When it comes to taking radical evolutionary paths, some animals don’t hold back. The common sea urchin, as it turns out, really drives this point home – boasting a spiny nervous system scattered throughout its body, which also includes light-sensitive cells similar to structures in human eyes.
Imagine ordinary sea urchins – round, spiky creatures doing their thing on the ocean floor. According to a recent Science Advances paper, this assemblage is the sea urchin’s entire body — or, rather, the head. That is, sea urchins don’t have a multi-part body, just a head, which is composed of a remarkably complex nervous system that forms a “whole-body brain,” as the researchers describe it in a statement.
Importantly, this nervous system is not a large cluster of neurons, but a sophisticated network of cells that closely resembles the brain structures of vertebrates. Additionally, the urchin head contains numerous light-sensitive cells, or photoreceptors, pointing to a previously unknown visual ability for these creatures.
“Our results show that animals without a conventional central nervous system can still develop brain-like organization,” Jack Ulrich-Lutter, lead author of the study and a researcher at the Natural Museum of Berlin in Germany, said in the statement. “This fundamentally changes the way we think about the evolution of complex nervous systems.”
Life of the pubescent sea urchin
Many animals, including humans, are bilaterally symmetrical, meaning that their bodies have right and left sides that generally mirror each other. But urchins and their relatives, like starfish or sea cucumbers, start out bilaterally symmetrical and undergo a “radical metamorphosis” and acquire pentaradial symmetry – simply, like a starfish, a five-part, spherically symmetric division.
Scientists have long been fascinated by the unique evolutionary pathways of these creatures, although according to the paper, genetic studies have so far focused mostly on pre-metamorphosis.
And so, the team initially set out to investigate the mysteries of the sea urchin genome during its dramatic transformation – that is, how could a single genome generate “two fundamentally different body plans”? To that end, they conducted a thorough genetic analysis of young sea urchins that had just metamorphosed, in the hopes of identifying which cells enable such an intense glow.
Forget aliens, sea urchins
In that process, the team discovered something even more extraordinary: an unexpectedly sophisticated network of neuronal cells spread throughout the urchin’s body. In fact, the body itself was merely a giant head; The genes that define internal organs in other creatures were all loaded inside, the paper said.
But according to the paper, these neuronal clusters were far more sophisticated than “a mere network of interconnected neurons and ganglia,” challenging previous beliefs that creatures like urchins were “simple” because they lacked a centralized brain.
However, new analysis revealed a surprisingly diverse family of neuronal cells and highly specific neuropeptides. In addition, the team also identified photoreceptors that appeared capable of combining different proteins to detect and process light stimuli.
Basically, sea urchins are much more sophisticated than we thought. They can understand much more than we believe. In fact, the latest results are probably the tip of the iceberg for sea urchins. I look forward to the next part of sea urchin lore.