Choanoflagellates are micro-eukaryotes that live in aquatic environments. All choanoflagellates have a single-celled swimmer stage characterized by a single apical flagellum that caps an ovoid body with actin filled microvilli. The flagellum of these heterotrophic organisms generates currents that draw food particles, namely bacteria, into their collar where they are phagocytosed. Thus choanoflagellates are a key link between trophic levels in the ocean.

The cellular organization of the choanoflagellate swimmer bears striking similarity to sponge choanocytes & comparative genomics has confirmed that choanoflagellates are the closest living relatives of animals. Their study has yielded insights into the origins and cell biology of the last common ancestor of animals