The starfish, a marine invertebrate belonging to the class Asteroidea, is a familiar sight on the ocean floor, characterized by its radial symmetry and typically five or more arms. These animals are not permanently fixed to the substrate but are mobile creatures capable of purposeful movement. Starfish can traverse rocky surfaces, sandy bottoms, and even vertical structures using a unique biological system. This locomotion allows them to engage in necessary life functions, such as hunting and seeking refuge.
The Engine of Locomotion: Tube Feet and the Water Vascular System
The ability of a starfish to move is dependent on its specialized water vascular system, a network of fluid-filled canals that functions as a hydraulic power source. Seawater is drawn into this system through a small, sieve-like plate on the upper surface called the madreporite. The fluid then travels through a stone canal to a ring canal that encircles the central disc.
From the central ring canal, radial canals extend outward, running the length of each arm within the ambulacral groove. Connecting to these radial canals are hundreds or thousands of small, bulb-like sacs called ampullae, which are muscular organs. Each ampulla is connected to a single tube foot, or podium, which protrudes from the underside of the arm.
Movement is achieved when muscles surrounding an ampulla contract, squeezing the water from the sac down into the attached podium. This increase in hydraulic pressure causes the tube foot to extend and elongate, reaching out toward the substrate. The tube foot then attaches to the surface, often using a temporary chemical adhesive, and muscles within the foot contract to pull the animal forward.
Once the pull is complete, the longitudinal muscles of the podium relax, and the ampulla muscles relax and expand, drawing the water back into the sac. This retraction mechanism allows the tube foot to detach from the substrate and prepare for the next forward step. The continuous, coordinated extension and retraction of these numerous tube feet propel the starfish along the ocean floor.
Measuring Starfish Speed: Capabilities and Limitations
While the tube feet provide gripping and pulling power, starfish movement is generally slow and deliberate compared to most marine life. The average speed for many common species is measured in centimeters per minute, often ranging up to about 15 centimeters per minute. The actual velocity is highly dependent on the species and the environmental conditions.
Species like the Sand Star, Luidia foliolata, are significantly faster, capable of traveling up to 2.8 meters per minute when burrowing in sand, utilizing pointed tube feet rather than suckers. The substrate itself influences speed; some starfish move more efficiently over soft sand than over rough pavement or rubble. Larger individuals within a species can sometimes achieve faster rates of movement than smaller ones.
Movement is a steady, crawling action, not a rapid dash, and sustained high speeds are not possible. The slow pace is a direct consequence of the hydraulic mechanism, which requires the sequential extension and retraction of many individual podia. This speed is perfectly suited to their lifestyle, which does not require the swift pursuit of mobile prey or rapid escapes from predators.
Why Starfish Travel: Searching for Food and Shelter
The primary motivation for a starfish to activate its locomotion system is to locate and secure food. Most starfish are predators that feed on slow-moving or sessile invertebrates, such as bivalves like clams, oysters, and mussels. The starfish must travel to the location of its prey, using its tube feet to navigate the terrain in search of chemical cues indicating a meal.
Once prey is located, the tube feet are used to grip the two shells of the bivalve and exert a steady, powerful pull to pry them open. Starfish also travel to find suitable shelter, moving away from areas with strong currents or high predator density, sometimes seeking refuge under rocks or in crevices. They move toward nearby habitat structures when exposed on open sand.
Movement is also tied to reproductive behavior, with individuals sometimes traveling to aggregation points to find mates. Although the movement is slow, it is highly directional and purposeful, guided by sensory organs on the tips of their arms. These organs detect light and chemical changes in the water, allowing the starfish to efficiently cover ground for survival and reproduction.