Sponges (phylum Porifera) are simple, multicellular, sessile, filter-feeding organisms that lack true tissues or organs. These aquatic animals rely on a constant flow of water through their bodies for food and oxygen. To maintain the necessary shape and integrity for this water flow, sponges must possess a supporting structure. This framework is provided by microscopic, rigid elements called spicules, which act as the skeletal components in most species. Spicules are embedded within the sponge’s gelatinous mesohyl layer, giving the soft body form and stiffness.
Composition and Formation
Sponge spicules are biomineralized structures composed of one of two main chemical compounds: calcium carbonate or silica. Calcareous spicules, characteristic of the class Calcarea, are made of calcium carbonate, typically in the crystalline form of calcite or aragonite. The majority of sponges, including the classes Demospongiae and Hexactinellida, produce siliceous spicules, which consist of hydrated amorphous silica, or biological glass.
These skeletal elements are secreted by specialized, mobile cells called sclerocytes. Formation begins when the sclerocyte creates a minute organic filament, known as the axial filament, which acts as a template for mineral deposition. The sclerocyte then deposits the inorganic material—either calcium carbonate or silica—around this core. The rate of spicule elongation can be quite fast, measured in the range of 1 to 10 micrometers per hour in some species.
Structural Roles and Functions
The primary purpose of spicules is to provide mechanical support, acting as an internal skeleton. Spicules interlock or are dispersed throughout the mesohyl, creating a scaffold that lends rigidity and maintains the precise architecture required for efficient water filtration. This structure ensures that the internal canals and chambers remain open for the continuous flow of water. In deep-sea glass sponges, siliceous spicules often fuse together to form an elaborate, rigid lattice structure, such as the cage of a Venus’ Flower Basket.
Spicules also play a significant role in defense against predators. The sharp, needle-like or multi-rayed structures make the sponge difficult or unpalatable for many animals to consume. Many sponges actively shed spicules, forming a dense, prickly carpet on the seafloor around them that deters bottom-dwelling predators like echinoderms from getting too close.
Beyond structural and defensive roles, the siliceous spicules of certain deep-sea sponges exhibit optical properties. These spicules function like biological fiber optic cables, trapping and transporting light from the dark seafloor. This ability may serve to illuminate symbiotic organisms or attract small crustaceans, such as the shrimp that inhabit the Venus’ Flower Basket. The layered design of these biosilica fibers also gives them exceptional mechanical strength, surpassing that of commercial fiber optics.
Classification by Shape and Size
Spicules are categorized into two size-based groups: megascleres and microscleres. Megascleres are the larger spicules that form the primary supporting framework and are often visible to the naked eye. Microscleres are smaller, more numerous elements scattered throughout the mesohyl, serving to reinforce the structure or provide additional protection.
Spicule morphology is highly diverse and is classified based on the number of axes or rays they possess. Monaxon spicules, such as simple rods (oxeas), grow along a single axis. Triaxon spicules, found only in glass sponges, have three axes that intersect at right angles to create six rays. Other types include tetraxon spicules, which have four rays, and polyaxon spicules, which have many equal rays radiating from a center. The specific combination, shape, and size of these spicules are the primary criteria used by scientists to classify and identify different sponge species.