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.

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