Dujardin, a French biologist interested in protozoan evolution, recorded 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 upheld by independent analyses of multiple unlinked sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Steenkamp, et al., 2006; Ruiz-Trillo et al. 2004; Ruiz-Trillo et al. 2006; Burger, et al., 2003; Mendoza, et al., 2002; Wainright, et al., 1993). 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, 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 free-living, single-cell and colony forming eukaryotes that are ubiquitous in aquatic environments. 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).
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)