Starfish possess a skeletal system that sets them apart from many other marine animals. Unlike vertebrates with internal bones or crabs with external shells, a starfish’s skeleton is an internal structure, though not true bone. This unique endoskeleton provides both support and flexibility, enabling the starfish to navigate its environment and perform various functions. It is comprised of numerous small, calcified plates embedded within the creature’s body wall, forming a complex internal framework.
The Ossicles
The primary calcified components of a starfish’s skeleton are known as ossicles. These are individual, small, irregularly shaped plates embedded within the dermis, or body wall. Ossicles serve as fundamental building blocks of the starfish’s internal support system, providing rigidity and protection to its internal organs. They vary considerably in size and shape across different species and within different regions of a single starfish, reflecting their specialized roles. These elements are infused with tissue, creating a three-dimensional structure that forms the body of the starfish.
Skeletal Structure and Arrangement
The individual ossicles are organized and interconnected to form the complete starfish skeleton, creating a flexible yet supportive internal framework. These elements are not rigidly fused but are linked by collagenous ligaments and muscles embedded within the body wall, allowing for remarkable variability in stiffness. Different types of ossicles contribute to specific anatomical regions and functions.
Dermal ossicles are embedded in the skin and often bear spines, contributing to the starfish’s spiny or granular texture. Ambulacral ossicles form the ambulacral grooves on the underside of each arm, housing the tube feet and supporting the water vascular system. Adambulacral ossicles are found alongside the ambulacral grooves, providing additional support. A larger, specialized ossicle, the madreporite, is typically visible as a distinct, button-like structure on the starfish’s upper surface, filtering water into the water vascular system. The arrangement of these ossicles can be a loose meshwork in some areas for flexibility, or more tightly packed for greater structural integrity.
Composition and Properties
Starfish ossicles are primarily composed of calcium carbonate, specifically calcite. These calcified elements possess a unique microstructure called “stereom,” a porous, sponge-like lattice. This stereom is formed from microscopic networks of calcite microcrystals arranged in a three-dimensional lattice. The porous nature of the stereom makes the ossicles lightweight while providing significant strength, contributing to the starfish’s overall support.
Living tissue, including specialized cells called sclerocytes, extends into this porous structure, allowing for growth and repair. The intricate arrangement of calcite crystals within the stereom enhances the ossicles’ strength, compensating for the inherent fragility of calcite. This design allows the starfish skeleton to adapt to mechanical loads, with regions experiencing higher stress often developing thicker stereom microstructures.