The All-Brain Sea Urchin: A Juvenile Cell Atlas Rewrites Nervous System Design

In this deep dive, we explore the first detailed single-nucleus atlas of Paracentrotus lividus at two weeks old. SnRNA-seq maps every active gene in every cell and reveals a surprisingly complex nervous system: 48 cell clusters, 29 neuronal families, and a full suite of signaling—dopaminergic, serotonergic, cholinergic, GABAergic—distributed across the body. Genes long associated with the head are found throughout the nervous system and epidermis, supporting an 'all-brain' view. We unpack 15 photoreceptor types, including image-forming cells that co-express melanopsin (opsin 4) with go-opsin (opsin 3.2)—a novel dual-opsin arrangement that likely enhances spectral tuning in dim marine environments. The study hints that ancient brain-building programs are repurposed across the body and may extend across echinoderms, reshaping how we think about nervous systems in seemingly simple animals.

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