View to a Krill: Secrets of Plankton Eyes

Nov. 19, 2008 -- Biologists on Wednesday explained how the larvae of marine zooplankton can see with just two cells, using what is believed to be the world's simplest vision system.

Zooplankton are tiny creatures such as copepods and krill that drift in the ocean's water columns, swimming up from the depths towards the light in order to graze on marine plants called phytoplankton near the surface.

This movement, called phototaxis, is the biggest biomass displacement in the world.

In a study published by the British-based journal Nature, European scientists looked at the larvae of the marine ragworm Platyneris dumerilii to try to explain how plankton are able to do the phototaxis trick.

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The larva has just two eye cells, consisting of a pigment cell and a light-sensitive cell, say the investigators.

The cells are unable to form images but enable the plankton to sense the difference between light and dark and send appropriate signals to its swimming mechanism, say the investigators.

First, the pigment cell absorbs light and casts a shadow over the photoreceptor cell. The shape of the shadow varies according to the position of the light source.

The photoreceptor cell then converts this light signal into electricity, sending it in a signal along a nerve that connects to a band of cells endowed with thin hairs, called cilia, that beat to displace water.

The basic but effective system could explain how the very first eyes in evolution may have worked, say the team from the European Molecular Biology Laboratory (EMBL) and the Max Planck Institute.

"For a long time, nobody knew how the animals do phototaxis with their simple eyes and nervous system," said EMBL's Detlev Arendt.

"We assume that the first eyes in the animal kingdom evolved for exactly this purpose. Understanding phototaxis thus unravels the first steps of eye evolution."