Fish physiology is a little more complicated than you learned in grade school. Some fish are warm-blooded—and some fish have lungs. 

A recent study in Nature Communications confirmed that a rare order of deep water fish called coelacanths have a lung, though the respiratory organ is no longer functional. The ancient species of fish, dating back about 410 million years, was long thought to be extinct until a living species was found near South Africa in 1938. 

Although scientists have known that coelacanth fossils show a large calcified organ, it was previously thought to be a bladder or some other type of tissue. But researchers, led by a zoologist from Rio de Janeiro State University, recently dissected several of these living fossils from the species Latimeria chalumnae, to confirm that the organ in question is a lung—at least in the living fish. Using X-ray imaging, the experts also reconstructed five different developmental stages of the fish, showing how the organ grows. 

Reconstructions of the fish lung at different developmental stages. Image Credit: Brito et al., Nature Communications (2015)

While early-stage embryos have a well-developed lung that might be functional, by the time the fish is fully grown, the organ isn't operating (the species breathes through its gills). In fact, as the fish becomes a juvenile and an adult, the lung ceases to grow and eventually becomes useless. 

The researchers also found small, hard, flexible plates surrounding the fish’s lung, which might be analogous to the calcified lung in the fossils. These hard plates might have once helped regulate lung volume millions of years ago when the fish lived in shallow, muddy waters, and potentially breathed air (compared to breathing through its gills, as it now does in deep-water environments).