Sponges have a simple body plan and rely on a constant flow of water for survival. This filter-feeding lifestyle is made possible by specialized cells called choanocytes, or collar cells. These cells are responsible for the sponge’s method of feeding, respiration, and reproduction. Collar cells line the interior walls of sponges and are central to their life-sustaining functions.
Anatomy of a Collar Cell
A collar cell, or choanocyte, lines the inner chambers of a sponge and has three main parts. The most prominent feature is the flagellum, a long, whip-like appendage extending from the cell body into the sponge’s internal cavity. Its motion propels water through the animal.
Surrounding the flagellum’s base is the funnel-shaped collar that gives the cell its name. This collar is made of tiny, finger-like projections called microvilli that form a sieve for capturing food particles. The microvilli are interconnected by a membrane, creating a cohesive filtration surface.
The main body of the collar cell is embedded in a jelly-like matrix called the mesohyl. This part contains the organelles needed for cellular function, including a nucleus and mitochondria. After food is trapped by the collar, it is engulfed by the cell body through phagocytosis and enclosed in a food vacuole for digestion.
Generating the Water Current
The water current is created by the coordinated effort of thousands or millions of collar cells. The synchronized beating of their flagella generates a constant, one-way flow of water. This current draws water into the sponge through numerous small pores, called ostia, on its outer wall.
Inside, water travels through a network of canals and chambers lined with collar cells. After being filtered, the water is expelled through one or more large openings known as oscula. This system ensures water moves efficiently, maximizing contact with the cells for nutrient absorption and gas exchange.
The internal architecture of sponges affects water flow efficiency. The simplest asconoid type has a large central cavity lined with collar cells. More complex syconoid and leuconoid sponges have folded body walls or intricate chamber networks. The leuconoid arrangement is the most complex and efficient, enabling these sponges to grow much larger.
Primary Functions of Collar Cells
Collar cells facilitate the sponge’s filter-feeding process. After trapping microscopic food like bacteria and algae, the cell engulfs the particles. While some digestion occurs within the collar cells, they often pass partially digested nutrients to amoebocyte cells, which transport nourishment throughout the sponge.
The water current is also for other physiological functions. It facilitates gas exchange, allowing cells to absorb dissolved oxygen and release carbon dioxide waste. This current also removes other metabolic wastes and plays a part in sexual reproduction by capturing sperm from the water to fertilize eggs held within the mesohyl.
The Evolutionary Connection
The structure of sponge collar cells has implications for understanding animal evolution. These cells bear a resemblance to a group of free-living, single-celled organisms known as choanoflagellates. Choanoflagellates are protists that also possess a single flagellum surrounded by a collar of microvilli, which they use for locomotion and feeding.
This similarity is strong evidence that choanoflagellates and the earliest animals share a recent common ancestor. Molecular studies confirm that choanoflagellates are the closest living single-celled relatives to the animal kingdom. This suggests the first multicellular animals may have evolved from a colony of choanoflagellate-like cells that became specialized.
The sponge, with its choanocyte-lined interior, represents one of the earliest branches of the animal family tree. It provides a model for how specialized cells could transition from a colonial organism to a multicellular animal. The collar cell is therefore a link to the origins of animal life.