Starfish (class Asteroidea) are marine invertebrates that inhabit all the world’s oceans. The common assumption that these creatures can swim is a misconception. Adult starfish do not propel themselves through the water column using fins or coordinated swimming motions. Instead, the vast majority of species move slowly and methodically along the seafloor. They employ a complex hydraulic system that allows them to crawl across surfaces like sand, rock, and coral.
The Truth About Adult Starfish Locomotion
The adult starfish moves across the ocean floor using hundreds to thousands of tiny, flexible appendages called tube feet. These are located in grooves on the underside of their arms. Each tube foot operates like a small, independent leg that extends, attaches to the substrate, and then retracts to pull the body forward. The tip of most tube feet possesses a temporary adhesive, rather than true suction, which allows for a secure grip on various surfaces.
Movement is typically slow and deliberate, often only a few centimeters per minute. While most species are slow, a few specialized burrowing species, such as those in the genus Luidia, can move much more rapidly, sometimes reaching speeds closer to two meters per minute. When traveling, a starfish does not maintain a single leading arm, but shifts which arm is in front depending on its direction.
The coordinated action of the tube feet allows the animal to navigate complex terrain and climb vertical surfaces. When a starfish needs to turn over, it uses a few arms to anchor and leverage its body. This crawling gait is fundamental to their ecological role as bottom-dwelling predators and scavengers.
The Engine of Movement: The Water Vascular System
The power behind the tube feet is the water vascular system, a sophisticated network of water-filled canals. This hydraulic system begins with the madreporite, a small, porous plate on the upper surface that filters in seawater. The water flows through the stone canal, which connects to a ring canal encircling the central disk.
From the ring canal, five radial canals extend down the length of each arm. These canals branch into short lateral canals, each connecting to a single tube foot. At the base of every tube foot, inside the body, is a small, muscular sac called the ampulla.
To extend a tube foot, the ampulla muscles contract, forcing water down into the attached podium (the external part of the tube foot). This increase in hydrostatic pressure causes the podium to elongate and push against the surface. To retract the foot, longitudinal muscles in the podium contract, pushing the water back into the relaxed ampulla. This continuous cycle of hydraulic extension and muscular retraction allows for the synchronized, wave-like motion of crawling.
When Starfish Actually “Swim”: The Larval Stage
While adult starfish crawl on the ocean floor, they are true swimmers during their larval stage. The life of a starfish begins as a microscopic, planktonic larva, typically passing through the Bipinnaria and Brachiolaria stages. These larvae are released into the water column and are bilaterally symmetrical, contrasting sharply with the adult’s radial symmetry.
The larvae move using thousands of tiny, hair-like structures called cilia that cover their bodies. The coordinated beating of these cilia allows the larva to propel itself through the water, swimming and drifting with the currents. This free-swimming stage is crucial for dispersal, enabling offspring to travel great distances from the parent population.
The larval phase lasts for weeks to months while the larva searches for a suitable place to settle. Once a location is found, the larva attaches to the substrate and undergoes metamorphosis, transforming into the familiar, slow-moving adult form. This early existence is the only time a starfish can be accurately described as swimming.