Innate behavioural response of Symsagittifera roscoffensis to selected environmental conditions The sandy eulittoral of the west coast of Europe is the habitat of the green worm Symsagittifera roscoffensis. It’s green colour results from an endosymbiosis with the micro algae Tetraselmis convolutae. The availability of light and nutrients plays a fundamental role for both partners. This theses elucidates the behaviour of the host S. roscoffensis under different light conditions and its ability to react to nutrients. S. roscoffensis chooses optimal light conditions for its symbiont and buries itself into the sand at high light intensities and protects its symbiont against photo damage.<eng>