by Patricio Estrela Sponge Bob Square Pants fame had already gone beyond the bounds of anatomical correctness, but it turns out that what we all thought about a starfish’s body composition is probably wrong, thanks to new genetic work that suggests the marine animal has no arms or legs, but it’s just a giant, crawling head.
Upending what biologists know about one of nature’s most curious creatures, researchers at the Chan Zuckerberg Biohub San Francisco, working with labs at Stanford University and the University of California at Berkeley, discovered that the sea star – commonly called the ‘starfish’ of the sea’, largely marine scientists’ ire – has genetic markers for a head, but almost none that encode a torso or tail.
Yes, those cute arms drawn in crayon by millions of children around the world are not really arms. These ends, genetically, are mostly heads.
“It’s as if the sea star completely lacks a torso and is best described as just a head crawling along the sea floor,” said Laurent Formery, lead author of the new study. “It’s not at all what scientists assumed about these animals.”
What’s more, the molecular signatures normally associated with the frontal region of the head were located in the middle of each arm of the starfish.
Laurent Formery
“These results suggest that echinoderms, and sea stars in particular, have the most dramatic example of dissociation of head and trunk regions that we know of today,” said Formery. “This just opens up a whole host of new questions that we can now start to explore.”
Humans, like most animals, have a body plan from head to tail that conforms to a shape of bilateral symmetry; if you folded it in half, you could create two almost equal segments. Sea stars, on the other hand, have challenged the status quo of the animal kingdom, along with their cousins in the phylum Echinodermata, such as sea cucumbers, sea urchins and sand dollars. Although their larvae have bilateral symmetry, at some stage of development they change to form a radial layout. Essentially, this means you can divide the star into identical segments starting from a center point, like a circular pie.
The most prevalent species of five-armed starfish have a pentaradial plane – five points around a central disc – which has led biologists to consider the middle region housing the mouth and anus as their ‘head’, and their external appendages like 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 body plan, and how can you compare any part of the starfish to ours? body plan?”
Previously, scientists suggested that sea stars’ head-to-tail axis could extend from their armored back to their belly, which houses their thousands of tube feet used for breathing, movement and traction. Others thought that each of a star’s arms might correspond to its own individual head-to-tail axis.
Using a specialized genetic sequencing technique, scientists were able to unravel the starfish’s makeup in a way never done before.
“The kind of sequencing that would take months can now be done in a matter of hours and is hundreds of times cheaper than just five years ago,” said co-senior author David Rank. “These advances meant that we could essentially start from scratch on an organism that is not normally studied in the laboratory and mount the kind of detailed study that would have been impossible 10 years ago.”
The team employed spatial transcriptomics, a technique that allowed them to identify which genes are active in specific regions of sea stars. Rather than finding any genetic evidence of a head-tail axis anywhere in the animal’s body, the team found the same gene expression seen in the forebrain of humans and other bilateral animals. It was not in a single, central location, but stretched out in the middle of the arms of the starfish.
The researchers also found that at the outer edges of the arms, gene expression matched that of the human midbrain. There was no evidence of trunk or head-to-tail progression; the animal appeared to be essentially all head.
And one of the genes associated with the animals’ proboscis was expressed, and that was at the edges of the starfish’s arms.
“These results suggest that echinoderms, and sea stars in particular, have the most dramatic example of dissociation of head and trunk regions that we know of today,” said Formery. “This just opens up a whole host of new questions that we can now start to explore.”
Yet somehow, this animal that may be “just a head crawling across the seabed” is just one of the notable aspects of its composition.
Members of the Echinodermata are the only animals that have an aquatic vascular system, which governs how they move, feed and breathe. As such, they lack blood, brains, and complex mouthparts for eating. To get around the latter, they crawl over their prey and push the cardiac stomach out through the mouth, placing it on the animal below (probably a bivalve, like a mussel) and use powerful enzymes to break down the flesh until it is soft enough. to accumulate in the stomach and ‘suck’ back into the body. (Fortunately, a long line of evolution has spared us this fate.)
Sea stars can also generate clones if they are cut in half, as long as there is enough viable central disc tissue to grow new arms. This is a useful asexual reproductive strategy if sexual partners are difficult to find. Some species can also tear off an arm and use it as bait to escape serious threats from predators.
An evolutionary marvel of sorts, the oldest evidence of its existence is a fossil dating back 480 million years, or the Ordovician period.
Formery now hopes to examine the evolution of the starfish’s nervous system, looking at what it can teach us about our own development. The team also hopes to investigate the specialized survival strategies employed by sea stars to not only survive but also thrive for so many millions of years. Researchers believe there may be a wealth of untapped knowledge that could lead to better ways to combat human disease.
“It’s certainly more difficult to work on organisms that are studied less frequently,” said co-senior author Daniel Rokhsar of UC Berkeley. “But if we take the opportunity to explore unusual animals that operate in unusual ways, it means we are broadening our perspective on biology, which will ultimately help us solve ecological and biomedical problems.”
The research was published in the journal Nature.
Source: Chan Zuckerberg Biohub SF
