Nervous system in older Patiria starfish anatomy
© Laurent Formery/Evident Image of the Year Award
Nervous system in older Patiria starfish
Scientists have long been perplexed by the anatomical arrangement of sea stars. Many experts assumed that sea stars didn't have heads because of their remarkable five-armed symmetry and the lack of a prominent head or tail.

However, it turns out that a starfish's entire body serves as a head.

This startling revelation comes from a recent study by researchers at Stanford University and UC Berkeley.

They uncovered gene signatures associated with head development in young sea stars after conducting a thorough gene analysis of the starfish's body. The expression of genes responsible for an animal's torso and tail portions, on the other hand, was noticeably lacking.

"It's as if the sea star is completely missing a trunk and is best described as just a head crawling along the seafloor. It's not at all what scientists have assumed about these animals," said Laurent Formery, lead author of the new study, in an official release.

Mystery surrounding sea stars' anatomy

The mystery surrounding sea stars' anatomy stems from their considerable divergence from the bilateral symmetry found in most creatures.

Sea stars are members of the invertebrate group known as echinoderms, which have a unique body plan with five equally symmetric parts.

Interestingly, in their evolutionary history, echinoderms can be traced back to an ancestor with a bilateral symmetry or two mirrored sides, much like a human's.

Sea urchins and sea cucumbers, both of which lack a recognizable head or tail, have a pentaradial (fivefold) body layout.

The genetic examination indicated that molecular markers usually associated with the frontal area of the head were found in the middle of each sea star arm.

Surprisingly, "these signatures becoming progressively more posterior moving out towards the arms' edges."

The observations led to the conclusion that sea stars may have heads. They may have experienced a metamorphosis along the course of evolution that resulted in organisms that are primarily constituted of heads.
starfish ultraviolet light nervous system genome
© Laurent Formery
By staining genetic material with fluorescent labels, researchers can examine how key genes behave across the sea star body.
Genome sequencing of starfish

For this study, the sea star's genome was sequenced using a sophisticated approach known as HiFi sequencing technology. This advanced method allowed them to map genomic activity in sea stars throughout their development.

"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."

According to the press release, there are several scientific proposals surrounding the head-to-tail axis in sea stars. Some have speculated that in sea stars, this axis may extend from the top to the lower side. Others, on the other hand, believe that each of the sea star's five arms is a duplicate of a regular head-to-tail axis.

The scientists used a technique known as spatial transcriptomics to find patterns that might suggest a head-to-tail axis.

This approach aids in locating activated genes in specific parts of the organism. It enabled the researchers to build a detailed representation of how genes are distributed inside the sea star's body.

"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. It just opens a ton of new questions that we can now start to explore," said Formerly.

In the fossil record, certain peculiar-looking ancestors of sea stars have been preserved, and these specimens seem to have a trunk-like structure.

Following this work, the researchers hope to look into the genetic patterning in sea urchins and sea cucumbers.

The study was published in the journal Nature.
Mrigakshi Dixit is a science journalist who enjoys writing about space exploration, biology, and technological innovations. Her professional experience encompasses both broadcast and digital media, enabling her to learn a variety of storytelling formats. Her work has been featured in well-known publications including Nature India, Supercluster, and Astronomy magazine.