choanoderm

Choanoderm is the inner tissue layer of sponges (phylum Porifera) that lines the sponge’s canal system and chambers with choanocytes, the flagellated collar cells responsible for feeding and water movement. These cells have a central flagellum surrounded by a collar of microvilli that traps suspended particles as water passes by.

In function, the choanoderm generates water currents by beating its flagella, drawing water through incurrent openings (ostia), across the choanoderm where food particles are captured, and out through the osculum. This water flow supports filter feeding and gas exchange. The choanoderm lines different internal structures depending on sponge architecture: in asconoid sponges it lines the spongocoel, while in syconoid and leuconoid sponges it lines the radial and/or chamber interiors that house choanocytes.

Structurally, the choanoderm is composed of choanocytes arranged to maximize contact with the flowing water, increasing the surface area available for particle capture. While often described as a specialized epithelial layer, sponges lack true true epithelia in the sense of higher animals; the choanoderm is best understood as a functional lining formed by choanocytes embedded in the mesohyl.

Evolutionarily, choanocytes and choanoderm are of particular interest because they resemble choanoflagellates, supporting hypotheses about the evolutionary relationship between choanocytes and the first animal lineages. The choanoderm thus plays a central role in sponge biology and in studies of the origins of multicellularity.