Long presumed to have no heads at all, sea stars may be nothing but

Seeing sea stars described as "just a head crawling along the seafloor" has given me a new perspective on these animals.

A summary:

For centuries, naturalists have puzzled over what might constitute the head of a sea star, commonly called a “starfish.”

The research, published Nov. 1 in Nature, suggests that, far from being headless, over evolutionary time sea stars lost their bodies to become only heads.

The researchers found that neither of the prominent hypotheses of sea star body plan structure was correct. Instead, they saw that gene expression corresponding to the forebrain in humans and other bilaterally symmetrical animals was located along the midline of sea stars’ arms, with genetic expression corresponding to that of the human midbrain towards the arms’ outer edges. While the genes marking different subregions of the head in humans and other bilaterians were expressed in the sea star, only one of the genes typically associated with the trunk in animals was expressed, at the very edges of the sea stars’ arms.

“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, a Biohub-funded postdoctoral scholar and lead author of the new study. “It’s not at all what scientists have assumed about these animals.”