Sea sponges are ancient, sessile multicellular organisms belonging to the phylum Porifera, a name that means “pore bearer.” These simple animals are fixed to a substrate, such as a rock or the seabed, and rely on the constant movement of water for survival and nutrition. Sponges operate as highly efficient biological filters, drawing water through their bodies to capture microscopic food particles. This unique strategy, known as filter feeding, is the defining characteristic of how the vast majority of sponges acquire nutrition.
The Primary Diet of Sea Sponges
The typical sponge diet consists almost entirely of microscopic life and debris suspended in the water column. Their primary food sources include picoplankton and bacterioplankton. These single-celled bacteria and archaea are often less than a micrometer in size, making them the most abundant food source for many sponge species. Sponges also consume nanoplankton and various forms of organic detritus. This detritus often takes the form of “marine snow,” a continuous shower of dead organic matter and fecal pellets drifting down from surface waters. Furthermore, many sponges have the ability to absorb dissolved organic matter (DOM) directly from the seawater, offering a substantial nutritional supplement to their particulate diet.
The Mechanics of Water Flow and Capture
The mechanics of feeding depend on a highly organized system of water circulation. Water is first drawn into the sponge through thousands of minute inlet pores, called ostia, which cover the outer surface of the body. The flow is propelled by specialized cells called choanocytes, or collar cells, which line the internal chambers of the sponge. Each choanocyte possesses a whip-like flagellum that beats rhythmically, creating a powerful negative pressure that pulls water through the canal system. The water slows dramatically as it passes into the choanocyte chambers, allowing for particle capture. Surrounding the base of the flagellum is a mesh-like structure composed of microvilli, which forms a collar. This collar acts as a fine sieve, trapping food particles, especially bacteria-sized particles smaller than 0.5 micrometers. Larger particles may be caught by other cells lining the incurrent canals. After the particles are captured, the filtered water is collected into larger excurrent canals and expelled from the sponge’s body through a larger opening called the osculum.
Intracellular Digestion
Digestion in sea sponges is a process that occurs entirely within individual cells, a trait known as intracellular digestion. Once a choanocyte has captured a food particle on its collar, the cell engulfs it through phagocytosis, internalizing the particle into a food vacuole. Choanocytes begin the digestive process by releasing enzymes into the vacuole to break down the food. The nutrients are then transferred to mobile, amoeba-like cells called archaeocytes, which reside in the mesohyl, the gelatinous matrix between the outer and inner cell layers. These archaeocytes function as nutrient transporters, moving throughout the sponge body to distribute the digested food. This distribution network is necessary because the sponge lacks a true circulatory system or a centralized digestive tract.
Specialized Feeding Strategies
While the vast majority of sponges are filter feeders, a small, specialized group has evolved a carnivorous feeding strategy. These species, primarily belonging to the family Cladorhizidae, live in deep-sea environments where the water is extremely clear and nutrient-poor, making traditional filter feeding inefficient. These carnivorous sponges have lost the typical water-pumping system and choanocyte chambers. Instead, they possess highly modified bodies featuring thin filaments or appendages covered with microscopic, hook-like spicules. These structures function like a sticky surface, passively trapping small organisms, such as copepods and other tiny crustaceans, that brush against them. Once prey is snared, the sponge cells slowly migrate to the site of contact, enveloping and digesting the immobilized animal over a period of several days.