Patrick Star of SpongeBob SquarePants fame had already stretched the boundaries of anatomical correctness, yet it turns out that what all of us think about the bodily composition of a sea star is most likely wrong, thanks to new genetic work that suggests the marine animal has no arms or legs but is just one giant, crawling head.
Upending what biologists know about one of nature’s most curious creatures, researchers from Chan Zuckerberg Biohub San Francisco, working with labs at Stanford University and the University of California Berkeley, have found that the sea star – commonly called a ‘starfish’, much to marine scientists' ire – is in possession of genetic markers for a head, yet next to none that code for a torso or tail.
Yep, those cute arms lovingly sketched in crayon by millions of children across the globe are not actually arms at all. Those extremities, genetically, are mostly 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 Laurent Formery, lead author of the new study. “It’s not at all what scientists have assumed about these animals.”
What’s more, the molecular signatures usually associated with the front region of the head were instead localized in the middle of each sea star arm.
“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. “It just opens a ton of new questions that we can now start to explore.”
Humans, like most animals, have a head-to-tail body plan that fits a bilateral symmetry format; if you were to fold in half, you could create two near-equal segments. Sea stars, on the other hand, have defied the animal kingdom’s status quo, along with their cousins in the phylum Echinodermata such as sea cucumbers, sea urchins and sand dollars. While their larvae possess bilateral symmetry, at some stage in development they switch out to form a radial layout. Essentially, this means you could split the star into identical segments from a central point, kind of like a circular pizza.
The most prevalent five-armed sea star species possess a pentaradial plan – five points around a central disk – which has led biologists to consider their middle region that houses the mouth and anus their ‘head’, and their outward appendages arms.
“This has been a zoological mystery for centuries," said senior author Christopher Lowe, a researcher at Stanford. "How can you go from a bilateral body plan to a pentaradial plan, and how can you compare any part of the starfish to our own body plan?"
Previously, scientists have suggested that the head-to-tail axis of sea stars might extend from its armored back to its underbelly, which houses its thousands of tube feet used for breathing, moving and traction. Others thought each of a star’s arms might correspond to its own individual head-to-tail axis.
Using a specialized genetic sequencing technique, the scientists were able to unravel the makeup of the sea star in a way that had never been done before.
“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-senior author David Rank. “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.”
The team employed spatial transcriptomics, a technique that allowed them to pinpoint which genes are active in specific sea star regions. Instead of finding any genetic evidence of a head-to-tail axis anywhere on the animal’s body, the team discovered the same gene expression seen in the forebrain of humans and other bilateral animals. It was not in a single, central location, but extended down the middle of the sea stars’ arms.
The researchers also found that, towards the arms’ outer edges, genetic expression matched that of the human midbrain. There was no evidence of a trunk, or head-to-tail progression; the animal looked to be essentially all head.
And one of the genes associated with the trunk in animals was expressed, and this was at the very edges of the sea stars’ 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,” said Formery. “It just opens a ton of new questions that we can now start to explore.”
Yet somehow, this animal that may be “just a head crawling along the seafloor” is just one of the remarkable aspects of its makeup.
Members of Echinodermata are the only animals that possess a water vascular system, which governs how they move, feed and breathe. As such, they lack blood, a brain and complex mouthparts for eating. To get around the latter, they will crawl onto prey and push their cardiac stomach out through their mouth, draping it over the animal below (most likely a bivalve, such as a mussel) and use powerful enzymes to break down the meat until it’s soft enough to gather up in the stomach and ‘suck’ back into their body. (Fortunately, a long line of evolution has spared us this fate.)
Sea stars can also spawn clones if cut in half, as long as there’s enough viable central disk tissue to grow new arms from. This is a handy asexual reproductive strategy if sexual mates are hard to come by. Some species can also tear off an arm and use it as a decoy to evade serious threats from predators.
Somewhat of an evolutionary marvel, the oldest evidence of their existence is a fossil that dates back 480 million years, or to the Ordovician period.
Formery now hopes to examine the sea star’s nervous system evolution, with a view to what it might teach us about our own development. The team also hopes to investigate the specialized survival strategies employed by sea stars in order to not just survive but thrive for so many millions of years. The researchers feel there might be a wealth of untapped knowledge that could lead to better ways of combating human diseases.
“It’s certainly harder to work in organisms that are less frequently studied,” said co-senior author, Daniel Rokhsar from UC Berkeley. “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.”
The research was published in the journal Nature.
Source: Chan Zuckerberg Biohub SF
