Wikipedia
Tetraselmis
Tetraselmis is a genus of phytoplankton. Tetraselmis has a very high lipid level; their amino acids stimulate feeding in marine organisms. Tetraselmis is green, motile, and usually grows 10 µm long x 14 µm wide.
The species T. convolutae is a symbiont of the acoelomate animal Symsagittifera roscoffensis (and can also live freely in the ocean).