Phototaxis, directional swimming towards the light, is a widespread behaviour in marine plankton. About 80% of all marine invertebrates with a dispersing larval stage show positive phototaxis at the beginning of the dispersal phase. Following this positive phototactic stage, older larvae often become negatively phototactic. We study the mechanisms of both positive and negative phototaxis in Platynereis
and other marine invertebrate larvae.
We have a comprehensive understanding of the mechanism of early larval positive phototaxis in Platynereis
. Positive phototaxis is mediated by a pair of very simple eyes, or eyespots. These eyespots are composed of two cells only: a photoreceptor and a shading pigment cell, and resemble Darwin's 'proto-eyes'
, considered to be the first eyes to appear in animal evolution. Many planktonic larvae possess these simple eyespots.
Eyespots cannot form images but enable the animal to sense the direction of light, because the pigment cell shades the photoreceptor from one side. In Platynereis
, selective illumination of one eyespot changes the beating of adjacent cilia by direct cholinergic innervation, resulting in locally reduced water flow. Computer simulations of larval swimming show that these local effects are sufficient to direct the helical swimming trajectories of larvae towards the light. The computer model also shows that axial rotation of the larval body is essential for phototaxis and that helical swimming increases the precision of navigation. Our findings in Platynereis
larvae provide a general mechanistic understanding of phototaxis in marine zooplankton larvae and its regulation by simple eyespots. It is possible that a similar direct coupling of light-sensing with ciliary locomotor control was a principal feature of the first animal eyes.