Non-visual photoreceptive brain specification in sea urchin larvae.

Centralized nervous systems enable animals to detect environmental cues and coordinate behavior, but their evolutionary origins in deuterostomes remain unclear. Among deuterostomes, echinoderms-such as sea urchins-have long been thought to lack brain-like structures, especially in larval stages. Although recent gene expression and neural activity studies suggest brain-like properties in sea urchin larvae, direct links to behavior are still emerging. Here, we identify a light-sensitive cluster of neurons in the posterior neuroectoderm of sea urchin larvae. These neurons express UV-sensitive Opsin5 and regulatory genes such as rx, otx, six3, and lhx6, which are conserved in the vertebrate diencephalon. We mapped this domain using single-cell RNA sequencing and in situ hybridization. Knockdown of Opn5L impaired light-dependent swimming, indicating an active role in photoreception. While further work is needed to fully establish circuit-to-behavior relationships, our findings add to growing evidence that sea urchin larvae possess a non-visual photoreceptive neural center with molecular features shared by vertebrate brain regions. This suggests that such domains originated in the deuterostome ancestor and contributed to the early evolution of brain function.