choanocyte

Choanocytes, also known as collar cells, are specialized, flagellated cells that line the interior chambers and canals of most sponges (phylum Porifera). Each cell bears a single, motile flagellum surrounded by a ring of microvilli that form a collar.

The beating of the flagellum generates a water current that draws water through pores (ostia) into the sponge’s canal system and exits via the osculum. This active circulation brings suspended food particles toward the collar, where they are captured by phagocytosis on the microvilli and then digested intracellularly. The collar cells also contribute to gas exchange and waste removal as water passes through.

In sexual reproduction, choanocytes can differentiate into sperm cells, providing a direct route to the germ line in some sponges; eggs are typically formed by other cell lineages such as archaeocytes. Choanocytes can also dedifferentiate and differentiate into other cell types as needed, participating in tissue renewal and regeneration.

Choanocytes are a defining feature of sponges and are central to their filter-feeding lifestyle. Their efficiency in trapping particles supports the sponge’s nutrition and energy budget, and their ability to transition into other cell forms contributes to the sponge’s developmental plasticity.