Choanoflagellates are microscopic aquatic organisms that offer insights into the diversity of life in watery environments. These single-celled creatures inhabit a wide range of aquatic habitats across the globe, from the polar regions to the tropics.
What Are Choanoflagellates?
Choanoflagellates are single-celled eukaryotic organisms distinguished by their distinctive cellular structure. Each cell typically has an ovoid or spherical body, measuring between 3 and 10 micrometers in diameter. A single, whip-like flagellum extends from one end of the cell, surrounded at its base by a ring of fine, interconnected protrusions called microvilli. This funnel-shaped arrangement gives them their name, which translates to “collar-flagellates.” The microvilli are so closely packed that they often appear as a solid, transparent collar under a light microscope.
While many choanoflagellate species exist as solitary individuals, some can form simple colonies. These colonies can take various shapes, such as spherical rosettes or linear chains, and some species may transition between solitary and colonial stages throughout their life cycle.
Life and Habits
Choanoflagellates primarily obtain their nutrition through filter-feeding, a process facilitated by their specialized collar and flagellum. The continuous beating of the flagellum creates a water current that draws bacteria and other small particles towards the collar. These food particles become trapped by the microvilli, which act like a sieve. Once captured, the particles are then ingested by the cell.
They are found globally in various aquatic environments, including marine, brackish, and freshwater habitats. They can be free-swimming, propelled through the water by their flagellum, or sessile, meaning they attach themselves to a substrate. In colonial species, individual cells within a colony cooperate in feeding, with the collective flagellar action enhancing water flow and food capture efficiency. Some colonial choanoflagellates can even coordinate their movements, such as the inversion of sheet-like colonies in response to light, demonstrating complex collective behaviors.
Their Place in Evolution
Choanoflagellates hold a significant position in the tree of life as the closest living relatives to animals. This close relationship is supported by genetic studies and striking morphological similarities. The feeding cells of sponges, known as choanocytes or “collar cells,” bear a remarkable resemblance to choanoflagellates, possessing a flagellum surrounded by a collar of microvilli. This structural likeness suggests a common ancestry between choanoflagellates and the earliest animals.
Studying choanoflagellates provides insights into how multicellularity might have evolved from single-celled organisms. The ability of some choanoflagellate species to form colonies, where individual cells remain attached after division, offers a living model for understanding the initial steps towards complex animal life. While the exact process of animal origins is still being investigated, the shared features and genetic toolkit between choanoflagellates and animals indicate that the ancestor of all animals likely resembled a choanoflagellate-like organism.