Where exactly is the head of a starfish? Zoologists, who have been puzzled for centuries, may finally have an answer.
According to a new study in the journal Nature, starfish don’t have a separate head. They are all head, and their “arms?” They’re just more heads too. It turns out, a starfish hardly has a body at all.
“It’s like the sea star is missing a middle section and is mostly just a head moving along the sea floor,” said Laurent Formery, a biologist at the University of Stanford and the study’s lead author. “It’s quite different from what scientists used to think about these creatures.”
The research might help scientists better understand creatures called echinoderms, such as sea cucumbers and sea urchins. These animals have strange bodies and their evolution has puzzled scientists for a long time.
Starfish and other echinoderms have a type of symmetry called radial symmetry, which means they look the same around a central axis, like a flower. This is unusual because most animals have bilateral symmetry, where one side mirrors the other, making it hard to understand how starfish’s bodies work.
What’s even weirder is that starfish aren’t born with this shape. They start as fertilized eggs, then become free-floating larvae, like plankton. After a few weeks, they sink to the ocean floor and change from having bilateral symmetry to having five points.
“This has been a mystery for zoologists for a long time,” said study co-author Christopher Lowe, a biologist at Stanford University, in a press release. “How can an animal change from having two-sided symmetry to having five-sided symmetry, and how can we compare starfish to animals like us?”
Although starfish may seem very different from us, genetics can reveal surprising similarities if you know where to look.
The researchers used a method called RNA tomography to make a 3D map showing how genes were working as the starfish grew. They focused on genes that control the starfish’s outer layer, which includes its nervous system and skin, and marked them with special dyes.
They found that genes linked to head development were active all over the starfish, even in its “arms.” But genes responsible for making a trunk or body were hardly active at all.
“These findings suggest that starfish, and echinoderms in general, have a very unique way of separating their head from their body compared to other animals,” said Formery.
This discovery raises a lot of new questions about how echinoderms evolved. Their fossil record shows that their ancestors had a body, so why do starfish look so different now?
