works of Louis Martin refer to the presence of S. roscoffensis in the interstitial seawater flows at low tide nights showing bright reflection of the Sun on the Moon. The latter has also shown that the worms, captive in the laboratory, gather mostly on the surface of the container when the tide is low and migrate to the bottom when the tide is high. In the thermo and photo-regulated chambers where the worms do not physically perceive the rising or falling tide, this vertical oscillation persists for 4 to 5 days and is then gradually lost and evolves in fine in a random occupation of the space in the container. Another movement characteristic to the species, sometimes observed in puddles of seawater and in the laboratory, brings together hundreds or even thousands of individuals to realize a circular procession. Modeling work shows that this social behavior is self-organized and is initiated by the movement of an individual who drives the worms to its proximity, thereby propagating the movement to an increasing density of worms that move in a coordinated way. Symsagittifera roscoffensis is thus a model for studying and understanding how individual behavior can lead to collective behavior. As early as 1924, zoologists observed the behavior of S. roscoffensis in response to the acidification of its environment. After forced diffusion of CO2 (carbon dioxide) gas into seawater, the dissolved CO2 concentration increases until the medium is saturated and generates carbonic acid and releases an H+ ion (CO2+ H2O ↔ H+ + HCO3-). The increase in H + ions causes a decrease in pH and therefore an increase in acidity. Under the effect of the abrupt and pronounced decrease of the pH, S. roscoffensis contracts, turns on itself, then gradually expels its algal partners. The worm gradually loses its green color and usually dies after a few hours. This experience mimics the effects of ocean acidification, which is today the major consequence (with the rise in temperatures) of anthropogenic activities, particularly the combustion of fossil fuels, which generates very high concentrations of CO2. This photosymbiotic system "S.roscoffensis / T. convolutae" allows to explore and evaluate the effect of acidification on oceanic photosymbiotic species, the most emblematic of which are corals. Coral bleaching, which results mainly from the rise in ocean temperature but which is aggravated with acidification, results in the dissociation of the algae / animal partnership leading to the expulsion