Fish Lungs

Coelacanths are celebrities in the animal kingdom for being a species that slipped through the cracks of history. Once thought to be extinct—for sixty-six million years, no less—they are now known to be very much alive, several specimens having popped up in fishing nets in the last century. The coelacanth has some fascinating relations: it belongs to the class of lobe-finned fishes, which are more closely related to the tetrapods (reptiles, amphibians, and mammals) than to any other extant fishes, and it has an interesting lobe-finned cousin, the lungfish, itself famous for being able to breathe air.

Take a rare fish with a bizarre lineage and you get a very excited ichthyologist! Biologists have known for centuries that coelacanth fossils had an unusual organ, sheathed in calcium. Without a living specimen to study, they were left to guess at its purpose—a swim bladder, perhaps? In 2010, someone speculated that this curious compartment could be a primitive lung. This makes sense when you consider that ancient coelacanths are among the closest relatives to today’s land animals. But what about today’s coelacanths? Do they have the same mystery organ? Camila Cupello, a zoologist at the Universidade do Estado do Rio de Janeiro, decided to search for evidence of this organ in modern fish, and the results are published today in Nature.

With careful dissection, Cupello and colleagues confirmed that this mystery organ exists in living specimens. Furthermore, it’s distinctly different from the swim bladders of other fishes!

By studying the coelacanth’s ontogenesis, or development from an embryo, the team could watch the lung grow along with the infant fish. The growth of the lung organ through embryonic stages, stopping when it becomes a juvenile, informed Cupello that the lung may have been useful in the past. The team notes that “These animals lived in shallow brackish, fresh or marine environments,” and in the less oxygenated oceans of the past, “air breathing might have been an essential respiratory need.”

Modern coelacanths, however, don’t need this organ, and they ditched it millions of years ago. “In the Mesozoic Era, adaptation of some coelacanths to deep marine water,” continue the authors, “may have triggered the total loss of pulmonary respiration, the marked reduction of the lung,” as an “adaptation to deeper environments.”

In considering the coelacanth’s family history, it’s no wonder that what is now a small vestigial organ turned out to be an ancient lung in the coelacanth’s ancestors. Although our paths diverged 390 million years ago, studying these animals gives us insight into the development of how we, as mammals, came to be. So take a deep breath—and thank the coelacanth for the path its ancestors helped explore for land-dwellers worldwide.

Image: Laurent Ballesta /www.andromede-ocean.com /www.blancpain-ocean-commitment.com