Starfish are just "one big head" crawling on the seafloor without an actual body

For ages, people have wondered whether sea stars, commonly called starfish, have a head or not. Unlike creatures like worms or fish, where it’s clear which end is which, starfish have five identical arms that can all move independently.

This has made it hard to figure out where their front or back might be.

“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 postdoctoral scholar and lead author of a new study.

Starfish symmetry is a mystery

Formery works in the labs at Stanford University and UC Berkeley, both led by investigators from the Chan Zuckerberg Biohub San Francisco.

Their research has found that, contrary to previous beliefs, starfish are now assumed to be “all head” and no traditional body at all.

The researchers discovered that genes associated with head development are active throughout the starfish’s entire “body,” while genes related to the torso and tail are mostly absent.

New look at an old question

Most animals, including humans, have a body plan that’s symmetrical on both sides and runs from head to tail.

Scientists have long understood how genes control this body plan in creatures like fruit flies and worms.

But starfish, which belong to a group called echinoderms, have puzzled researchers because of their unique five-point symmetry and lack of an obvious head or tail.

Echinoderms like starfish, sea urchins, and sea cucumbers have always been a bit of a mystery. Their unusual body structure doesn’t fit neatly into the typical patterns seen in most animals.

Some scientists thought that perhaps each arm of a starfish was like a separate head-to-tail axis. Others speculated that the axis might run from the top to the bottom of the sea star.

Studying starfish heads with cutting-edge genetics

To get to the bottom of this, the researchers used advanced genetic sequencing techniques. They partnered with PacBio, a company that develops genome-sequencing devices.

Using a method called HiFi sequencing, they were able to read long stretches of DNA quickly and accurately. This allowed them to see which genes were active in different parts of the starfish’s body.

“The kind of sequencing that would have taken months can now be done in a matter of hours, and it’s hundreds of times cheaper than just five years ago,” said David Rank, a former PacBio Scientific Fellow and co-senior author of the study.

Genes of starfish are only associated with heads

What they found was unexpected. Genes associated with the front part of the head in other animals were active in the middle of each starfish arm.

As you moved toward the edges of the arms, genes associated with more rear parts of the head became active. There was little to no expression of genes linked to the torso and tail.

“These results suggest that the echinoderms, and starfish in particular, have the most dramatic example of decoupling of the head and the trunk regions that we are aware of today,” Formery said.

Evolutionary curveball

This suggests that starfish might have lost their bodies over evolutionary time, ending up as creatures that are essentially all head.

The team is excited about what this means for understanding evolution. They hope to investigate whether similar patterns are found in other echinoderms like sea urchins and sea cucumbers.

“If we take the opportunity to explore unusual animals that are operating in unusual ways, that means we are broadening our perspective of biology, which is eventually going to help us solve both ecological and biomedical problems,” said Daniel Rokhsar of UC Berkeley, an expert on the molecular evolution of animal species.

Many unanswered questions remain

Formery is also interested in learning more about the starfish’s nervous system, which remains a bit of a mystery.

Starfish walk by moving water through thousands of tube feet and digest their prey by extruding their stomachs outside of their bodies.

It only stands to reason that these unusual creatures have also evolved completely unexpected strategies for staying healthy.

“It just opens a ton of new questions that we can now start to explore,” Formery added.

Questions like how their unique body plan affects their behavior and interaction with the environment are next on the list.

What happens next?

Christopher Lowe, a marine and developmental biologist at Stanford University, highlighted the importance of collaborative efforts.

“The Biohub’s willingness to take risks and provide support for a joint position between our labs has been critical for the success of this project,” he said.

The collaboration between institutions and the use of innovative technology have paved the way for this breakthrough.

The researchers utilized spatial transcriptomics to pinpoint which genes are active in precise locations within the starfish’s body, providing a detailed map of genetic activity.

Starfish, heads, and humanity

To sum it all up, this study flips our understanding of starfish on its head — literally.

By revealing that starfish are essentially all head and no body, the researchers have opened up a whole new way of thinking about these fascinating creatures.

“Exploring these unique animals broadens our perspective of biology,” Rokhsar concluded.

Their unique genetic makeup challenges what we thought we knew about animal evolution and body structures.

Looking ahead, this discovery unravels some of the mysteries surrounding starfish while setting the stage for exploring other unusual animals.

Studying these amazingly unique creatures might even lead to breakthroughs in medicine and new approaches to fighting diseases.

It’s exciting to think about what other secrets the starfish holds and how we’re going to figure them out.

The full study was published in the journal Nature.

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