Sea star bodies are not really bodies, study says

The heads of most animals are easily identifiable, but scientists have not been able to say the same for their heads. starfish until now.

A sea star has five identical arms with a layer of “tube feet” beneath them that may help the sea creature move along the sea floor, making naturalists wonder if sea stars have defined front and rear ends – and even whether they have heads.

But new genetic research suggests the opposite – that sea stars are largely heads without torsos or tails and likely lost these features evolutionarily over time.

Researchers said the bizarre fossils of sea star ancestors, which appeared to have a kind of torso, make much more sense in evolutionary terms in light of the new discoveries.

The findings were published Wednesday (1st) in Nature magazine.

“It is as if the starfish is completely trunkless and is best described as just a head crawling along the sea floor,” said the study’s lead author, Laurent Formery , postdoctoral researcher at Stanford University and the University of California, Berkeley, in a statement. “It’s not at all what scientists assumed about these animals.”

The revelations, made possible by new genetic sequencing methods, could help answer some of the biggest remaining questions about echinoderms, including their shared ancestry with humans and other animals that look nothing like them.

A unique body plan

Sea stars belong to a group called echinoderms, which includes sea urchins, sand dollars and sea cucumbers. The unusual animals have unique body plans arranged in five equal sections that differ greatly from the symmetrical head-to-tail bodies of bilateral animals, which have mirrored left and right sides.

Sea stars begin as fertilized eggs that hatch into larvae that float in the ocean, like plankton, for weeks to months before settling on the ocean floor. There, they undergo a process that transforms a bilateral body into a star shape, or pentaradial body.

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

The bilateral body plan that most animals have derives from molecular-level genetic actions 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 a similar genetic programming. This discovery received the Nobel Prize in Physiology or Medicine in 1995.

But echinoderms also share a common ancestor with bilaterian animals, which contributes to the questions researchers are trying to resolve.

“How the different body parts of echinoderms relate to those we see in other groups of animals has been a mystery to scientists for as long as we have studied them,” said study co-author Dr. Jeff Thompson professor at the University of Southampton.

“In their bilateral relatives, the body is divided into head, trunk and tail. But just by looking at a starfish it is impossible to see how these sections relate to the bodies of bilateral animals.”

Cracking the echinoderm code

The researchers behind the new study used micro-computed tomography to capture an unprecedented three-dimensional view of the shape and structure of sea stars.

Team members then used advanced analytical techniques to identify where genes were expressed in the tissue and identify specific RNA sequences in the cells. Gene expression occurs when information within a gene becomes functional.

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

“If you take the skin off an animal and look at the genes involved in defining the head and tail, the same genes code for these body regions in all groups of animals,” Lowe said.

“So we ignore the anatomy and ask: is there a molecular axis hidden beneath all this strange anatomy and what is its role in a starfish forming a pentaradial body plan?”

Together, the data created a 3D map to determine where genes were expressed as the sea stars developed and grew. 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 the development of a head have been detected in all sea stars, especially concentrated in the center of the star and in the center of each limb.

But gene expression for the trunk and tail sections was largely absent, revealing that sea stars “have the most dramatic example of dissociation of the head and trunk regions that we know of today,” said Formery, who is also researcher at the Chan 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 It is Mark Zuckerberg as well as by NASA, the National Science Foundation and the Leverhulme Trust.

“When we compared gene expression in a starfish to that of other groups of animals, such as vertebrates, it appeared that a crucial part of the body plan was missing,” Thompson said. “The genes that are normally involved in the patterning of the animal’s trunk were not expressed in the ectoderm. It appears that the entire echinoderm body plan is approximately equivalent to the head in other animal groups.”

Sea stars and other echinoderms likely evolved their unique body plans when their ancestors lost their trunk region, allowing them to move and feed differently than other animals.

“Our research tells us that the echinoderm body plan evolved in a more complex way than previously thought and there is still much to learn about these intriguing creatures,” said Thompson. “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 insights

Animal research mainly targets those that share similarities with humans. But studying groups like echinoderms could solve some of the most complex mysteries about the evolution of life on Earth.

“Most animals don’t have spectacular nervous systems and are chasing prey – they are modest animals that live in burrows in the ocean. People aren’t usually attracted to these animals, but they probably represent how life began,” Lowe said.

Understanding how animals like sea stars developed could also provide insights into the diverse ways in which different species remain healthy.

“It’s certainly more difficult to work on organisms that are studied less frequently,” said study co-author Daniel Rokhsar, professor of genetics, genomics, evolution and development at the University of California, Berkeley, and a researcher at the Chan Zuckerberg BioHub.

“But if we take the opportunity to explore unusual animals that operate in unusual ways, it means we are broadening our perspective on biology, which will ultimately help us solve ecological and biomedical problems.”