Sea stars are among the world’s most bizarre marine creatures. They have no blood and no brains. They eat by vomiting their stomach out of their mouth and engulfing prey within it. They move around on thousands of tiny tube feet. And many have the extraordinary ability to regenerate lost body parts—some can even form an entirely new sea star from a single “arm” and part of the central disk.
The anatomy of these enigmatic animals has puzzled researchers for years. With no discernable head, scientists had suggested sea stars don’t have one. Now, new research published last week in the journal Nature indicates the opposite is true—that sea stars are all 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,” lead researcher Laurent Formery, a biologist from Stanford University and the University of California, Berkeley, says in a statement from Chan Zuckerberg Biohub, which funded the study. “It’s not at all what scientists have assumed about these animals.”
Sea stars, also known as starfish, are part of a group of animals called echinoderms, which includes sea urchins and sand dollars. These creatures are often characterized by a classic five-fold symmetry—meaning you could split them up into five identical sections. Humans and most other animals, on the other hand, have bilateral symmetry—meaning we can be separated down the middle into two equal parts. Yet, oddly, sea stars start their lives as larvae with bilateral symmetry, before changing into their radial adult form.
“This has been a zoological mystery for centuries,” senior author Christopher Lowe, a marine and developmental biologist at Stanford University, says in a statement. “How can you go from a bilateral body plan to a pentaradial plan, and how can you compare any part of the starfish to our own body plan?”
To answer these questions, scientists could look at a starfish’s DNA and take note of where genes that code for its head or trunk are expressed in the body. But traditional methods used to detect gene expression in other animals don’t work well in the tissue of young sea stars, per the statement. These traditional methods involve chopping genetic material into small pieces. But now, using new technology, the team was able to pull data from intact, gene-sized DNA strands and create a 3D map to pinpoint which genes are active at precise locations in a sea star.
The team compared the genetic expression of the sea star species Patiria miniata to a closely related species of acorn worm. They found that genetic expression corresponding to head development in acorn worms and other vertebrates was located all around the sea stars, especially at the center and in the middle of each “arm.” But genes associated with the trunk—or torso—in other animals were largely missing.
Because the ancestors of sea stars do have trunks, this suggests that somewhere along their evolutionary journey, sea stars lost these genes—though when that happened is still a mystery.
“It’s an elegant and compelling study, with important implications for the evolutionary history of sea stars and other echinoderms,” Imran Rahman, a paleontologist at the Natural History Museum in London who was not involved in the research, tells Dino Grandoni of the Washington Post. “Ultimately, this could enable us to uncover the sequence of evolutionary changes that gave rise to the head-like body plan.”
Next, the research team plans to look back in the fossil record to pinpoint exactly when sea stars lost their torsos, writes Scientific American’s Lori Youmshajekian. They also plan to see whether the “all head” pattern holds up for sea urchins and sea cucumbers, too.