Researchers believe a starfish body is just one giant head (Credit: Nick Hobgood/ CC-BY-SA-3.0/

As far as anatomy goes, there have been few animals as puzzling to researchers as the starfish. Most animals have easily identifiable heads, bodies, and legs. However, starfish bodies are made up of five identical arms with a layer of "tube feet," to help them propel forward. The lack of a discernable head led some to even suggest they did not have one. But, a new study asserts that the opposite is true. The sea star's body is just one big head and no tail!

"It's as if the sea star is completely missing a trunk, and is best described as just a head crawling along the seafloor,” said Laurent Formery, lead author of the new study. “It’s not at all what scientists have assumed about these animals.”

Starfish belong to a group of animals called echinoderms. They include sea cucumbers, sand dollars, and sea urchins. Echinoderms have bodies arranged in five equal sections. In contrast, humans and most other animals have bilateral symmetry. This means they can be separated down the middle into two equal parts.

For their study, published in Nature on November 1, 2023, the researchers used a new technology called HIFI sequencing. It allowed them to create a 3D map to determine which genes are active at precise locations in a sea star. To their surprise, they found the gene expressions corresponding to a head all over the sea stars. This was especially true in the center of the sea star's body and the center of each arm. However, there were no signs of genes associated with the trunk or torso.

Starfish use tube feet to move (Credit: Rpillon/ CC-BY-SA 3.0/

"These results suggest that the echinoderms, and sea stars in particular, have the most dramatic example of decoupling of the head and the trunk regions that we are aware of today," said Formery.

The researcher added that ancient fossilized starfish appeared to have had trunks. This suggests that they eventually evolved to lose these genes. The team next plans to find out exactly when the change occurred. They also want to investigate if other echinoderms, like sea urchins and sea cucumbers, are "all heads," too.