Many people might imagine starfish as soft, simple creatures dwelling on the seafloor. However, beneath their often colorful and patterned skin lies a sophisticated internal framework that provides structure and enables their unique way of life. These marine animals possess a complex internal skeleton, which allows for both protection and movement in their aquatic environment.
Composition of a Starfish Endoskeleton
A starfish’s skeleton is an endoskeleton, meaning it is an internal structure, unlike the external shells of crabs or insects. This internal framework is composed of numerous small, interlocking calcareous plates called ossicles, which are embedded within the dermis of the body wall. These ossicles are formed intracellularly by specialized cells known as sclerocytes.
Each ossicle features a unique, porous, mesh-like microstructure referred to as stereom. This stereom is made up of microscopic networks of calcium carbonate crystals, specifically magnesium calcite, arranged in a three-dimensional lattice. This honeycomb-like structure contributes to the skeleton’s strength while also keeping it relatively lightweight.
The Skeleton’s Role in Support and Movement
The network of ossicles provides a flexible yet protective framework for the starfish’s internal organs and gives the animal its characteristic shape. These skeletal elements are not rigidly fused together in starfish, but are instead interconnected by collagenous ligaments and muscles embedded within the body wall. This arrangement allows the starfish to exhibit remarkable variability in stiffness, enabling it to be both pliable for navigating uneven surfaces and rigid for holding onto prey or resisting currents.
The skeleton plays a direct role in locomotion by interacting with the water vascular system. The ossicles in the ambulacral grooves, located on the underside of each arm, contain small openings or pores. Through these pores, the tube feet, which are external projections of the water vascular system, extend. The ossicles provide a stable scaffold for the water vascular system’s hydraulic pressure, enabling the tube feet to extend and retract for movement across various substrates.
Growth and Regeneration of the Skeleton
The starfish skeleton is a dynamic structure that grows throughout the animal’s life. Growth occurs by the addition of new, smaller ossicles, particularly at the tips of the arms. As the starfish grows, new skeletal material expands the existing network.
Starfish are well-known for their capacity to regenerate lost limbs, and this process includes the complete regrowth of the skeletal structure within the new arm. If a starfish loses an arm, cells at the wound site begin to repair the tissue, and new plates of calcium carbonate are deposited into a developing collagen network. This remarkable ability involves the formation of new ossicles and stereom, ensuring the regenerated arm has a fully functional skeletal system, mirroring the original structure.