Two American teams have just revealed the details and physiological and molecular origins of one of nature's incongruities: The six legs of the Triglidae, better known by their common name of gurnards. In the same issue of Current Biology, published on September 26, they have published two articles that are as rich as they are astonishing.

Nicholas Bellono, from Harvard University, and his colleague David Kingsley, from Stanford, had no intention of studying this family of fish. The former was collecting squid from the Woods Hole Aquarium in Massachusetts for his experiments. The latter was simply visiting. But aquarium manager Scott Bennett enjoys showing off his strangest residents to his guests. "He told us that they are so good at detecting and discovering prey buried on the seabed that other fish follow them," said Bellono. Intrigued, the researcher took off with a few specimens to test their abilities.

In the Harvard laboratory, Prionotus carolinus, the northern sea robin, performed wonders. It walked and dug up small prey buried by researchers, and it also detected mussel extracts or simple molecules in capsules placed under the sand. The team then launched experiments to try and understand the cellular and molecular basis of this particularity but without success. Bellono persisted, ordering new fish. This time, the newcomers didn't even dig. "We thought we'd done something wrong, but it turned out to be another species of gurnard," he said.

They turned an accident into a windfall. Now the team could compare. They discovered that while the two species had the same legs, they didn't have the same feet. One was covered with sensitive papillae, the other was devoid of them. Drawing on the same thread, the Harvard sleuths revealed sensory neurons and the epithelial cells of taste. In other words, the chemical perception involved in P. carolinus was absent in its non-burrowing cousin, P. evolans. A comparative study of gurnard species around the world has showed that only a few species dig and that the associated chemical perception is only a recent evolution.

Meanwhile, at Stanford, Kingsley delved into genetics to study the development of the famous legs. It was already known that they had evolved from the two pectoral fins, from which three rays detached, gained strength and developed muscles and an independent nervous system, finally becoming fully-fledged limbs. But the genes and proteins involved remained totally unknown.

The team deciphered the genome and, thanks to various experiments involving the creation of hybrids, was able to highlight the molecular processes at work. A protein called tbx3a plays an essential role in the formation of legs and papillae. As it happened, the associated tbx3a gene has been conserved throughout evolution in a large number of species, from fish to mammals, including humans.

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