Overlooked fish fin stirs the waters of evolution

A few hundred million years ago, fish fins morphed into the arms and legs of terrestrial animals, according to evolutionary theory. So, you’d think science would know just about everything about them by now.

It turns out that some fins are more important than others. The tiny adipose fin, a fatty nub between the dorsal fin and tail of some 6,000 species of fish, has been the ugly stepchild of anatomy. Most scientists shrugged and dismissed it as a withered vestige of a second dorsal fin present in a common ancestor, but jettisoned by its progeny as they radiated into new species.

But a new study published online in the journal Proceedings of the Royal Society B finds these fins arose independently at least three times, in a remarkable example of convergent evolution that suggests the fin has an important adaptive function that science has all but ignored.

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Thomas A. Stewart at the University of Chicago has anchored his nascent research career on that tiny fin. The PhD candidate in organismal biology and anatomy, and lead author of the study, poured cold water on the current hypothesis about the fin’s origin by building an ancestry model from 620 species of ray-finned fish, or teleosts, gathered from online libraries and the collection of Chicago’s Field Museum.

“It was thought that they didn’t serve any function at all because they’re small and they don’t have much by way of muscles or skeleton,” Stewart said. “For that reason, it was assumed they didn’t do anything and we never tried to figure out what they might be doing.”

What the fins do that may help a fish adapt remains uncertain, and likely will fall to biomechanics experts. Stewart suggests they might mediate turbulence behind the dorsal fin, or serve as some kind of sensory organ for the tail.

“I think now we know that it is not just a modified version of a second dorsal fin, like you see in sharks or Coelacanths, that is assumed to be primitive to jawed vertebrates,” Stewart said. “We know that that’s not the case now.”

It’s not uncommon for vertebrates to lose appendages. Eels, for instance, lost the rear fins on their underside. And a fossil of a 90-million-year old snake, found in Patagonia, suggests serpents had rear appendages when they slithered out of the water.

“Things are lost all the time, but the fact that it is maintained is suggestive of a use in these animals,” Stewart said. “A lot of times when fish develop to become predators, the dorsal fin will slide backward, so it’s right by the tail, and then the adipose fin disappears.”

University of Chicago biologist Michael I. Coates, who oversees Stewart’s work, said the multiple emergence of the adipose fin “breaks the constraint” on the body plan of the gnathostomata, or jawed vertebrates, which includes fish, amphibians, reptiles, birds and mammals.

“It’s rare that we can see an example of a vertebrate where it’s throwing out a new structure,” Coates said. “It’s unique to that group; it breaks a constraint, and that makes it interesting because it gives us an opportunity to think about how many ways there are of doing this, of generating a new kind of appendage in a new position.”

That leaves Stewart with a lot of big questions about a small fin. “It’s always exciting to investigate, but it’s also daunting to see how much there is to do,” Stewart said. “The genetics isn’t touched, the biomechanics isn’t touched.”