from algae ... they represent however, in their current state of adaptation, a specific tissue of the worm dedicated to assimilation". It is in an academic book entitled Plant-Animals published in 1912 by Frederick Keeble, the co-discoverer of the nature and the origin of micro-algae, where a set of works is describing the specific relationship between Convoluta roscoffensis and its photosynthetic partners. Luigi Provasoli, a pioneering micro-algologist in the art of isolating and cultivating micro-algae, maintained for several years between 1965 and 1967 in New York (Haskins Laboratories, now in Connecticut) generations of S. roscoffensis directly received from the Station Biologique de Roscoff. Thus generations of non-symbiotic juveniles were born in New York and were used to test the specificity of association between S. roscoffensis and different species of micro-algae. A century after its formal description, in 1991, thanks to the techniques of molecular biology and the emerging tools of bioinformatics, the phylogeny of acoela was revisited and modified: the green Convoluta of Roscoff, Convoluta roscoffensis became Symsagittifera roscoffensis, the "Roscoff worm". Biotope and ethology Symsagittifera roscoffensis lives in the tidal balancing zone, preferentially in a sandy substrate, poor in organic matter. The accumulation of the latter generates reduced, hypoxic or anoxic conditions, which promote the development of anaerobic bacteria releasing compounds such as hydrogen sulfide (H2S) incompatible with the oxygen requirements of the worm. Circatidal rhythms (dynamic alternation of high and low tides) influence the behavior of these animals that migrate vertically at each tide: at rising tide the colonies sink into the sand, they rise to the surface at the beginning of the ebb tide and become visible in interstitial seawater flowing. Sand particle size is an important factor because it determines the ease of vertical movements of the worms. The latter always remain in contact with seawater when the interstitial flows dry up during the ebb tide, and are also protected (buried in the sand) at rising tide, from dispersive effects of the waves. The colonies are located on the upper part of the foreshore, which is the least long-term covered layer of water during the tidal cycle. Colonies of S. roscoffensis are therefore in this place theoretically exposed to the longest light exposure to maximize the photosynthetic activity of micro-algae partners. Light is an essential biotic factor since the photosynthetic activity of algae in hospite is the only contribution to nutrient intake for the animals. The