With fleshier fins than, say, salmon or trout, coelacanths resemble the fish that made the transition onto land around 360-390 million years ago, which became the ancestor of all tetrapods (four-limbed vertebrates like us). Nowadays, you can find these rare, two-meter-long giants living in rocky habitats between 110 and 400 meters deep in the coastal waters of the Mozambique Channel and Sulawesi, Indonesia. Unlike fossil coelacanths, living species lack a lung that’s calcified, or sheathed in bony plates – these were thought to be an adaptation to shallow water. As coelacanths lost their lung over time there was a parallel development of a unique adaptation to deep water environments: a fatty organ employed as a buoyancy control. 

Researchers didn’t know if any remnants of the lung had been retained in modern coelacanth anatomy. So to investigate, a team led by Paulo Brito from the Universidade do Estado do Rio de Janeiro used x-ray tomography to create 3D reconstructions of five developmental stages (from embryo to adult) of a living coelacanth species called Latimeria chalumnae (pictured below).

The researchers found a potentially functional, well-developed lung in the earliest embryo stage. However, lung growth is dramatically slowed during later embryonic, juvenile, and adult stages – eventually becoming functionless, or vestigial. “The lung now has no function at all,” Brito told the Los Angeles Times, “just like our appendix.” 

Furthermore, with dissections, 3D reconstructions, and tissue studies, the researchers also found small, flexible plates scattered around the vestigial lung in adults. These structures might be similar to the calcified lung of extinct coelacanths, helping them regulate lung volume. They were eventually lost as the fish became adapted to deep-water environments. This adaptation may have helped some coelacanths survive when the (non-avian) dinosaurs and shallow-water coelacanths didn’t. They were safer from the post-asteroid environmental crisis. 

^{Three-dimensional reconstructions of the pulmonary complex of L. chalumnae at different ontogenetic stages. Yellow, esophagus and stomach; green, fatty organ; red, lung. Brito et al. Nature Communications}