Starfish, or sea stars, are marine invertebrates classified as echinoderms, defined by their radial symmetry and spiny skin. While the visual image of a star-shaped creature might suggest lightness, the adult form is tied directly to the sea floor. Understanding the animal’s physical structure and movement mechanisms provides a definitive answer to whether they can float.
The Adult Body Plan and Density
Adult starfish are designed to be bottom-dwellers, and their anatomy reflects this benthic lifestyle. They do not float primarily because of the composition of their internal skeleton, which is heavy. This endoskeleton is embedded beneath the skin and is made up of thousands of tiny, interlocking calcium carbonate plates called ossicles.
These ossicles are composed of magnesium-rich calcite, forming a porous, lattice-like structure known as stereom. Despite this internal architecture, the density of the mineralized material gives the adult starfish a specific gravity greater than that of seawater. This structural weight ensures the creature remains firmly anchored to the substrate, such as sand, mud, or rock. The body plan is built for stability, making passive floating impossible.
Locomotion: The Water Vascular System
Since the adult starfish cannot float, its movement relies on an intricate hydraulic system called the water vascular system. This complex network of canals and reservoirs uses seawater pressure to power locomotion. Water enters the system through a small, porous plate on the upper surface of the body, known as the madreporite.
From the madreporite, water flows down the stone canal to a circular ring canal at the center of the body. Radial canals branch off the ring canal, extending along the length of each arm. These radial canals connect to hundreds of tiny, muscular extensions called tube feet, or podia, which line the underside of the arms.
Each tube foot has a bulb-like reservoir, the ampulla, at its base inside the body. When the ampulla contracts, it forces water into the tube foot, extending it and allowing the suction cup at the tip to adhere to a surface. Sequential contraction and relaxation of these ampullae and tube feet create a coordinated, slow crawling motion across the ocean floor. This hydraulic mechanism is the sole method of movement for the adult.
The Exception: Floating Larval Stages
While the adult is a sinker, the starfish life cycle includes a stage where it floats as part of the ocean’s plankton. Reproduction involves the release of gametes into the water column, where fertilization occurs. The resulting embryo develops into a microscopic, free-swimming larva, often passing through stages like the bipinnaria and brachiolaria.
These larval forms possess bilateral symmetry, a contrast to the adult’s radial body plan, and are equipped with bands of cilia that help them swim and feed. Their movement is primarily driven by ocean currents, meaning they are planktonic and float passively for weeks or months. This drifting stage allows for species dispersal over vast distances before the larva undergoes metamorphosis and settles onto the ocean floor as a miniature adult.