Researchers have determined the mechanism by which the world’s simplest vision system works. A team of biologists spent a decade studying the larvae of the marine rag-worm Platynereis, a tiny creature with just two cells that respond to light and direct the worm to swim towards it. The rag-worm and other zooplankton like krill 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 [AFP].
The rag-worm has two cells that work together as “proto-eyes”: one pigment cell and one light-sensitive cell. 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 [AFP]. So although the worm sees no images, it can sense the difference between light and dark and swim in the right direction.
In the study, published in Nature [subscription required], lead researcher Detlav Arendt shows that the worms have a direct connection between their vision cells and their movement cells: The photoreceptor axons [connect] directly to neighboring ciliated cells in Platynereis larvae. “Essentially there is no nervous system in between” the photoreceptors and ciliated cells, Arendt said. “The axons contact ciliated cells in the neighborhood” [The Scientist].
The movement of a tiny worm is of deep interest to biologists because it provides insight into how sophisticated vision may have begun evolving in mammals. Photobiology expert John Spudich commented that Arendt’s “elegant study” identifies a photosensory mechanism that may be the “key evolutionary intermediate suggested by Charles Darwin’s reflections on the evolution of human vision” [The Scientist]. Darwin famously marveled at the evolution of the eyeball, writing that at first its development through natural selection seemed “absurd in the highest possible degree,” but then added that it could be understood “if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist.”
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Image: European Molecular Biology Laboratory