Scientists finally solved the centuries-long mystery of where a starfish's head is

• For centuries, scientists have wondered: Where is the head of a starfish?
• Researchers mapped starfish genes to solve the mystery, and it wasn't what they expected.

In a game of pin the tail on the donkey, scientists have finally pinpointed the head on a starfish. Or more accurately — the heads on starfish.

Turns out starfish, aka sea stars, don't just have one head sitting at the center of their bodies. These unusual creatures instead have head-like regions in each of their limbs, according to a paper published in Nature.

The discovery has been centuries in the making. It's a mystery that "dates from the very beginning of zoology," Laurent Formery, lead author of the study, told Insider.

Why it took centuries to identify a starfish head

Formery, who is a postdoc at Stanford School of Humanities and Sciences, said it took so long to solve this mystery because, "I think it's really because they are so different, right?" he said, referring to sea stars.

Many animals like insects, reptiles, birds, and mammals — including humans — have what's called bilateral symmetry, which means they're symmetric along a midline.

This helps scientists identify the location of the head, trunk, limbs, and so on, Formery said.

On the other hand, sea stars with five limbs, lack bilateral symmetry and instead have what's called five-fold radial symmetry.

If you look at a sea star "there is nothing that looks like your head. You just have no idea," Formery said.

Despite appearances, however, sea stars actually have more genetically in common with animals like humans than meets the eye, according to Stanford News.

Mapping a starfish's genetic code

Animals have a genetic code that unfurls and activates different regions as we develop — sticking a leg here, an arm there, and a head over there.

It's those genetic similarities that Formery and his colleagues studied. But they couldn't have done it without the help of modern technology, Formery told Insider.

"For a very long time, we just had access to anatomy and morphology. And they're not very helpful when you look at a sea star," Formery told Insider. "And recently, we can kind of ignore the morphology and just focus on the molecular aspects of development."

The researchers analyzed the different genes within individual starfish limbs. Initially, they were expecting to find some genes that resembled a head and some that resembled a trunk — similar to most animals. But, instead, they found mostly head-like genes.

The genes typically associated with the trunk of an animal were absent in starfish, said co-author Jeff Thompson, a lecturer at the University of Southampton, in a statement, adding that the whole starfish "body plan is roughly equivalent to the head in other groups of animals."

The discovery was "very weird," and unexpected, Formery added. "Nobody really had [this] envisioned before we had access to this data."

Do starfish have a brain?

Looking ahead, Formery said that he's interested in analyzing the starfish more to determine whether its multiple heads may also contain multiple brains.

"When you look at textbooks, they say sea stars have a very simple nervous system, and it's like they don't have a brain," Formery said.

But Formery said that what he and his colleague found was that some of the genes responsible for developing a sea star's nervous system are also "the genes that are involved during our own development to make the brain."

So if starfish have similar genes to the ones that help brain development in humans, that raises some very interesting questions, he said.

"Questions about what is the nervous system of a sea star? And is it a brain? Is it just a brain basically?" Formery asked.