Choanoflagellates are single-celled aquatic organisms that have garnered significant attention in biology due to their unique placement in the tree of life. They inhabit both marine and freshwater environments globally, acting as filter feeders that consume bacteria and small detritus. Understanding choanoflagellates requires clarifying their physical characteristics, addressing the outdated nature of the term “protist,” and examining the genetic evidence linking them to the earliest forms of animal life.
What are Choanoflagellates?
Choanoflagellates are microscopic eukaryotes named for their distinctive morphology: a single, whip-like appendage called a flagellum surrounded by a “collar” of thin, actin-filled microvilli. This structure is the organism’s primary tool for survival. The flagellum beats to create a water current that directs fluid toward the collar.
The collar of microvilli functions as a sieve, capturing bacteria and other small food particles suspended in the water current. Once trapped, these food particles are ingested by the cell body through phagocytosis. Choanoflagellates are found as free-swimming, solitary cells, or they may live as sessile organisms attached to a substrate. Some species, such as Proterospongia and Salpingoeca rosetta, also exhibit colonial lifestyles, forming clusters of cells on a stalk or in planktonic clumps.
Addressing the Protist Classification
The short answer is that choanoflagellates are often referred to as protists, but this term is largely a non-phylogenetic descriptor used for convenience. Historically, any single-celled eukaryote that was not a plant, animal, or fungus was placed into the now-obsolete Kingdom Protista. Choanoflagellates fit this description because they are eukaryotes that lack the tissue organization of animals, the cell walls of fungi, and the complex photosynthetic structures of plants.
Modern molecular phylogeny, which relies on genetic evidence rather than just physical appearance, has reorganized the tree of life into “supergroups.” Under this system, choanoflagellates are placed within the supergroup Opisthokonta, a clade that also includes animals (Metazoa) and fungi. The Opisthokonta are defined by the shared trait of having a single posterior flagellum in their motile cells, such as in choanoflagellate cells or animal sperm. This placement signifies that choanoflagellates are more closely related to animals than they are to many other organisms traditionally called protists, such as amoebas or algae.
The Unicellular Link to Animal Life
The placement of choanoflagellates as the closest living relatives to animals (Metazoa) is their most significant biological feature, offering a glimpse into the ancestors of multicellular organisms. The first line of evidence is a morphological resemblance: the choanoflagellate cell structure is virtually identical to the choanocyte, or “collar cell,” found in sponges, which are considered the most primitive living animals. This similarity suggests that the choanoflagellate form represents the type of cell that gave rise to the first animals over 600 million years ago.
Beyond physical form, genetic analysis provides substantial support for this evolutionary link, revealing that choanoflagellates possess many gene families previously thought to be exclusive to multicellular animals. They share genes responsible for cell adhesion, such as those that code for cadherins, which are necessary for cells to stick together and form tissues in animals. They also possess genes involved in cell-to-cell signaling, including the full three-part system for phospho-tyrosine signaling proteins. This complex signaling system is used by animal cells for communication in processes like immune response and hormone stimulation, suggesting the molecular machinery for multicellularity was already present in the unicellular ancestor.
Further supporting this link is the ability of some choanoflagellates, like S. rosetta, to switch their lifestyle from solitary to colonial in response to environmental cues, such as the presence of specific bacterial lipids. These colonial forms, or “rosettes,” are temporary multicellular arrangements that resemble the early developmental stages of animal embryos. This ability to form simple colonies, coupled with the presence of animal-specific adhesion and signaling genes, indicates that the genetic toolkit required for the evolution of complex animal body plans was inherited from a choanoflagellate-like ancestor.