Sponges, members of the Phylum Porifera, represent some of the most ancient multicellular organisms on Earth. They lack true tissues, organs, or a nervous system, relying instead on specialized cells to perform all life functions. The answer to how these organisms feed without a mouth lies in their highly efficient aquiferous system. This system is engineered to continuously move vast quantities of water through the body, filtering out microscopic food particles.
The Unique Body Plan of Sponges
The body of a sponge is built around a system of pores, canals, and chambers that collectively facilitate water flow. The outer surface is perforated by thousands of tiny openings called ostia, which serve as the entry points for the surrounding water. The water passes through an intricate network of channels, eventually reaching the central cavity, known as the spongocoel, or specialized flagellated chambers. The filtered water is then expelled through one or more large exit openings called oscula.
The structure is maintained by three primary cell types working in concert. Pinacocytes form the flattened, skin-like outer layer, providing structure and a boundary with the environment. Wandering cells, known as amoebocytes, move throughout the gelatinous matrix, or mesohyl. The feeding mechanism is executed by the choanocytes, or collar cells, which line the internal chambers where filtration takes place.
Generating the Current The Pumping System
The feeding operation depends on the coordinated action of millions of choanocytes embedded within the sponge’s interior. Each choanocyte possesses a single, whip-like projection called a flagellum that beats rhythmically. The synchronized movement of these flagella acts as a biological pump, generating the negative pressure that pulls water into the sponge. This action draws water through the ostia and pushes it through the internal canal system toward the exit.
This continuous pumping creates a powerful current, allowing a sponge to process an astonishing volume of water relative to its size. Some sponges are capable of filtering their entire body volume of water every few seconds, or up to 1,000 times their volume in a single hour. This high rate of turnover ensures a constant supply of oxygen and food particles, while also flushing out waste products. This high filtration capacity underscores the importance of the choanocyte flagella.
The water current slows significantly as it reaches the choanocyte chambers, which allows the filtration process to occur efficiently. Once the water passes through the feeding chambers, the channels narrow again, accelerating the spent water as it exits the osculum. This rapid expulsion helps prevent the sponge from immediately refiltering the same water, increasing the overall efficiency of the system. The sponge can also regulate the flow rate by contracting cells around the osculum or ostia to control the openings.
Capturing and Processing Nutrients
The choanocyte is responsible for both generating the current and capturing the microscopic organisms and particulate matter that comprise the sponge’s diet. Surrounding the base of the flagellum is a fine, mesh-like structure called the collar. This collar is composed of microvilli, which act as a sieve to trap bacteria, phytoplankton, and small organic debris suspended in the water. These particles are typically less than one micrometer in size, which is the preferred food source for many species.
Once a food particle is trapped on the collar, it slides down to the main body of the choanocyte. The cell then engulfs the particle in a process called phagocytosis, forming a food vacuole for intracellular digestion. While larger particles may be ingested directly, smaller particles like bacteria are the primary target. The choanocytes partially digest the captured nutrients before passing them on to the amoebocytes.
The amoebocytes, or wandering cells, function as the circulatory and distribution system of the sponge. These cells move through the mesohyl, collecting partially digested food packages from the choanocytes. They complete the digestive process and transport the processed nutrients to all other cells. This internal transport system ensures that every part of the organism receives energy.