A fish species found in the Indian Ocean has a vestigial lung, suggesting that its ancestors had working lungs before they shifted to life in deep waters.

The coelacanth fish Latimeria chalumnae is descended from ancient coelacanths that lived in shallow waters. Paulo Brito at Rio de Janeiro State University in Brazil and his colleagues studied the fish at different stages of development, and found that a lung developed early in the embryo but then slowed its growth as the embryo matured. As the lung shrank in size relative to the growing embryo, a fatty organ that helps fish to control their buoyancy developed.

This is further evidence that ancestral coelacanths could breathe air in shallow waters, and that they lost the use of the lung as it was replaced by the fatty organ — an important adaptation to the deep ocean.

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