Symsagittifera roscoffensis is a plathelminth living in symbiosis with the green algae Tetraselmis convolutae. Host and symbiont are a model system for the study of endosymbiosis, so far mainly focused on their biochemical interactions. S. roscoffensis is well known for its positive phototaxis that is hypothesized to optimize the symbiont's light perception for photosynthesis. In this study, we conducted a detailed analysis of phototaxis using light sources of different wavelength and brightness by videotracking. Furthermore, we compared the behavioral data with the electron transfer rate of the photosystem from cultured symbiotic cells. The symbiotic algae is adapted to low light conditions showing a positive electron transfer rate (ETR) already at a photosynthetically active radiation (PAR) of 0.112 µmol m−2 s−1 (mol photons per square meter and second), and S. roscoffensis showed a positive phototactic behaviour for light intensities up to 459.17 µmol m−2 s−1 which are not optimal regarding the needs of the symbiotic cells and even may harm host and symbiont. Red light can not be detected by the animals and therefore their eyes seem to be not suitable for measuring the exact photosynthetically active radiation to the benefit of the photosymbionts.

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