Choanophyta

Choanophyta, commonly known as choanoflagellates, are a group of free-living unicellular or colonial flagellate eukaryotes. They are predominantly aquatic, found in marine and freshwater environments worldwide. Their most distinctive feature is a single flagellum surrounded by a collar of microvilli. This collar is used for filtering food particles from the water, making choanoflagellates important consumers of bacteria and dissolved organic matter.

The structure of the choanoflagellate cell is highly specialized. The flagellum beats, creating a current that draws water towards the cell. As water passes through the microvillar collar, food particles are trapped and ingested. This feeding mechanism is remarkably similar to that of choanocytes, the feeding cells found in sponges. This similarity has led to the long-standing hypothesis that choanoflagellates are the closest living relatives of animals. Recent molecular phylogenetic studies strongly support this idea, placing choanoflagellates as the sister group to the Metazoa.

Choanoflagellates exhibit diverse life cycles, often involving both asexual reproduction through budding or fission and sexual reproduction. Many species can exist as solitary cells, while others form colonies, which can be simple or complex. The colonial forms are particularly relevant to their proposed evolutionary link to multicellularity, as they display some degree of cell differentiation. Research into choanoflagellates continues to shed light on the origins of animal life and the evolution of complex multicellular organisms.