Sponges are multicellular, aquatic animals with a porous body and an internal framework, or endoskeleton. This skeleton provides structural integrity, allowing the soft-bodied organism to maintain its form in the water. The composition and organization of this internal skeleton vary greatly across different sponge species.
Composition of the Sponge Skeleton
A sponge’s skeleton is built from microscopic elements produced by specialized cells. The primary structural units are spicules, which are crystalline, needle-like structures that provide rigidity. Spicules vary greatly in shape, from simple rods to intricate star-like forms, and their specific shape and size are unique to each species.
The mineral content of spicules is a main identifier. Some sponges produce spicules from calcium carbonate, while others use silica, a form of natural glass. Siliceous spicules are formed around a central organic filament called spiculin.
Many sponges also incorporate a protein called spongin into their skeletons. Spongin is a collagen that forms a flexible network of fibers, which can hold mineral spicules together or form the entire skeleton. Skeletons made entirely of this protein result in a soft and elastic structure.
Major Types of Sponge Skeletons
The combination of spicules and spongin defines the major classes of sponges. Classification depends on whether the skeleton is built from calcium carbonate, silica, or spongin, and how these elements are arranged. This skeletal architecture is a factor in the biology and evolution of these animals.
The class Calcarea is defined by skeletons made exclusively of calcium carbonate spicules. These spicules are simpler in form compared to their silica counterparts and can be arranged in a loose meshwork or fused into a rigid structure. These sponges are found only in marine environments.
The class Hexactinellida, or glass sponges, has skeletons made of silica spicules. These spicules often have six points and are fused into intricate, three-dimensional lattices. This fusion creates a strong, glass-like skeleton that can resemble a delicate basket or vase.
Demospongiae is the largest and most diverse class of sponges, exhibiting great variety in skeletal composition. Some have skeletons of only silica spicules that are not fused. Others have a composite skeleton of both silica spicules and spongin fibers. A third arrangement is a skeleton made entirely of spongin fibers, which includes commercial bath sponges.
The Skeleton’s Role in a Living Sponge
The skeleton’s most apparent role is providing structural support, which gives the sponge its characteristic shape and prevents its soft body from collapsing. This rigidity is necessary for the organism to stand upright in water currents and maintain its overall form.
Beyond support, the skeleton is a primary means of defense against predators. The sharp, needle-like spicules that permeate the sponge’s body make it an unpalatable and potentially harmful meal for most marine animals. These spicules function as a deterrent.
The skeleton’s internal architecture is also linked to the sponge’s filter-feeding lifestyle. Sponges pump large volumes of water through a system of internal canals to capture food. The rigid skeletal framework ensures these channels remain open, allowing for the efficient flow of water necessary for feeding and respiration.
Significance Beyond the Sponge
Sponge skeletons also influence human activities and marine ecosystems. For centuries, humans have harvested certain demosponges for domestic use. The soft, absorbent skeletons of these sponges, consisting only of a dense network of spongin fibers, are processed and sold as natural bath sponges.
When sponges die, their mineral skeletons can impact the seafloor environment. In some deep-sea areas, large quantities of discarded silica spicules from glass sponges accumulate over time. These dense mats of spicules alter the sediment’s texture and stability, creating a habitat for other marine organisms.
The intricate structures of some sponge skeletons have captured the interest of scientists and engineers. The skeletons of glass sponges, for instance, are composed of silica spicules with properties similar to fiber-optic cables that efficiently transmit light. This has led to biomimicry research, where these natural designs provide inspiration for new materials and technologies.