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Monday, December 11, 2023 | Back issues
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Sea stars have heads after all, researchers say

After centuries of speculation, it turns out that the whereabouts of sea star heads have been staring scientists in the face all along.

(CN) — Scientists on Wednesday revealed the answer to an age-old question on what constitutes the head of a sea star or “starfish”: it’s the marine organism’s entire body.

The new study in the journal Nature comes from a collaboration of scientists led by Chan Zuckerberg Biohub San Francisco Investigators, whose labs at Stanford University and the University of California, Berkeley, traced gene signatures on juvenile starfish to discover the true whereabouts of the invertebrate’s extremities and its seemingly missing head.

Focusing on the species Patiria miniata, a starfish with five symmetrical arms, the researchers found gene signatures associated with head development all over the organism’s body. There was little to no gene expression for an animal torso or tail.

The location of molecular signatures associated with head development also surprised researchers: Signatures that typically indicate the front portion of heads were found in the middle of starfish arms, while those corresponding to a rear placement were increasingly condensed near arm edges.

“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 lead author Laurent Formery in a statement. “It’s not at all what scientists have assumed about these animals.”

Until now, the bodies of starfish species have challenged scientists’ understanding of animal evolution. Instead of displaying the typical bilateral symmetry of most animals, including humans, adult sea stars and related echinoderms have a fivefold axis of symmetry without a distinct tail or head.

Finding technology capable of detecting gene expression in starfish tissue has also been an issue, the researchers said, though recent developments from PacBio — a Silicon Valley company that builds genome-sequencing devices — finally made their goal of studying starfish genetics possible.

“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 co-author David Rank in a statement. “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.”

PacBio’s HiFi sequencing also allowed the researchers to use an approach called spatial transcriptomics, which let them map active starfish genes at precise areas of the organism with fluorescent dye labels.

What the researchers found debunks prevailing hypotheses about starfish body plans, one being that the species’ head-to-tail axis extended from its armored back to its underbelly and the other assuming that the starfish’s five arms corresponded to a copy of a typical head-to-tail axis.

Instead, the researchers concluded that the gene expression associated with bilateral animal forebrains is located toward the middle of starfish arms, whereas genes affiliated with animal midbrains became more condensed toward the starfish’s outer arms.

As for active genes correlated with animal trunks, the researchers determined one that exists at the very tip of sea star arms.

“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,” Formery said, adding that some fossilized sea stars appear to have had a trunk before and that the discovery “opens a ton of new questions that we can now start to explore.”

Moving forward, the researchers hope to compare the genetic pattern of starfish with other echinoderms like sea urchins and sea cucumbers and to learn whether starfish genetics can reveal more about the evolution of the nervous system — a subject Formery says is poorly understood in echinoderms.

Given the starfish’s unusual traits — like digesting prey with its stomach outside of its body — the researchers believe further studies will help fill in gaps about what is known of animal evolution and potentially inspire innovations in medicine.

“It’s certainly harder to work in organisms that are less frequently studied,” said co-author Daniel Rokhsar, a professor of genetics, genomics, evolution and development at UC Berkeley, in a statement. “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.”

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