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Abstract

Eyes and nervous systems evolved in a marine environment at the dawn of animal life and diversified during the Cambrian explosion, one of the most spectacular events in the history of life. Little is known about early stages of eye and visual circuit evolution. Simple marine planktonic organisms, in particular ciliated larvae of various marine invertebrates, can give us insights into how simple eyes and circuits of marine organisms function and may have evolved. We investigate the nervous system of the marine annelid model, Platynereis dumerilii. The ciliated, planktonic larvae of Platynereis have three pairs of eyes forming simple reflex circuits. The eyes control phototactic swimming, a key behavior regulating larval depth in the water column. We use a combination of behavioral, molecular genetic and ultrastructural studies to map and characterize phototactic circuits in Platynereis larvae. We believe that the simple circuitry we uncover in these ciliated larvae could give us insights into how neural circuits function and may have evolved.