Flagellates are single celled animals, protozoans, that are characterised by the presence of a whip-like flagellum, which they use for locomotion. Choanoflagellates are a subgroup which are distinguished by having a single flagellum surrounded by a collar of microvilli, which are tiny hair-like structures. Choano, in fact, means collar. Basically the organism consists of a cell body with a flagellum at the end surrounded by the microvilli. As the flagellum beats, it sets up water currents that move the animal forward and propel bacteria, detritus and plant cells into the microvilli where they can be digested. There are about 150 species of choanoflagellates described world-wide.
There is still much to be learned about the life cycles of choanoflagellates. Most reproduce by simple fission. It is not known if any species also use any form of sexual reproduction. Some species are known to encyst when conditions turn unfavourable and then excyst when conditions improve.
Choanoflagellates are common and widely distributed, occurring in marine, brackish and fresh water environments. They have been found from the Arctic to the tropics to the Antarctic and from the surface to deep water. Their numbers increase where there are plankton blooms. Some are free swimming while others are more sessile, attaching themselves to detritus in the water column and waiting for prey items to come to them. A few species are colonial, where groups of individuals band together for the protection that added size gives them. Being so numerous, they are undoubtedly an important part of the planktonic food chain which is the basis of all oceanic life.
They are important in another, more theoretical, way as well. They appear to be close relatives to the cells that were the ancestors of all multicellular animals. The similarity between the choanoflagellates and the cells in sponges was first noted as early as the 1840s by the French biologist, Dujardin. The flagellated cells in sponges are called choanocytes and have both flagellae and microvilli, much like the choanoflagellates. In the sponges, which are one step above colonial animals, the choanocytes act to move water currents through the sponge where the microvilli can capture food items, in the same way that free-living choanoflagellates feed. When evolutionary biologists look at choanoflagellates they are looking at living examples of the types of protozoa that banded together, first in colonies and later in complex multicellular organisms that are our own direct ancestors.
for more information: http://tolweb.org/Choanoflagellates http://www.ucmp.berkeley.edu/protista/choanos.html