Marine sacoglossa sea slugs, mainly of the genus Elysia, have developed a unique functional nutritional mode in which they gain the capacity for phototrophic-mediated carbon acquisition by sequestering macroalgae plastids into tubule cells of their digestive diverticula (kleptoplasty). as occuring in algae and plants, exposure to excessive light is expected to be a major cause of stress to the photosynthetic apparatus of photosynthetic symbioses. When the absorbed light energy exceeds the capacity of photochemical pathways, reactive oxidative species accumulate in the cell and cause damages to the photosynthetic apparatus (photoinhibition), mainly through the degradation of protein D1 in photosystem II. To cope with high ambient light levels, algae and plants have developed a range of physiological photoprotective processes, the most important being the xanthophyll cycle, the antioxidant enzyme system and the de novo synthesis of D1 protein. In the case of marine photosynthetic symbioses typically inhabiting surface waters or intertidal habitats, the efficient functioning of photoprotective mechanisms may be of crucial importance for their survival as photoinhibition reduces the amount of photosynthetically fixed carbon made available to the animal host and damage the photosynthetic apparatus in the chloroplast. Anatomical and behavioural traits can also enhance the performance and functional longevity of chloroplasts in different light regimes. This adaptation would be functionally equivalent to chloroplast avoidance movements and leaf fold described in plants, and the migratory behaviour of motile microalgae, shown to provide effective photoprotection against photoinhibition. This work will provide new insights into the photophysiology and photobehaviour of a marine photosynthetic symbiose model occurring in North-eastern Atlantic temperate waters, advancing the knowledge on the performance and functional longevity of kleptoplasts.