Sea stars may actually be walking heads with no bodies

For centuries, the anatomical structure of sea stars, commonly referred to as “starfish,” has left scientists stumped. Given their unique five-armed symmetry and lack of any discernible head or tail, many assumed that sea stars might be headless. However, a recent study reveals that sea stars might, in fact, be all head.

Not headless after all

Experts at Stanford University and UC Berkeley labs, led by Chan Zuckerberg Biohub, discovered that while gene signatures related to head development were prevalent throughout juvenile sea stars, those related to the torso and tail were almost entirely absent. 

More surprisingly, molecular signatures generally linked to the frontal part of the head were identified at the center of each sea star arm, with these signals gradually becoming more posterior towards the extremities of the arms.

The research indicates that sea stars might not be headless after all. Instead, over evolutionary time, they may have lost their bodies, transforming into beings that are essentially all head. 

“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 study lead author Laurent Formery, a Biohub-funded postdoctoral scholar. “It’s not at all what scientists have assumed about these animals.”

A decade of research 

The collaborative work between Christopher Lowe from Stanford University and Daniel Rokhsar from UC Berkeley has been ongoing for ten years. Their efforts were supported by the CZ Biohub SF’s Intercampus Research Awards.

“The work we proposed to do together was very ambitious and the kind of thing that generally does not play very well with traditional funding mechanisms,” Lowe said. “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.”

Sea star head: Mysterious anatomy

The mystery surrounding the sea star’s anatomy originates from its stark departure from the bilateral symmetry seen in most animals, which can be split evenly along an axis stretching from head to tail. 

Sea stars, along with echinoderms like sea urchins and sea cucumbers, exhibit a five-fold axis of symmetry, devoid of a clear head or tail.

Studying sea star heads

The researchers overcame the challenge of studying sea star genetics using PacBio’s HiFi sequencing technology. This advanced method empowered the scientists to map genetic activity across developing sea stars. 

“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, co–senior author of the study. “These advances meant we could start essentially from scratch in an organism that’s not typically studied in the lab and put together the kind of detailed study that would have been impossible 10 years ago.”

Using spatial transcriptomics, they identified active genes in specific areas of the organism, painting a detailed picture of gene distribution within the sea star body.

What the researchers learned 

The findings countered popular hypotheses about the sea star’s body plan. Instead, genetic expressions seen in the human forebrain were discovered along the sea star arms’ midlines. 

Furthermore, while genes representing different head subregions in bilaterally symmetrical animals were present in the sea star, only one gene usually linked to the trunk in animals was identified. This gene is located at the sea stars’ arm edges.

“These results suggest that the echinoderms, and sea stars in particular, have the most dramatic example of decoupling of the head and the trunk regions that we are aware of today,” said Formery, adding that some bizarre-looking sea star ancestors preserved in the fossil record do appear to have had a trunk. “It just opens a ton of new questions that we can now start to explore.”

Implications about sea star heads

The study paves the way for further exploration, including whether sea urchins and sea cucumbers display similar genetic patterning. Formery is particularly interested in what sea stars can reveal about the evolution of the nervous system. 

The researchers said that learning more about the sea star and its relatives will not only help solve key mysteries of animal evolution, but could also inspire innovations in medicine.

Given the sea star’s unique bodily functions, including digesting prey by extruding their stomachs, it is believed they might have evolved innovative strategies to stay healthy. Such strategies could expand our approaches to combating human disease, noted the study authors. 

“It’s certainly harder to work in organisms that are less frequently studied,” said Rokhsar. “But 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.”

More about sea stars

Sea stars, often referred to as starfish, are among the most recognizable and intriguing marine invertebrates. With their unique shape and diverse range of species, sea stars capture our imaginations and remain a vital component of ocean ecosystems.

Anatomy and physiology of sea stars

Sea stars possess a distinct, radially symmetrical body structure. While most people associate them with having five arms, some species can have up to 40 arms. Also, as previously discussed, there is still much debate about whether or not sea stars have heads.

Water vascular system

At the heart of a sea star’s anatomy lies the water vascular system. This hydraulic system pumps seawater through the sea star’s body, enabling it to move. Tiny tube feet, which are extensions of this system, protrude from the sea star’s underside. They play pivotal roles in movement, feeding, and respiration.

Regeneration abilities

One of the most remarkable features of sea stars is their regenerative abilities. If a sea star loses an arm due to predation or injury, it can often regenerate a new one. Some species can even regenerate an entire body from just a portion of a lost limb, given the central disc remains intact.

Feeding habits and diet

Sea stars are primarily carnivorous and have developed interesting ways to consume their prey. Some sea stars actually eat coral, causing more problems for coral reefs on top of the climate change threat.

Stomach extrusion

Many sea stars use an extraordinary method to feed. They can extrude their stomachs out of their bodies to envelop and digest prey, often bivalves like clams and mussels. After digesting the soft parts of the prey, they retract their stomachs back into their bodies. A chemical signal tells them when they are full.

Diverse diets

Depending on the species, sea stars can consume a range of foods, from algae to small fish. Some species, like the crown-of-thorns starfish, prey on coral polyps. This can lead to significant ecological concerns if their populations become too large.

Reproduction and lifecycle

Sea stars have a diverse range of reproductive strategies, with both sexual and asexual reproduction observed. They have also been observed to cross breed.

Spawning

Many sea stars reproduce by spawning. Males and females release sperm and eggs into the water, where fertilization occurs. This results in free-swimming larvae, which eventually settle on the ocean floor and metamorphose into juvenile sea stars.

Asexual reproduction

In certain conditions, sea stars can reproduce asexually. This usually involves the sea star dividing its body and regenerating the missing parts.

Ecological importance

Sea stars play a crucial role in marine ecosystems. They often act as keystone species, meaning their presence or absence can have a disproportionately large impact on the environment. By controlling populations of other marine creatures, they help maintain balance and biodiversity.

In summary, sea stars, with their captivating biology and significant ecological roles, are much more than just pretty faces in the ocean. Their ability to regenerate, unique feeding mechanisms, and importance in marine ecosystems highlight their intricate relationship with the environment.

As we continue to study and understand these creatures, we gain deeper insights into the wonders of marine life and the delicate balance of our oceans.

The study is published in the journal Nature.

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