When it comes to taking hardcore evolutionary paths, some animals don’t hold back. The common sea urchin, as it turns out, really drives this point home—boasting a spiky nervous system scattered throughout its body, including light-sensitive cells resembling structures in human eyes.
Imagine ordinary sea urchins—roundish, spiky creatures minding their own business on the sea floor. According to a recent Science Advances paper, this assemblage is the sea urchin’s entire body—or, rather, head. That is, sea urchins don’t have a multi-part body, just a head, which is comprised of a remarkably complex nervous system forming an “all-body brain,” as the researchers describe it in a statement.
Importantly, this nervous system isn’t a hodgepodge bunch of neurons but rather a sophisticated network of cells that highly resemble the brain structures of vertebrates. What’s more, urchin heads have several light-sensitive cells, or photoreceptors, hinting at a previously unknown visual capacity for these creatures.
“Our results show that animals without a conventional central nervous system can still develop a brain-like organization,” Jack Ullrich-Lüter, study lead author and a researcher at the Natural Museum of Berlin in Germany, said in the statement. “This fundamentally changes how we think about the evolution of complex nervous systems.”
The life of pubescent sea urchins
Many animals, including humans, are bilaterally symmetric, meaning they have bodies with right and left sides that generally mirror each other. But urchins and their relatives, such as starfish or sea cucumbers, start out bilaterally symmetric and undergo a “radical metamorphosis” and gain pentaradial symmetry—simply, a five-part, circular symmetrical divide, like a starfish.
Scientists have long been fascinated by the unique evolutionary pathways of these creatures, although genetic studies thus far have mostly focused on them pre-metamorphosis, according to the paper.
And so, the team initially set out to investigate the mysteries of the sea urchin genome during its dramatic transition—that is, how could a single genome produce “two radically different body plans”? To that end, they conducted a thorough genetic analysis of young sea urchins who had just finished transforming, hoping to identify which cells enabled such a drastic glow-up.
Forget aliens, sea urchins
In that process, the team discovered something far more extraordinary: an unexpectedly refined network of neuronal cells extending throughout the urchins’ bodies. In fact, the body itself was just one giant head; genes that in other creatures would define internal organs were all crammed inside, the paper noted.
But these neuronal clusters were much more refined than “a mere network of interconnected neurons and ganglia,” according to the paper, challenging previous beliefs that creatures like urchins were “simple” since they lacked a centralized brain.
The new analysis, however, revealed a strikingly diverse family of neuronal cells and highly specialized neuropeptides. In addition, the team also identified photoreceptors that also appeared capable of combining different proteins to detect and process light stimuli.
Basically, sea urchins are a lot more sophisticated than we thought. They may also sense more than we believed. In fact, the latest results are probably the tip of the iceberg for sea urchins. I eagerly await the next piece of sea urchin lore.
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