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Abstract

Animal 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 the evolution of neuronal circuits and nervous systems. Simple marine planktonic organisms, in particular ciliated larvae of various marine invertebrates, can give us insights into how simple nervous circuits of marine organisms function and may have evolved. We investigate the nervous system of the marine annelid model, Platynereis dumerilii. In the ciliated, planktonic larvae of Platynereis we have found several multifunctional sensory-motor neurons. These simple neurons are involved in the regulation of larval swimming depth and larval phototaxis. Sensory information is thus translated into motor output on locomotor cilia via a very simple neural circuitry. Other neurons are sensory-neurosecretory, providing a direct link between environmental stimuli and neuroendocrine regulation. We propose that the simple circuitry found in these ciliated larvae represents an ancestral state in nervous system evolution.