Sponges, classified under the phylum Porifera, are among the simplest and oldest known multicellular animals on Earth. They are sessile organisms and do not actively pursue food. Instead, the majority of sponges are highly efficient filter feeders, constantly drawing in water to capture suspended particles for nutrition. This unique feeding strategy is supported by a specialized body architecture that functions as a biological pump and sieve. Understanding what sponges eat requires an exploration of the plumbing system that moves the water, the cells that capture the food, and the types of microscopic particles they consume.
The Anatomy of Water Flow
The feeding process begins with the sponge’s unique body structure, which is riddled with microscopic openings. Water enters the sponge through thousands of tiny pores on the outer surface called ostia, which lead into an internal network of canals. These canals branch and narrow, directing the flow toward specialized internal chambers.
The water current moves through these passages, eventually reaching the large, central cavity known as the spongocoel. From the spongocoel, the water is expelled through one or more large openings called oscula. This unidirectional movement, from the numerous small ostia to the fewer, larger oscula, prevents the re-ingestion of waste.
Cellular Mechanisms of Food Capture
The mechanism for generating the water flow and capturing food lies with specialized cells called choanocytes, or collar cells. These cells line the internal surfaces of the sponge’s chambers and possess a single, whip-like flagellum. The continuous beating of the choanocyte flagella creates a negative pressure that draws water into the sponge and propels it through the canal system.
Each choanocyte is equipped with a delicate collar of microvilli surrounding the base of the flagellum, which acts as a minute sieve. As water is drawn past the choanocyte, food particles become trapped in the mucus-coated microvilli collar, and the cell then engulfs the particle through phagocytosis. Since sponges lack a true digestive system, this process constitutes intracellular digestion, occurring within food vacuoles.
Once digested, the nutrients are transferred to another type of mobile cell, the archaeocyte, which resides in the gelatinous middle layer of the sponge body. These amoeboid cells act like the circulatory system, moving throughout the sponge to distribute the captured nutrients for energy and growth. Archaeocytes also consume particles too large to pass through the ostia, which they engulf from the outer surface.
Primary Diet: Microbes and Particulate Matter
Most sponges are microphagous, meaning their diet is composed of small particles suspended in the water column. Their primary food source is bacteria, which typically measure between 0.2 and 1 micrometers in size. Choanocytes are so effective that bacteria-sized matter often accounts for the largest portion of a sponge’s food supply.
The diet also includes other microscopic organisms like picoplankton and nanoplankton, such as small unicellular algae and protists. Sponges can filter out particles up to approximately 50 micrometers. They are remarkably efficient; some reef sponges remove nearly 99% of bacteria from the water they process. To meet energy needs, a single sponge can process thousands of liters of water every day.
The Exception: Carnivorous Sponges
A small, evolutionarily distinct group of sponges, primarily in the family Cladorhizidae, has abandoned the filter-feeding lifestyle. These carnivorous sponges are typically found in the nutrient-poor deep sea, where the density of suspended microbes is too low to support filter-feeding. These species lack the choanocytes and the complex canal systems that define their filter-feeding relatives.
Instead of filtering water, these predatory organisms use specialized structures to capture small animals, such as tiny crustaceans. Their bodies are covered in fine, hook-like spicules that function like a biological Velcro, ensnaring any prey that brushes against them. Once a small invertebrate is captured, the sponge’s amoeboid cells migrate to the site and slowly engulf the prey. Digestion is then performed externally, with the cells forming a temporary cavity around the prey and breaking it down before absorption.