A microscopic yellow-green alga that lives symbiotically within the cells of some marine invertebrates: Zooxanthellae are plantlike flagellate protozoans in the order Dinoflagellida, photosynthesizing symbiotes inside the cells of sponges, corals, and others.

These unicellular yellow zooxanthellae contain xanthophyll and live as symbionts in the tissues of higher organisms, such as sponges, coelenterates, worms, etc.