The food-trapping cells of sponges (phylum Porifera) are known as choanocytes. These simple aquatic animals are filter feeders, relying on specialized cells to capture nutrients from the surrounding water. Choanocytes are the primary agents in this feeding process, drawing water through the sponge body and trapping microscopic particles.
The Food-Trapping Cell Choanocytes
The specialized food-trapping cells are formally called choanocytes, which translates to “collar cells” due to their unique structure. Each choanocyte consists of a round cell body embedded in the mesohyl, the sponge’s gelatinous middle layer. Protruding from the cell body are a single flagellum and a delicate collar.
The collar is a cylindrical arrangement of microvilli, which are tiny, finger-like projections of the cell membrane. Choanocytes are strategically positioned, lining the internal chambers and canals of the sponge body.
The Mechanism of Filtration
The feeding process begins with the rhythmic beating of the flagellum at the center of the choanocyte collar. This motion creates a negative pressure, generating a current that pulls water into the sponge through thousands of small pores called ostia. The water flows through the internal canal system and chambers lined with choanocytes before exiting through a large opening called the osculum.
While this water movement delivers oxygen and carries away waste, its primary function is to supply food. As water passes over the choanocytes, the collar acts as a fine-meshed sieve. The microvilli collar traps suspended food particles, including bacteria, tiny plankton, and organic debris, typically smaller than \(0.5\) micrometers in size.
Once trapped, these microscopic particles are often coated in mucus, helping them adhere to the microvilli. The particles slide down the collar toward the cell body where they are engulfed by the choanocyte in a process called phagocytosis. This engulfment forms a food vacuole inside the cell, marking the initial stage of intracellular digestion.
The Role of Other Cells in Digestion
Choanocytes capture and initially process food, but they do not manage the full digestive and nutrient distribution workload alone. Partially digested food vacuoles are transferred to mobile cells known as amoebocytes (or archaeocytes). These motile cells wander throughout the mesohyl.
Amoebocytes complete the intracellular digestion of the food particles. After full digestion, they function as nutrient transporters, distributing absorbed molecules to all other cells within the sponge body. This transport is necessary because the sponge lacks a formal circulatory system.
Amoebocytes also act as undifferentiated stem cells in sponge biology. They can transform into various other cell types as needed for repair, growth, or structural support.