The study reveals that starfish are actually “crawling heads under the sea.”

(CNN) — Most animal heads are easily recognizable, but until now scientists couldn’t tell about starfish.

A starfish has five similar arms, with a layer of “tube feet” underneath that helps the marine creature move along the ocean floor. This has naturalists wondering if starfish have defined front and back ends… and if they even have heads.

But new genetic research suggests the opposite: starfish are mostly heads without bodies or tails, and they may have evolved to lose those features over time. Strange fossils of starfish ancestors that appeared with some kind of torso make a lot of evolutionary sense, according to new findings, researchers say.

The results were published in the journal Wednesday Nature.

“The starfish seems to be missing its trunk entirely, and it’s like dragging a simple head across the seafloor,” study lead author Laurent Fourry, a postdoctoral fellow at the University of Massachusetts, said in a statement. and the University of California at Berkeley. “There is nothing that scientists have assumed about these animals.”

Discoveries made possible by new genome sequencing methods will help answer some key questions that remain about echinoderms, including their shared ancestry with humans and other animals.

Unique body structure

Starfish belong to the group of echinoderms, which includes sea urchins, sand dollars (or sea biscuits), and sea cucumbers. These unusual animals have a single body, arranged in five equal segments, which greatly differs from the symmetrical bodies of bilateral animals, whose left and right sides mirror each other.

Starfish begin as fertilized eggs that hatch into floating larvae that take weeks or months to settle on the ocean floor. There they go through a process that transforms a dihedral body into a star-shaped or pentaradial body.

“This has been a zoological mystery for centuries,” study co-lead author Christopher Lowe, a marine and developmental biologist at Stanford University, said in a statement. “How can we go from bilateral to a pentaradial body plan, and how can we compare any part of a starfish to our own body plan?”

The bilateral body structure that most animals have comes from genetic activity at the molecular level that can be traced to the head and trunk, or main body, regions, which is why many vertebrates, including vertebrates such as humans and insects, share it. Genetic programming. This finding was presented Nobel Prize in Physiology or Medicine In 1995.

However, echinoderms share a common ancestor with amphipods, adding to the puzzle that researchers are trying to solve.

“How the different body parts of echinoderms are related to the body parts we see in other groups of animals has been a mystery to scientists since our study,” said Dr Jeff Thomson, study co-author and professor at the University of Southampton. “In their bilaterian relatives, the body is divided into a head, trunk, and tail. But by looking at a starfish, you can’t see how these segments relate to the bodies of bilaterian animals.”

Cracking the echinoderm code

In the new study, researchers used micro-computed tomography to obtain an unprecedented three-dimensional view of the starfish’s shape and structure.

Next, team members used advanced analysis techniques to determine where genes are expressed within tissues and to detect specific RNA sequences within cells. Gene expression occurs when the information contained in a gene is acted upon.

Specific molecular markers act as blueprints for the body, directing each cell to the part of the body it belongs to.

“If you peel back the skin of an animal and look at the genes that define the head of the tail, the same genes encode these body parts in all groups of animals,” Lowe explains. “So we ignore the anatomy and ask: Is there a molecular axis hidden beneath all this strange anatomy and what is its role in creating a pentaradial body structure in a starfish?”

Together, the data created a 3D map to determine where genes are expressed as the starfish develops and grows. The team was able to determine the genes that control the development of the starfish’s ectoderm, which includes the skin and nervous system.

Genetic signatures associated with head development were found in all starfish, particularly concentrated in the center of the star and in the center of each limb. However, gene expression of trunk and tail segments is largely absent, with starfish “presenting the most dramatic example of the unraveling of head and trunk segments as we know them today.” “said Fourie, a researcher at the Zuckerberg BioHub, a nonprofit research institute in San Francisco.

The research was funded by the Chan Zuckerberg BioHub, co-founded by Dr. Priscilla Chan and Mark Zuckerberg in 2021, and by NASA, the National Science Foundation, and the Leverhulme Fund.

The researchers stained the starfish’s genetic material with fluorescent tags, allowing scientists to map the behavior of these animals’ genes. (Credit: Laurent Fourie)

“When we compared starfish gene expression to other animal groups, such as vertebrates, there was an important part of the body structure that was missing,” Thompson explained. “The genes normally involved in shaping the animal’s trunk are not expressed in the ectoderm. The entire body structure of echinoderms appears to be more or less equivalent to the head of other groups of animals.”

Starfish and other echinoderms developed their unique body structures after their ancestors lost their trunks, allowing them to move and feed differently than other animals.

“Our research suggests that the body structure of echinoderms has evolved in a more complex way than previously thought and that there is much more to be learned about these enigmatic creatures,” Thompson said. “As someone who has studied them for the past ten years, these discoveries have radically changed the way I think about this group of animals.”

New discoveries

Research with animals has primarily focused on sharing similarities with humans. However, the study of groups such as echinoderms can solve some of the most complex mysteries about the evolution of life on Earth.

“Most animals don’t have spectacular nervous systems and go out to hunt prey: they’re tame animals that live in holes in the ocean. People aren’t usually fascinated by these animals, although they reflect how they started most of their lives,” Lowe says.

Understanding how animals like starfish evolved can provide insight into the different ways different species stay healthy.

“It’s definitely more difficult to work in species that are less frequently studied,” said Daniel Roxar, a professor of genetics, genetics, evolution and development at the University of California, Berkeley and a San Zuckerberg Biohub researcher. , said in the statement..

“But if we take advantage of the opportunity to study unusual animals that behave in unusual ways, we broaden our perspective on biology, which will ultimately help us solve ecological and biomedical problems.”