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Marine biotops are still largely unexplored ecosystems and many scientific questions remain unanswered. One of the great miracles in oceans is the daily mass migration of plankton in the vertical direction towards the light. Little is known about the mechanism how these myriads of small organisms find their way to the light. Scientists from the European Molecular Biology Laboratory (Heidelberg, Germany) and from the Max-Planck-Institute of Developmental Biology (Tuebingen, Germany) now discovered how the larvae of invertebrate sea dwellers navigate towards the light. The larvae of a marine annelid (an aquatic worm) served as model organism. The larva of Platynereis dumerilii (scientic name of the annelid) has two primitive eye spots for light detection and cilia for movement. When the scientists spotted light on one of the eye spots the larva changed the frequency of its cilia and moved towards the light source. Between the eye spots and the cilia exists a direct neural connection which allows a 1 to 1 ratio between stimulus (light) and reaction (moving towards the light). The scientists suggest that the earliest eyes in evolution could have worked alike.
Larvae of invertebrate marine animals like crustaceans, sponges or starfishes show the most basic eyes within the animal kingdom. These eyes spots consist of a photo receptor for light detection and a pigment cell. This eye type was called by Charles Darwin as ancestral eye, the first eyes in Evolution.
Phototaxis essential for the first organisms
These eyes can detect only the direction from where the light comes. This ability in primitive organisms to move towards a light source is called phototaxis. In the marine ecosystems for the invertebrate animals of the plankton phototaxis is essential for the daily vertical movement from the depth of the oceans towards the light under the surface of oceans. Here the zooplankton finds its food the phytoplankton which needs light for growth.
First eyes developed in the oceans
"Until now it was not clear that animals with their basic eyes and nerve cells can perform directed navigation towards the light" says Detlev Ahrendt, the leader of the research projekt at the European Molecular Research Laboratory in Heidelberg. "We suppose that the first eyes in evolution developed for this purpose", concludes the scientist.
Because the annelid Platynereis dumerilii is a living fossile which dwells in oceans nearly unchanged since millions of years it can be assumed that the situation in early evolution of eyes was similar to what the scientists now revealed in their research.
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