Choanocyten

Choanocytes, also known as choanocyten, are flagellated collar cells that line the inner surfaces of sponges (Porifera). Each cell bears a single long flagellum surrounded by a ring of microvilli forming a collar. The beating flagellum creates a water current through the sponge's canal system, drawing in food particles and oxygen-rich water.

Food particles such as bacteria and small organic detritus are trapped by the collar and taken into the cell by phagocytosis. The resulting food vacuoles are digested intracellularly, and some nutrients are transferred to archaeocytes for distribution throughout the sponge. In addition to feeding, the water flow aids gas exchange and waste removal.

Choanocytes participate in reproduction: in many sponges, they or archaeocytes differentiate into sperm or eggs, contributing to sexual reproduction. In asexual and regenerative contexts, choanocytes can dedifferentiate into other cell types, such as archaeocytes, demonstrating cellular plasticity within the sponge body plan.

Phylogenetically, choanocytes are characteristic of sponges and bear resemblance to choanoflagellates, a group of unicellular or colonial flagellates. This similarity has informed hypotheses about the origin of multicellularity in animals and the early evolution of Metazoa.

Choanocytes are widespread in most sponge species, forming the choanoderm lining the internal canals and chambers that constitute the sponge’s aquiferous system.