Choanoflagellates are unicellular eukaryotes that form colonies and are the closest living single-celled organisms to animals. They filter feed using a collar and are found in aquatic environments, including 100m below Antarctic ice sheets. They are consumed by larger organisms and are related to specialized sponge cells called choanocytes.
Choanoflagellates (meaning “collared organism with a tail”) are thought to be similar to the single-celled organism from which all animals evolved, and the closest living single-celled to animals. Choanoflagellates are a type of protozoa, i.e. a unicellular eukaryote (complex celled organism), in contrast to other unicellular organisms such as bacteria and archaea. Like many other microbes, choanoflagellates form colonies, which may have been the precursors of the first multicellular organisms.
Most choanoflagellates are sessile, meaning they stay in one place, attached to the ocean floor with a stalk. The flail whips rapidly, pushing the water up. This flagellum is surrounded by microvilli which form the “collar” that gives the microbe its name. As the flagellum pushes up to create a suction current, causing bits of food to get stuck in the microvilli. This strategy is called filter feeding and is also used by fin whales, krill, sponges and other marine organisms. The cell body of the choanoflagellate is also covered by a shield called the theca.
Like many other microorganisms, choanoflagellates are ubiquitous in aquatic environments. They have also been found 100 m (328 ft) below Antarctic ice sheets.
The genome of the choanoflagellate has been sequenced and subsequent analyzes have shown that they are closely related to animals. In their behavior, choanoflagellates are among the most complex of protozoa. They even build small flint “houses” called lorica (meaning “armour”). For further evidence of the close relationship between choanoflagellates and animals, we see that there are specialized sponge cells called choanocytes (“collar cells”) that bear a great resemblance to choanoflagellates. Choanocytes can also be found in nematodes.
Coanoflagellates are a variety of nanoplankton, which refers to the smallest (unicellular) form of organisms in the sea. As such, they are consumed by a variety of larger organisms, including krill and many others. When a choanoflagellate dies, it is rapidly removed from the ocean floor, floating around until it is consumed.
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