The starfish’s body is a single head – France Culture

You know, I imagine, the famous saying: “No guns, no chocolate.” Well, the starfish, for their part, will soon have to do without it: the latter’s five branches would not be arms, but extensions of their heads.

To reach this conclusion, a team of biologists from Stanford University and UC Berkeley tried to answer a question: But why do these sea creatures have this star shape?

An unusual shape

Now for comparison: Humans and, in a broader sense, vertebrates have a so-called bilateral symmetry, which means they have two sides: we have a stomach side and a back side, a head and a torso. The anatomy of starfish is much rarer; it is based on five-axis symmetry, also called “pentaradium”.

Scientists therefore tried to understand how the starfish ancestor, which had bilateral symmetry, transformed into an animal with five identical parts. This is what Laurent Formery, researcher in the field of molecular biology and co-author of this study published in the journal Nature, explains:

“Echinoderms, this group of animals that includes sea urchins and starfish, you should know that they are close to us from an evolutionary point of view and at the same time extremely different. And that’s really a problem.” has fascinated zoologists for decades, even centuries. Where does this five-axis symmetry come from that is so original and unique in animals? You should know that what we really understand today about molecular biology and developmental biology comes from the study of We have discovered and found a small group of species, so-called model species, which include in particular the mouse, the zebrafish and the vinegar fly, that the molecular mechanisms during embryonic development are actually extremely similar in all of these animals. This really begs the question of how, over the course of evolution, could we have animal forms as diverse as what we see today? And finally we said to ourselves that the answer to this problem is to find out which animals are the strangest. And we said, Starfish, that could be a good example. And we’ll look at what happens to these mechanisms of embryonic development when we look at the development of an animal that’s completely different from the animals we might be used to working with… Starfish are really interesting because of their evolutionary origins From our point of view, they are close to us and at the same time completely different. So it really begs the question: What happened during evolution to get here?

To understand how a starfish’s branches form, scientists examined the genes that determine the anatomy of these animals. They sorted the front active genes, which indicate the head, and the rear genes, which indicate the trunk.

No stem-related genes

According to Laurent Formery, the result of this genetic mapping greatly surprised the authors of the study: there are no genes linked to the starfish phylum: “We can really only establish a connection between the five-axis symmetries of the starfish by looking at the gene level starfish and the bilateral symmetry of other animals. We have not found any territories in the starfish that correspond to what we call the hull. So there is actually no rear territory in starfish. All that’s left is the front part, which we could simplify by saying it’s just the head.

So starfish are actually a single head with five projections. And this discovery already raises several questions.

At first we tend to imagine that a starfish moves with its “arms.” In reality, so-called podia are used, small soft tubes that can be retracted. They are located under its branches and are unique to this species. Since in vertebrates the trunk is associated with movement, we can therefore ask whether the starfish would not have just evolved pods to compensate for the lack of a trunk?

Ultimately, scientists believed that the bands of neurons in the starfish’s arms were a relatively simple system. However, since these are actually their heads, this nervous system may well share a common evolutionary origin with our own brains.