Sponges belong to the Phylum Porifera, one of the most ancient lineages of animals. They exist primarily in marine environments, where they are permanently attached to a substrate, a lifestyle known as sessile. Sponges possess a simple body plan, lacking true tissues, organs, or a nervous system. Their existence revolves around maintaining a constant flow of water through their porous bodies.
Sponges as Heterotrophs
Sponges are classified as obligate heterotrophs, meaning they must obtain carbon and energy by consuming organic matter produced by other organisms. Their diet is composed entirely of organic material suspended in the water column. Primary food sources include microscopic particles like bacteria, small plankton, and various forms of organic detritus. Some sponges are particularly efficient at utilizing dissolved organic matter (DOM), which can account for up to 90% of their overall heterotrophic diet.
The Specialized Water Flow System
The movement of water delivers food and oxygen to the sponge’s cells. Water enters the sponge body through thousands of tiny inlet pores called ostia, which cover the outer surface. These small openings lead into a complex network of internal passageways known as the canal system. Water is directed through various canals and into specialized internal chambers, the complexity of which varies greatly among sponges. Ultimately, the filtered water collects and is expelled through one or more large exit holes, called oscula, usually located at the top of the sponge body.
The Cellular Mechanics of Filter Feeding
The actual capture of food occurs within the internal chambers, which are lined by specialized cells called choanocytes, or collar cells. Each choanocyte possesses a single, whip-like projection called a flagellum that beats constantly to generate the water current.
The coordinated beating of millions of these flagella across the internal chambers pulls water through the sponge and creates a localized flow past each choanocyte. Surrounding the base of the flagellum is a delicate ring of microvilli, known as the collar. This collar acts as a fine mesh sieve.
As the water current is driven through the collar, microscopic food particles, particularly bacteria less than 0.5 micrometers in size, become physically trapped in the microvilli mesh. Once a particle is secured on the collar, the choanocyte engulfs it whole through a process called phagocytosis.
Phagocytosis involves the cell membrane extending pseudopods to surround the particle, forming a membrane-bound food vacuole inside the choanocyte. Choanocytes typically capture up to 80% of the sponge’s total food supply.
Post-Capture Nutrient Processing
Once the food particle is enclosed within a vacuole inside the choanocyte, the process shifts from capture to intracellular digestion. The choanocyte begins to break down the organic matter using digestive enzymes contained within the vacuole.
However, the choanocyte often does not complete the digestive process itself. Instead, the partially digested food vacuoles are passed to mobile cells, known as amoebocytes or archaeocytes, which reside in the gelatinous mesohyl layer between the outer and inner cell layers.
These amoeboid cells move throughout the sponge body, completing the digestion of the food within their cytoplasm. They then act as nutrient delivery vehicles, distributing the fully processed nutrients to all other cells that make up the sponge structure.
Metabolic waste products are released by the individual cells into the exiting water current and carried out through the osculum.