green cells in hospite were in fact flagellated microalgae in the free living state and that they were the "infective" factor causing the green coloration of adults (absent in non-symbiotic juveniles). Thus, there is no vertical transmission of the symbionts (transmitted by the parents) but a horizontal acquisition at each new generation (i.e., the symbionts are in the environment). Tetraselmis convolutae belongs to the class Chlorodendrophyceae within the division Chlorophyta. This alga has remarkable characteristics, including four flagella, a theca (polysaccharide envelope) and a vacuole (stigma or "eyespot") that contains photo-receptor molecules. T. convolutae lives in the free living state in the water column but is mainly benthics. Thus, in hospite, the alga does not have the same phenotype as in the free living state: it no longer has its flagella, its theca and stigma. These phenotypical differences did not allow Geddes, Delage and Haberlandt to deduce that the green cells in the tissues could have been micro-algae. Life cycle Modus vivendi: a highly integrated partnership—not only additive Few works describe the trophic nature of the exchanges between the animal and its photosynthetic partners. The photosynthetic activity provides, in addition to oxygen, various organic compounds: amino acids, proteins, polysaccharides and fatty acids. Micro-algae recycle uric acid (coming from the worm's nitrogen metabolism) for the synthesis of its amino acids / proteins. The assimilation of uric acid by S. roscoffensis has attracted the attention of early experimenters, including Louis Destouches aka Louis Ferdinand Céline who carried out work on the physiology of these animals and who concluded in 1920 "that it is therefore very likely that symbiotic exchanges lead to the transformation of uric acid into nitrogenous food for zoochlorella". Douglas has formally shown that endogenous uric acid from the worm (derived from its metabolism) is a source of nitrogen for micro-algae and that non-symbiotic juveniles contain uric acid crystals that disappear 15 to 20 days after the establishment of symbiosis. However, the author proposes that if exogenous uric acid is not used by the worm and that if its concentration in the medium culture decreases over time it is because of a bacterial activity associated with the animals. Near resurgences rich in nitrate, in intertidal areas where live S. roscoffensis, worms are able to assimilate significant amounts of nitrate depending on the exposure and intensity of light. This quantity is ten times greater than that absorbed by the alga in