Choanocytes (Collar Cells)

Choanocytes, also known as collar cells, are specialized cells (2 to 10 μm in diameter) that line the inner body of sponges (phylum Porifera).¹ They are a defining feature of the group and play a central role in both feeding and nutrient circulation. These cells collectively make up the innermost cellular layer of a sponge’s body, the choanoderm.

The name ‘collar cell’ stems from a collar-like ring of tiny, finger-like projections called microvilli located at the base of the cell’s body. This collar surrounds a whip-like structure known as the flagellum (plural: flagellae).²

While choanocytes are found in all sponges, their density varies depending on the sponge’s body plan. While sponges with complex body forms have a greater concentration of choanocytes, those with simpler body forms typically have fewer and more scattered choanocytes.

Function

Sponges are typically sessile and cannot move around in search of food. Thus, they rely on the water current generated by the choanocytes for feeding.

• Water enters through tiny pores (ostia) on the sponge’s body.
• The flagella generate a water current towards the choanoderm.
• The collar traps food particles from the passing water.
• The particles are packed into vesicles.
• The vesicles are transferred to amoebocytes, where they are digested.
• The amoebocytes then distribute nutrients throughout the sponge’s body.
• The filtered water exits through a large pore, the osculum.

For more details on how sponges feed using choanocytes, refer to sponge feeding mechanism.

Additional Roles

When water is driven in by choanocytes, it carries along dissolved oxygen with it. Once inside, gaseous exchange takes place, with oxygen being absorbed and carbon dioxide expelled along with the outgoing water. Additionally, choanocytes help sift out large, unwanted particles and ammonia, helping in the expulsion of waste.

In many sponges, these cells can also undergo morphological changes and differentiate into spermatocytes, which produce sperm for reproduction.⁴