Choanocytes: The Feeding Cells of Sponges

Choanocytes (collar cells) are the flagellated, phagocytic cells lining the internal chambers of sponges (Porifera). They drive both feeding and water flow.

Structure and Function

• Collar of microvilli: Surrounds a single flagellum; traps bacteria, phytoplankton, detritus (0.1–10 µm).
• Flagellum: Beats rhythmically, creating unidirectional current: ostia → canals → choanocyte chambers → osculum.
• Phagocytosis: Captured particles engulfed into food vacuoles.
• Digestion: Lysosomal enzymes break down food; nutrients diffuse to adjacent cells (archaeocytes/amoebocytes) for distribution.

Additional Roles

• Gamete production (in some species)—choanocytes differentiate into sperm.
• Secrete spicules (skeletal elements) in calcareous sponges.

Significance

Choanocytes exemplify the evolutionary link between unicellular choanoflagellates and animal multicellularity. Their collar-flagellum apparatus is homologous to choanoflagellate feeding structures.