The
choanoflagellates are a group of free-living unicellular and colonial
flagellate eukaryotes considered to be the closest living relatives of the
animals. Choanoflagellates are collared flagellates having a funnel shaped collar of interconnected
microvilli at the base of a flagellum. They have a distinctive cell
morphology characterized by an ovoid or spherical cell body 3–10
µm in diameter with a single apical flagellum surrounded by a collar of 30–40
microvilli (see figure). Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap
bacteria and
detritus against the collar of
microvilli, where these foodstuffs are engulfed. This feeding provides a critical link within the global
carbon cycle, linking
trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in
animals. As the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of
animals.
