Starfish, recognized by their star-like shape, are marine invertebrates found in oceans worldwide. Commonly inhabiting diverse marine habitats from shallow intertidal zones to abyssal depths, they display a wide array of colors and forms across nearly 2,000 known species. Their adaptability allows them to thrive in warm tropical waters and frigid polar regions. Many are curious about how these creatures move and their classification within the animal kingdom.
The Simple Answer: No Fins
Starfish do not possess fins; they are not fish. Instead, starfish belong to a group of marine animals called echinoderms, which also includes sea urchins, sand dollars, and sea cucumbers. Unlike fish, starfish lack the anatomical features associated with swimming, such as gills, scales, or a backbone. Their method of locomotion relies on a unique hydraulic system rather than fins.
How Starfish Really Move
Starfish move using a sophisticated system called the water vascular system, which powers thousands of tiny appendages known as tube feet. Water enters this system through a sieve-like plate on the starfish’s upper surface, called the madreporite. From the madreporite, water flows through internal canals, including a stone canal, a ring canal, and radial canals that extend into each arm. This network of fluid-filled canals creates hydraulic pressure, allowing the starfish to extend and retract its tube feet.
Each tube foot functions like a small suction cup, capable of attaching to surfaces. By coordinating the extension and retraction of these tube feet, the starfish can slowly glide across the seafloor or climb vertical surfaces. This system also enables them to exert considerable force, such as when prying open the shells of bivalves for feeding.
Are Starfish Even Fish?
Despite the common name “starfish,” these animals are not fish. This misconception arises because they live underwater. Starfish are classified as invertebrates within the phylum Echinodermata, a group distinct from fish. Fish are vertebrates, meaning they possess a backbone, gills for breathing, scales covering their bodies, and fins for propulsion.
Starfish, conversely, lack all these characteristics. They do not have a backbone, instead relying on an internal skeleton made of calcium carbonate plates embedded in their skin. They breathe by absorbing oxygen directly from the water through their tube feet and small skin gills called papulae. While fish exhibit bilateral symmetry with a distinct head and tail, adult starfish display radial symmetry, typically with five or more arms radiating from a central disc.