The presence of a lung in living coelacanths, deep-water fish once thought extinct, is confirmed in a paper published this week in Nature Communications. Although this lung is no longer thought to be functional, it provides clues about how its ancient relatives might have lived some 410 million years ago.
Coelacanths are large, lobe-finned fish that were believed to be extinct until a living species, Latimeria chalumnae, was discovered off the coast of South Africa in 1938, earning them the status of so-called ‘living fossils’. Compared to fossil species, Latimeria lacks a characteristic ‘calcified lung’, thought to be an adaptation to shallow water, and it is not currently known whether any remnants of these fossil species have been retained in the coelacanth’s present-day anatomy.
Paulo Brito and colleagues create 3D reconstructions of five developmental stages of the lung of the living coelacanth species Latimeria chalumnae using an imaging technique called X-ray tomography. They confirm that although this species possesses a well-developed, potentially functional lung in early-stage embyros, growth of the lung is slowed considerably at later embryonic, juvenile and adult stages, eventually becoming functionless (vestigial).
The authors also report the presence of small, flexible plates scattered around this vestigial lung in the adult Latimeria specimens and suggest they are comparable to the ‘calcified lung’ of fossil coelacanths. While these structures are no longer of use in living species that breath using gills, in fossil species these plates are thought to have played a role in regulating lung volume, eventually being lost as species became adapted to deep-water environments.