Sponges are ancient, multicellular animals found in aquatic environments, primarily marine but also freshwater. Sessile, they attach to an underwater surface throughout their adult lives. Despite their plant-like appearance, sponges are animals, representing one of the earliest branches on the animal evolutionary tree. Their unique adaptations allow them to acquire nutrients effectively without complex organ systems.
The Unique Feeding Mechanism of Sponges
Most sponges obtain food through filter feeding. Water enters the sponge through tiny pores called ostia. This water then flows through a system of internal canals and chambers. Specialized cells known as choanocytes drive the water movement.
Choanocytes line the internal chambers and canals, each possessing a whip-like flagellum. The beating of these flagella creates a constant water current, pulling water into the sponge. Surrounding each flagellum is a collar that acts like a sieve. As water passes through this collar, microscopic food particles become trapped. The filtered water then exits the sponge through a larger opening called the osculum.
Dietary Preferences of Sponges
Filter-feeding sponges primarily consume microscopic organisms and organic matter suspended in the water column. Their diet includes bacteria, single-celled algae (phytoplankton), and tiny zooplankton. Sponges also ingest particulate organic detritus, which consists of small fragments of decaying organic material. Some species may also absorb dissolved organic substances directly from the water.
Once food particles are trapped by the choanocytes, they are ingested through phagocytosis. Digestion then occurs intracellularly, inside food vacuoles within the choanocytes. Enzymes are released into these vacuoles to break down the food into simpler substances. After digestion, nutrients are absorbed and transported by mobile cells called amoebocytes to other parts of the sponge’s body.
Beyond Filter Feeding: Carnivorous Sponges
While most sponges are filter feeders, a small group of deep-sea sponges has evolved to be carnivorous. These sponges typically live in nutrient-poor deep-sea environments where filter feeding is less efficient. Instead of filtering water, carnivorous sponges directly capture small prey, such as crustaceans and other invertebrates.
These predatory sponges often possess specialized structures like sticky spicules or filaments that act as Velcro-like hooks to ensnare prey. The harp sponge, Chondrocladia lyra, is a notable example, using its elaborate, branched structure to maximize prey capture. Once prey is caught, sponge cells migrate to the area, surrounding and slowly engulfing the captured organism. Digestion occurs externally, as the sponge secretes a membrane and enzymes to break down the prey before absorbing the nutrients.