It sounds like a children's game or an absurd question: Where is the starfish's head? In reality, the enigma has been occupying specialists for over a century, given the unusual structure of the animal and all its cousins in the echinoderm group (sea urchins, sea cucumbers and so forth). While all species from the Deuterostomia super-branch are axially symmetrical (right-left) with a so-called anteroposterior distribution (head to tail), the starfish and its relatives are nothing like this. All exhibit five-fold radial symmetry. As for the location of the head and tail, no one knew until now.

Researchers from American (Stanford, Berkeley and Columbia), British (Southampton) and Japanese (Okinawa) universities have just unraveled the mystery. "Basically, we can say that the starfish is just a head," summed up Laurent Formery, a French postdoctoral student at Stanford and lead author of the Nature article announcing this result. To establish this, the scientists relied on molecular studies. Where anatomy had hitherto failed, genes have made it possible to draw a conclusion.

Formery and his colleagues set out to map gene expression in the animal. Using two cutting-edge techniques – RNA tomography and in situ hybridization – they were able to track the activity of some of these genes and see how the traditional head/trunk/tail distribution observed in bilateral species was reflected in starfish. In fact, the same genes code for these different major body parts throughout living things.

Unexpected results emerged. The prevailing hypothesis, that each arm should reproduce the classic head-to-tail unfolding, was overturned. Instead, the researchers observed that the head genes were expressed along the entire length of the animal's arms. Head everywhere, but trunk and tail nowhere. In fact, they found none of the molecular markers associated with those body parts. But by following the genes coding for the different parts of the head, they found the equivalent of an anteroposterior axis, extending from the center to the periphery of each arm.

This discovery will enable us to be a little more equipped in diving into two black boxes that were previously hermetically sealed. Firstly, to try to understand how the animal can go from a classically bilateral larval stage to an adult stage with five-fold symmetry. Secondly, to determine its evolution. We know that our common ancestor, some 600 million years ago, was, like us, bilateral. Why and how did echinoderms branch out into this extraordinary form? Formery, who wrote his thesis on sea urchins, is now looking for answers in sea cucumbers.