Choanoflagellates (Choanoflagellida) are free-living, single-celled, and colony-forming eukaryotes that are ubiquitous in aquatic environments. They are believed to be the sister group to the animals (kingdom Animalia). As their name implies, choanoflagellates (collared flagellates) 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. 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 (see this animation from the University of Alberta).
Dujardin, a French biologist interested in protozoan evolution, noted the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841 (Leadbeater and Kelly 2001). Over the past decade, this hypothesized relationship between choanoflagellates and animals has been supported by independent analyses of multiple unlinked sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Wainright et al. 1993; Mendoza et al. 2002; Burger et al. 2003; Ruiz-Trillo et al. 2004, 2006; Steenkamp et al. 2006). Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa and negate the possibility that choanoflagellates evolved from metazoans (Lavrov et al. 2005). Finally, recent studies of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes (King et al. 2003). Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their last common ancestor and the earliest events in metazoan evolution.
Choanoflagellates are found globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones. Although most sampling of choanoflagellates has occurred between 0 m and 25 m, they have been recovered from as deep as 300 m in open water (Thomsen 1982) and 100 m under Antarctic ice sheets (Buck and Garrison 1988). Many species are hypothesized to be cosmopolitan on a global scale, while other species are reported to have restricted regional distributions (Thomsen et al. 1991).
(slightly modified from ChoanoWiki contributors 2011)