Charles Darwin spent a great deal of time puzzling over how the complex machinery of the eye arose. In 1859, he proposed the theory of 'proto-eyes', or very simple light-sensing cells, as a starting point for eye evolution. Since then, surprisingly little progress has been made in understanding the neural functioning of the most rudimentary eyes. Now, researchers in Germany have established a link between light perception and locomotion in the larvae of a marine worm, Platynereis dumerilii, confirming that Darwin was on the right track.

Many tiny marine invertebrates such as P. dumerilii larvae migrate vast distances from the deep sea to light-drenched surface waters in order to disperse. Researchers suspected that simple light-sensing eyespots mediate this navigation towards the light — a phenomenon known as phototaxis — but it wasn't clear how sensory information received by the eyespots translated to locomotion. While a postdoc with Detlev Arendt at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Gáspár Jékely set out to find an answer.

Jékely and his colleagues began by asking where a neural impulse generated by illumination of an eyespot might travel. P. dumerilii larvae are spherical, with a belt of hair-like cilia at their midpoint. Eyespots are situated on either side, just above the cilia, and consist of just two cells: a light-sensing 'photoreceptor' nerve cell and a pigmented cell that shades the photoreceptor so that it detects light from only one half of the potential field of view.

In animals with complex nervous systems, neural impulses are sent from photoreceptor cells to the brain's visual centres. When the researchers labelled a P. dumerilii larva's photoreceptor and traced the path of its nerve fibre, they were surprised to find that, rather than connect to the larva's simple brain, it links directly to the nearby cilia belt. These cilia serve as the swimming motor for the larvae, propelling them with a beating motion. The finding revealed a direct coupling between the light-sensing eyespots and the phototactic swimming of marine larvae (see page 395). “We expected some more complex neurobiology behind it and it was just fascinating to see that it was so simple,” says Jékely, now at the Max Planck Institute for Developmental Biology in Tübingen, Germany.

The team also used fast video microscopy to detail how the eyespots mediate navigation. By stimulating one eyespot with a light beam, they observed that the cilia adjacent to the eyespot slow their beating in response to the light. The cilia on the opposite side do not slow, so propel the larvae with greater force, steering them towards the light much as differential pull on the oars of a canoe causes it to change direction.

Next, the group enlisted the help of co-author François Nédélec, a physicist at EMBL, to create a computerized model of the swimming behaviour. The resulting program showed that the larvae must continually turn on their axis so that one eyespot is shaded and the other is turned to the light. This explains why almost every type of marine plankton, from sponge and jellyfish larvae to single-celled protists, adopt a spiral motion when swimming towards light. “Basically, every simple organism that is able to follow light in open water will use this strategy,” says Jékely. Some of the finer details may differ, he says, “but I suspect in most cases there will be a very simple coupling between eye and cilia”.