Phototaxis Light Sets the Beat

Nancy R. Gough

Science Signaling, AAAS, Washington, DC 20005, USA

One of the largest movements of biomass on Earth is the vertical migration of marine plankton, which swim toward the light. Jékely et al. investigated the mechanism by which phototaxis is achieved by the zooplanktonic larvae of the marine annelid Platynereis dumerilii and found a direct sensory-motor coupling between the eyespot and the locomotor cilia. The two-cell eyespot of this marine invertebrate is one of the simplest animal eyes and consists of a photoreceptor cell and a pigmented cell that shades the photoreceptor, defining its view angle. Jékely et al. found that the eyespots of P. dumerilii were independently functional and that most larvae phototaxed after ablation of one eyespot. Electron microscopy revealed that the photoreceptor cell extended an axon that joins the nerve ring and forms synapses with the steering cilia. The photoreceptor expressed VAChT, encoding the vesicular acetylcholine transporter, suggesting that it was a cholinergic neuron, and nicotinic acetylcholine receptors composed of 9 and 10 subunits were detected on the ciliated cells. Phototaxis was inhibited by nicotinic acetylcholine receptor antagonists. Illumination of one eyespot triggered a decrease in the ciliary beating frequency that was blocked by acetylcholine receptor antagonists and that produced a decrease in fluid flow. Ciliary cells closest to the eyespot showed the strongest response. Zooplankton and other phototactic single-celled organisms exhibit spiral swimming. Modeling suggested that axial rotation was required for proper phototaxis because it allows the organism to scan all directions for light and thus ensure proper orientation toward the light.

G. Jékely, J. Colombelli, H. Hausen, K. Guy, E. Stelzer, F. Nédélec, D. Arendt, Mechanism of phototaxis in marine zooplankton. Nature 456, 395–399 (2008). [PubMed]