The adipose fin, which sits between the dorsal fin and the tail on many fishes, might have evolved separately in different fish lineages rather than once from a single ancestor. This suggests that the fin (pictured with arrow) has an adaptive purpose and can evolve into various forms, contrary to previous thinking.

Credit: Ammit/Alamy

Thomas Stewart at the University of Chicago, Illinois, and his colleagues reconstructed the evolutionary relationships of 232 fishes, looking at the presence and absence of adipose fins. They also studied the skeletons of 620 fish species from 55 families. The team concludes that these fins have a wide variety of skeletal structures and have repeatedly evolved some of the same features, such as fin rays — rods of bone or cartilage that support the fin membrane.

Although the purpose of these fins is not clear, they could be a powerful tool for studying vertebrate limb evolution, the authors suggest.

Proc. R. Soc. B 281, 20133120 (2014)