The coelacanth uses its gills to extract oxygen from water just like all other fishes that exist today but millions of years ago prior to the age of the dinosaur, the ancestors of this so-called "living fossil" breathed using their lungs.
Findings of a new study revealed that the species, which was once thought to be extinct, had an obsolete lung in its abdomen.
Researchers of the study published in the journal Nature Communications on Sept. 15 said that the organ may have helped the fish survive in shallow waters with low oxygen level millions of years ago.
Just like the human appendix, however, the coelacanth's lung was rendered defunct by evolution as the fish moved to live in deep water.
It is believed that some species of the coelacanth moved into deeper water during the Mesozoic era and eventually stopped using their lungs as they started to rely solely on gills to breathe.
"By the Mesozoic Era, adaptation of some coelacanths to deep marine water, an environment with very low variations of oxygen pressure, may have triggered the total loss of pulmonary respiration," said Paulo Brito, from Rio de Janeiro State University in Brazil.
The researchers said that the adaptation of some species of the coelacanth to deep water likely contributed to their survival when an asteroid stuck the Earth and killed the dinosaurs. The other species that relied on gills and lungs were not as lucky.
During the Late Cretaceous period, the coelacanths that live in shallow waters disappeared from the fossil record.
Brito and colleagues examined the lungs of the Latimeria chalumnae, a surviving species of coelacanth, at five stages of development using X-ray tomography, which allowed them to take several X-rays of the organ and come up with 3D reconstructions.
The researchers discovered that the dysfunctional lung of the fish is proportionally much larger in the embryo compared with that in the adult, which means that the growth of the lungs slows down as the marine animal gets older.
"Our results demonstrate the presence of a potentially functional, well-developed lung in the earliest known coelacanth embryo, and its arrested growth at later ontogenetic stages, when the lung is clearly vestigial," the researchers wrote.
Small and flexible plates that were scattered around the vestigial lung were also found in the adult specimens, and indication that the organ is comparable to the calcified lung of the fossil coelacanth.
Photo: Todd Huffman | Flickr
