The phylum Porifera, commonly known as sponges, represents a group of simple, multicellular aquatic animals that diverged early in the evolutionary history of the animal kingdom. The term Porifera, meaning “pore bearer,” reflects their body plan, which is permeated by numerous small openings that facilitate a constant flow of water. Unlike common kitchen sponges, which are synthetic materials, the biological organisms are built from a unique architecture of specialized cells and a complex, often mineral-based, internal skeleton. This construction allows the sponge to function as a sessile filter feeder, a lifestyle that has proven highly successful across diverse marine and freshwater environments. Sponges lack true tissues or organs, relying instead on the coordinated action of individual components.
The Basic Cellular Organization
Sponges are classified as parazoans, highlighting their lack of true tissues. Their structural organization is defined by three distinct layers of cells and non-living material that form the body wall around a central cavity. The outermost layer is the Pinacoderm, composed of flattened pinacocytes that cover the exterior surface, protecting the organism and regulating water flow into internal channels.
Beneath the Pinacoderm lies the Mesohyl, a gelatinous, non-living matrix that serves as the connective substance. This matrix contains protein fibers and various mobile, specialized cells. The innermost layer is the Choanoderm, which lines the internal chambers and water canals. This layer is constructed from choanocytes, which drive the sponge’s filter-feeding mechanism.
The Supportive Skeleton: Spicules and Spongin
The physical integrity and shape of a sponge are maintained by an internal skeleton formed from one or both of two primary non-living materials: spicules and spongin. Spicules are microscopic, crystalline skeletal elements that provide rigidity and defense, often resembling tiny needles or complex, star-like shapes. Their chemical composition varies by species, consisting either of calcium carbonate (calcite) or amorphous silica (silicon dioxide). These mineralized structures are secreted by specialized cells within the mesohyl and form a supportive framework throughout the body.
In contrast, spongin is a tough, flexible protein, a specialized form of collagen. This organic material is woven into a fibrous network that contributes elasticity and flexibility to the structure. Spongin is responsible for the “spongy” texture of natural bath sponges. Many species use a combination of both spongin fibers and spicules, while others rely solely on one or the other to maintain their three-dimensional form.
Specialized Cells That Build and Maintain
Choanocytes, also known as collar cells, line the internal chambers. Each choanocyte possesses a flagellum surrounded by a collar of microvilli. These structures work together to generate the water current and filter microscopic food particles, such as bacteria, from the water. Once captured, food is often internalized by the choanocyte for initial digestion.
Amoebocytes (or archaeocytes) are mobile cells that wander throughout the gelatinous mesohyl matrix. These versatile cells distribute nutrients absorbed by the choanocytes to other parts of the sponge body. Amoebocytes also play a direct role in creating the supportive skeleton, differentiating into sclerocytes to secrete mineral spicules and spongocytes to secrete spongin fibers. These cells are totipotent, meaning they can transform into other cell types, allowing sponges to regenerate lost parts or rebuild their structure after fragmentation.
How Composition Determines Sponge Types
The composition of skeletal materials is the primary characteristic used to categorize the Phylum Porifera into major classes. The Class Calcarea includes sponges defined by their skeleton of calcium carbonate spicules. These spicules are typically three- or four-rayed and are often restricted to shallower marine waters where calcium carbonate is readily available for biomineralization.
The Class Hexactinellida, or glass sponges, is characterized by spicules made exclusively of silica. These spicules are often six-rayed, giving the class its name, and are frequently fused together to form a rigid, lattice-like internal skeleton. The largest and most diverse group is the Class Demospongiae, which comprises about 90% of all sponge species, including all freshwater forms. Demosponges have a skeleton made of siliceous spicules, spongin fibers, or a combination of both.