Starfish are actually “heads crawling on the bottom of the sea.”

(CNN) — The heads of most animals are easily recognizable. But until now scientists haven’t been able to say the same about starfish.

The starfish has five identical arms and a layer of ‘tube feet’ Below helps the sea creature move along the sea floor. This has led naturalists to wonder whether starfish have defined front and back ends, and whether they have a head.

But new genetic research suggests the opposite: that starfish are largely heads, lacking a trunk or tail, and likely lost those features evolutionarily over time. Researchers say strange fossils of starfish ancestors, Which appears to have some kind of trunk, and makes more evolutionary sense according to the new findings.

The results were published on Wednesday in the journal nature.

“It’s as if the starfish has completely lost its proboscis “It is best described as a simple head dragging along the seafloor,” Laurent Formieri, lead author of the study and a postdoctoral fellow at Stanford University and UC Berkeley, said in a statement. The condition that scientists assumed regarding these animals.

Results made possible by New genetic sequencing methods They could help answer some key remaining questions about echinoderms, including their shared ancestry with humans and other animals that look nothing like them.

The body structure is unique

sea ​​Star They belong to the echinoderm group, Which include sea urchins, sand dollars (or sea biscuits), and sea cucumbers. These unusual animals have a unique body, They are arranged in five equal sections, which differ greatly from the symmetrical bodies of bilateral animals, which have the sides, left and right, mirrored in each other.

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Starfish start out as fertilized eggs that hatch into larvae that float in the ocean, like plankton, for weeks or months before settling on the seafloor. There they go through the process of transforming the binary body into a star-shaped, or pentagonal, body.

“This has been a mystery in zoology for centuries.” Christopher Lowe, co-lead author of the study and a marine and developmental biologist at Stanford University, said in a statement. “How do we go from a two-sided body plan to a five-sided body plan, and how does any part of a starfish compare to the body structure?”

The bilateral body structure of most animals comes from genetic actions at the molecular level that can be traced back to the head and trunk or main body regions, which is why vertebrates, such as humans, and many invertebrates, including insects, share similar genetic programming. This discovery won an award Nobel Prize in Physiology or Medicine In 1995.

However, echinoderms also share a common ancestor with bilaterian animals, adding to the mystery researchers are trying to solve.

“How are the different parts of the body of echinoderms related “The ones we see in other groups of animals have been a mystery to scientists ever since we studied them,” Dr. Geoff Thompson, study co-author and professor at the University of Southampton, said in a statement.In its bilateral relatives, the body is divided into the head, trunk, and tail. “But just by looking at a starfish, it’s impossible to see how these parts relate to the bodies of bilateral animals.”

Breaking the echinoderm code

In the new study, researchers used computed tomography to get an unprecedented 3D view of the shape and structure of starfish.

Next, team members used advanced analytical techniques to discover where genes are expressed within tissues and identify specific RNA sequences within cells. Gene expression occurs when information in a gene becomes active.

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

“If you peel the skin off an animal and look at the genes that define the tip of the tail, the same genes encode these body regions in all groups of animals,” Lowe explains. “So we ignore the anatomy and ask ourselves: Is there a molecular axis hidden beneath all this strange anatomy and what is its role in the starfish’s formation of a pentagonal body structure?“.

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

Genetic signatures associated with head growth have been detected in all sea stars. It is particularly concentrated in the center of the star and in the center of each limb. However, gene expression for the trunk and tail segments was largely absent, revealing that the starfish “represents the most dramatic example of the separation of head and trunk regions that we know of today.” Zuckerberg BioHub, a nonprofit research organization in San Francisco.

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

“When we compared starfish gene expression with gene expression in other animal groups, such as vertebrates, an important part of their body structure appeared to be missing,” Thompson explained. “The genes normally involved in the formation of 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 structure of the head in other groups of animals.”

Starfish and other echinoderms likely developed their unique body structures once their ancestors lost the trunk region, which allowed them to move and feed differently from other animals.

“Our research tells us that the body structure of echinoderms evolved in a more complex way than previously thought, and that there is still a lot to learn about these interesting creatures,” Thompson said. “As someone who has studied these animals for the past 10 years, these findings have radically changed the way I think about this group of animals.”

New results

Research on animals focuses primarily on those that share similarities with humans. However, studying groups such as echinoderms could solve some of the most complex mysteries about the evolution of life on Earth.

“Most animals don’t have amazing nervous systems They go out to hunt prey: humble animals that live in burrows in the ocean. Generally, People are not attracted to these animals However, it probably represents how much life has begun,” Lowe says.

Understanding how animals like starfish evolve can also provide insight into the different ways different species maintain their health.

“Working on organisms that are not frequently studied is certainly more difficult.”Daniel Rokhsar, co-author of the study and professor of genetics, genomics, evolution and development at UC Berkeley and researcher at the Chan Zuckerberg BioHub, said in a statement.

“But if we take the opportunity to do so Explore unusual animals That work in unusual ways, meaning we broaden our perspective on biology, which will ultimately help us solve environmental and biomedical problems.

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