Symsagittifera roscoffensis

Symsagittifera roscoffensis, also called the Roscoff worm, the mint-sauce worm, or the shilly-shally worm, is a marine acoel worm. The origin and nature of the green color of this worm stimulated the intrigued zoologists in the 1870s. It was discovered that the coloring resulted from the symbiosis between the animal and a green micro-alga, the species Tetraselmis convolutae, hosted under its epidermis. It is the photosynthetic activity of the micro-algae in hospite (in the host's body) that provides the essential nutrients for the worm. This partnership is called photosymbiosis, from "photo", "light", and symbiosis, "that which lives with". These photosynthetic marine animals live in colonies (up to several million individuals) in the tidal zone.