Sea stars, commonly known as starfish, do not possess bones in the way that humans or other familiar animals do. In the biological sense, they lack the true bony tissue found in vertebrates. Their body structure and movement system rely on a unique, non-bony framework that provides both support and flexibility.
Sea Stars are Invertebrates
The fundamental reason sea stars do not have bones is their classification as invertebrates. Bones are complex organs made of living cells embedded in a matrix of collagen and calcium phosphate, and they are exclusive to the phylum Chordata. Sea stars belong to the phylum Echinodermata, a group of marine animals that includes sea urchins and sea cucumbers.
As echinoderms, sea stars lack a spinal column, which is the defining characteristic of vertebrates. They are instead built around a five-pointed radial symmetry, radiating from a central disc. Their body support system evolved along an entirely different path, using an internal skeleton that is chemically and structurally distinct from true bone.
The Endoskeleton of Ossicles
The structural support system of the sea star is an internal endoskeleton, but it is not composed of bone. This skeleton consists of thousands of small, interlocking plates known as ossicles, embedded within the dermis layer of the body wall. These ossicles are made primarily of calcium carbonate, arranged in a unique three-dimensional lattice structure.
This honeycomb-like arrangement makes the material both lightweight and strong. The individual plates vary in form, ranging from flat plates to granules and spines, which cover the entire upper surface of the animal. Connective tissue and muscle fibers link these ossicles together, allowing the sea star’s arms to bend and twist around the contours of the ocean floor.
The degree to which the ossicles are fused determines the rigidity of the sea star’s body. In some species, the plates are tightly packed, while in others, they are loosely connected, allowing for greater flexibility. This internal framework provides necessary protection and shape without the jointed, load-bearing characteristics of a vertebrate skeleton. The ossicles even form specialized structures, such as the pincer-like pedicellariae, used to keep the sea star’s surface clean.
Movement Without Bones
The movement of a sea star is powered by a completely separate mechanism known as the water vascular system, which operates as a hydraulic network. This system is a complex series of fluid-filled canals that extends throughout the body and into each arm. Seawater enters this network through a porous plate on the upper surface called the madreporite.
The hydraulic system supplies the thousands of tiny, suction-cupped appendages on the underside of the arms, known as tube feet. When small, bulb-like organs called ampullae contract, they force water into the attached tube feet, causing them to extend and press against the ground. The sea star moves by coordinating the extension and retraction of these tube feet, powered by water pressure instead of muscle contraction against rigid bone joints.
The water vascular system does more than just facilitate movement; it also assists in feeding, gas exchange, and waste transportation. The tube feet allow the sea star to grip surfaces, crawl along the seabed, and use suction to pry open the shells of prey like clams and mussels. This unique hydraulic locomotion system bypasses the need for a bony skeleton and complex joint structures.