Sea Urchins Aren’t Brainless — Their Whole Body Functions as a Brain

Sea urchins — the spiny, slow-moving echinoderms that creep over rocks using hundreds of tiny tube feet — have long been considered to possess only a simple, diffuse nervous system. New molecular and cellular analyses show a very different picture: adult sea urchins host a highly diversified, body-wide neural network that researchers now describe as an "all-body" brain.

An international team led by marine biologist Periklis Paganos (Stazione Zoologica Anton Dohrn, Naples) and molecular biologist Jack Ullrich-Lüter (Museum für Naturkunde Berlin) reached this conclusion after mapping gene expression and cell types in juvenile and adult urchins. Genes typically associated with trunk or torso development in other animals are repurposed in urchins and are active in internal organs such as the gut and the water vascular system.

Rather than a simple nerve net, adult urchins contain hundreds of distinct neuronal types. Neurons are the most abundant cell type in juveniles, making up more than half of identified cell clusters. These neurons produce a broad palette of neurotransmitters — including dopamine, serotonin, histamine, GABA, glutamate and acetylcholine — and some cell populations are linked to complex functions such as learning and memory.

The researchers write that the diversity of neuronal cell types and the integration of multiple photoreceptor systems support the idea that the sea urchin nervous system comprises an "all-brain" rather than a "no-brain" state.

Gene-expression mapping shows that gene programs normally tied to different body regions in other animals appear together in single cell clusters in urchins. The authors interpret this as a merging of positional programs rather than a one-to-one mapping between a gene and a single tissue type. Head-patterning genes are broadly expressed across the body and are prominent in the oral nerve ring (the neural center around the mouth) and the five radial nerve cords that extend outward and connect around the oral ring.

Sea urchins lack eyes but are remarkably light-sensitive: their surfaces are covered with photoreceptor cells that express multiple classes of opsins, the light-sensitive proteins that convert photons into electrical signals. Much of the animal’s surface can detect and respond to light, effectively making it appear as if the urchin is covered in tiny eyes. Those photoreceptors feed into the distributed neural network, contributing sensory input across the animal’s body.

These findings, published in Science Advances, suggest that echinoderms never needed to evolve a single centralized brain because sensory processing and neural integration are distributed throughout the body. In other words, sea urchins may not lack a brain so much as possess a different kind of brain — one that spans their entire form.

Implications: The study challenges long-standing assumptions about nervous-system simplicity in echinoderms, informs our understanding of how nervous systems can be organized and may shed light on alternative evolutionary paths to complex sensory processing.