of the algae. Although it may be reversible, bleaching is a direct illustration of the environmental changes that are threatening more and more coral reefs and the many associated species. Anatomy of the adult and reproduction of the species The adult worm is about 4 to 5 mm long. In the anterior part (the head), we distinguish a statocyst or otolith: a gravito-sensor that allows the worm to orient itself in space and to show a negative geotropism (a mechanical stimulus against the wall of a tube containing worms triggers their active dive to the bottom of the tube). The worm has two photoreceptors flanking the statocyst: the perception of light allows the worm to move towards enlightened environments: one speak of positive phototropism. This adaptation likely maximizes the probability of encounter between the free microalga and the non-symbiotic juvenile which also presents this positive phototropism. S. roscoffensis, exposed to different light intensities tends to move and expose itself to higher intensities than those at which free micro-algae perform optimal photosynthesis. Other works have shown experimentally that if it has the choice, the photosymbiotic worm avoids exposing itself to extreme light conditions: either too weak or too strong. In its environment, it is suspected that a temporary burying allows to escape to too strong intensities (photoinhibition). This worm has a central (brain) and peripheral nervous system. When one amputates the anterior part ("the head" containing the brain), one can observe a capacity of regeneration of the whole central nervous system in about twenty days with a joint recovery of the normal behavior. However, the different biological functions are not regenerated at the same speed: if the phototropism, which is associated with the regeneration of the photoreceptors, is recovered quickly, the geotropism, which is associated with the regeneration of the statocyst, is not recovered for several weeks. S. roscoffensis does not have a circulatory blood system: the diffusion of oxygen through the tissues is passive. Part of this oxygen also comes from the photosynthetic activity of the in hospite algae. The surface of the animal is abundantly ciliated and strewn with numerous mucus-secreting glands. Mucus provides a physical network allowing the worms to move in the seeps of seawater. An in-depth study on the behavior of S. roscoffensis in situ explains that the only possibility for the worm to move horizontally is to make a support (invisible to the naked eye)