Sclerites are hardened body parts found in many invertebrate animals. Derived from the Greek word “sklēros” meaning “hard,” these distinct, rigid structures provide a framework for these organisms, contributing to their overall integrity.
What Are Sclerites
Sclerites appear in various forms across different invertebrate groups, such as plates, spicules, or scales. In arthropods like insects and crustaceans, sclerites are often plate-like segments forming the exoskeleton, connected by flexible membranes. These segments include dorsal tergites, ventral sternites, and lateral pleurites.
Sclerites also appear in other invertebrates. Sponges, for instance, have internal spicules, which are microscopic, needle-like structures providing skeletal support. Soft corals incorporate sclerites within their tissues for rigidity. Some mollusks, like chitons, possess articulated plates as part of their external shell, while others, such as cone shells, have isolated sclerites like a sting.
Sclerites are also found in fossilized organisms, often being the only preserved parts. Early animals like the armored slug-like Wiwaxia and the worm-like Halkieria possessed complex arrangements of sclerites. The complete set of these hardened parts in a fossil is known as a scleritome, offering insights into the body plans of ancient invertebrates.
Roles of Sclerites
Sclerites perform multiple functions for the survival of invertebrates. A primary role is structural support, acting as an internal or external skeleton that maintains the organism’s shape. This support is particularly evident in arthropods, where the segmented exoskeleton forms a framework for the body.
Sclerites also offer protection against predators and environmental hazards. Their hardened nature creates a defensive barrier, deterring potential threats. For example, an arthropod’s tough exoskeleton plates shield its soft internal tissues from injury. Similarly, spicules in sponges can make them unpalatable or difficult for predators to consume.
Beyond protection, sclerites aid in locomotion. In arthropods, the rigid plates serve as attachment points for muscles, allowing coordinated movement of limbs and body segments. The interaction between these hardened parts and flexible membranes facilitates various forms of movement, from walking and flying to swimming. This arrangement enables efficient transmission of muscle force.
Sclerite Composition and Formation
The composition of sclerites varies by invertebrate group, but they are primarily made of organic and inorganic materials. Common organic components include chitin, a complex carbohydrate polymer, which often forms a network with proteins. This chitin-protein complex provides a flexible yet strong base for the sclerites.
Inorganic materials, such as calcium carbonate or silica, are deposited within this organic matrix to provide rigidity. Calcium carbonate is a common mineral in many marine invertebrate sclerites, including those found in crustaceans, mollusks, and corals. This mineral can exist in different crystalline forms, such as calcite or aragonite.
The formation of sclerites involves biomineralization, where organisms biologically control the deposition of minerals. This process often begins with the secretion of an organic matrix, which serves as a template for mineral crystallization. For example, in soft corals, specific extracellular proteins, like ECMP-67, can induce calcite crystal formation.
The synthesis and deposition of these materials are carefully regulated by the organism’s cells. In crustaceans, chitin and proteins form an intricate network, and calcium carbonate then precipitates onto this framework, creating a rigid shell. This precise biological control ensures the formation of sclerites with specific shapes and mechanical properties tailored to the organism’s needs.