Sponges are ancient, multicellular organisms that inhabit aquatic environments around the globe. These creatures are unique for their sessile, or stationary, existence, attaching to surfaces and obtaining sustenance from the water flowing around them. Their feeding method, known as filter-feeding, is central to their biology and allows them to thrive in diverse marine and freshwater habitats.
How Sponges Capture Food
Sponges draw water into their bodies through a complex system of pores and canals. Microscopic pores, called ostia, dot the outer surface of the sponge, allowing water to enter. This water then flows through a network of internal canals that lead to larger chambers.
These internal chambers are lined with specialized cells known as choanocytes, or collar cells. Each choanocyte possesses a whip-like flagellum surrounded by a mesh-like collar of microvilli. The constant beating of these flagella creates a water current, drawing water into the sponge, through the choanocyte-lined chambers, and expelling it through a larger opening called the osculum. As water passes through the choanocyte collars, food particles are trapped and ingested by the choanocytes.
The Sponge Diet
The primary diet of most sponges consists of microscopic particles suspended in the water. These include various types of plankton, such as phytoplankton and small zooplankton. They also consume bacteria, viruses, and other microorganisms.
Beyond living organisms, sponges also feed on organic detritus, which is dead organic matter and debris. Some sponges can absorb dissolved organic substances directly from the water. While most sponges are filter feeders, there are some carnivorous species, particularly found in deep-sea environments, that capture small crustaceans using sticky threads or hooked spicules.
Processing and Utilizing Nutrients
Once food particles are captured by the choanocytes, they are ingested into food vacuoles within these cells. Digestion in sponges is entirely intracellular, meaning it occurs within individual cells. Digestive enzymes are released into these vacuoles, breaking down food into simpler substances.
After digestion, nutrients are absorbed by the choanocytes. Specialized mobile cells called amoebocytes transport these digested nutrients from the choanocytes to other cells throughout the sponge’s body. Sponges lack complex circulatory or excretory systems. Waste products from metabolism, such as nitrogenous waste, are released by individual cells into the water flowing through the sponge and expelled through the osculum. Sponges can also remove unwanted particles and debris through a slow contraction process, expelling mucus with aggregated waste.