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The heads of most animals are easily identifiable, but scientists haven’t been able to say the same for sea stars until now.
A starfish has five identical arms with a layer of “tube feet” beneath them that can help the marine creature move along the seafloor, causing naturalists to puzzle over whether sea stars have defined front and back ends — and if they have heads at all.
But new genetic research suggests the opposite — that sea stars are largely heads that lack torsos or tails and likely lost those features evolutionarily over time.
The researchers said the bizarre fossils of sea star ancestors, which appeared to have a kind of torso, make a lot more sense in evolutionary terms in light of the new findings.
The findings were published Wednesday in the journal Nature.
“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 study author Laurent Formery, postdoctoral scholar at Stanford University and the University of California, Berkeley, in a statement. “It’s not at all what scientists have assumed about these animals.”
The revelations, made possible by new methods of genetic sequencing, could help answer some of the biggest remaining questions about echinoderms, including their shared ancestry with humans and other animals that look nothing like them.
A unique body plan
Sea stars belong to a group called echinoderms, which includes sea urchins, sand dollars and sea cucumbers. The unusual animals have unique body plans arranged in five equal sections that differ greatly from the symmetric head-to-tail bodies of bilateral animals, which have left and right sides mirroring each other.
Sea stars begin as fertilized eggs that hatch and become larvae that float in the ocean, like plankton, for weeks to months before settling on the ocean floor. There, they go through a process that transforms a bilateral body into a star shape, or pentaradial body.
“This has been a zoological mystery for centuries,” said senior study coauthor Christopher Lowe, marine and developmental biologist at Stanford University, in a statement. “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?”
The bilateral body plan most animals have stems from molecular-level genetic actions that can be traced in the head and trunk, or main body, regions, which is why vertebrates, like humans, and many invertebrates, including insects, share similar genetic programming. This discovery was awarded the Nobel Prize in Physiology or Medicine in 1995.
But echinoderms also share a common ancestor with bilateral animals, which adds to the puzzle researchers are trying to solve.
“How the different body parts of the echinoderms relate to those we see in other animal groups has been a mystery to scientists for as long as we’ve been studying them,” said study coauthor Dr. Jeff Thompson, a lecturer at the University of Southampton, in a statement. “In their bilateral relatives, the body is divided into a head, trunk, and tail. But just looking at a starfish, it’s impossible to see how these sections relate to the bodies of bilateral animals.”
Cracking the echinoderm code
Researchers behind the new study used micro computed tomography scanning to capture an unprecedented three-dimensional look at the shape and structure of sea stars.
Then, members of the team used advanced analytical techniques to spot where genes were expressed within the tissue and pinpoint specific sequences of RNA within the cells. Gene expression occurs when the information within a gene becomes functional.
Specific molecular markers act like body plan blueprints, directing each cell to the body region where it belongs.
“If you strip away the skin of an animal and look at the genes involved in defining a head from a tail, the same genes code for these body regions across all groups of animals,” Lowe said. “So we ignored the anatomy and asked: Is there a molecular axis hidden under all this weird anatomy and what is its role in a starfish forming a pentaradial body plan?”
Together, the data created a 3D map to determine where genes were expressed as sea stars developed and grew. The team was able to determine the genes that control the development of the starfish’s ectoderm, which includes its skin and nervous system.
Genetic signatures associated with the development of a head were detected all over the sea stars, especially concentrated in the center of the star and the center of each limb. But gene expression for torso and tail sections were largely absent, revealing that sea stars “have the most dramatic example of decoupling of the head and the trunk regions that we are aware of today,” said Formery, who is also a researcher at the Chan Zuckerberg Biohub, a nonprofit research organization in San Francisco.
The research was funded by the Chan Zuckerberg Biohub, launched in 2016 by Dr. Priscilla Chan and Mark Zuckerberg, as well as NASA, the National Science Foundation and the Leverhulme Trust.
“When we compared the expression of genes in a starfish to other groups of animals, like vertebrates, it appeared that a crucial part of the body plan was missing,” Thompson said. “The genes that are typically involved in the patterning of the trunk of the animal weren’t expressed in the ectoderm. It seems the whole echinoderm body plan is roughly equivalent to the head in other groups of animals.”
Sea stars and other echinoderms likely evolved their unique body plans once their ancestors lost their trunk region, allowing them to move and feed differently from other animals.
“Our research tells us the echinoderm body plan evolved in a more complex way than previously thought and there is still much to learn about these intriguing creatures,” Thompson said. “As someone who has studied them for the last ten years, these findings have radically changed how I think about this group of animals.”
Unlocking new insights
Animal research largely targets those that share similarities with humans. But studying groups like echinoderms could solve some of the most complex mysteries about the evolution of life on Earth.
“Most animals don’t have spectacular nervous systems and are out chasing prey — they are modest animals that live in burrows in the ocean. People are generally not drawn to these animals, and yet they probably represent how much of life got started,” Lowe said.
Understanding how animals like sea stars have developed could also allow insights into the varied ways that different species remain healthy.
“It’s certainly harder to work in organisms that are less frequently studied,” said study coauthor Daniel Rokhsar, professor of genetics, genomics, evolution and development at the University of California, Berkeley, and researcher at the Chan Zuckerberg Biohub, 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.”
Correction: A previous version of this story misstated the launch date of the Chan Zuckerberg Biohub.