Starfish, often called sea stars, are marine invertebrates that inhabit diverse ocean environments, from shallow intertidal zones to deep abyssal plains. While their aquatic habitat might suggest otherwise, adult starfish generally do not swim like fish or other finned aquatic animals. Instead, their movement is a unique process. They rely on a specialized biological system for locomotion, navigating their underwater world without fins or powerful swimming muscles.
How Starfish Navigate Their Environment
Adult starfish primarily navigate their surroundings using thousands of small, flexible appendages called tube feet, located on the underside of their arms. These tube feet extend and attach to surfaces, then contract to pull the starfish forward. This process is typically slow and deliberate, enabling them to crawl along the seabed, climb rocks, and even scale vertical surfaces. The tips of the tube feet often have suckers, which allow them to grip firmly onto various substrates, providing traction. This coordinated action of numerous tube feet allows for a gliding motion across the ocean floor.
The Marvel of the Water Vascular System
The unique movement of starfish is powered by an intricate internal hydraulic network known as the water vascular system. This system is a series of fluid-filled canals containing seawater, which facilitates locomotion, food transportation, and gas exchange. Water enters this system through a sieve-like plate on the starfish’s upper surface, called the madreporite. From the madreporite, water flows through a stone canal, which connects to a circular ring canal located around the central mouth.
Branching off from the ring canal are radial canals that extend into each arm of the starfish, running along the ambulacral grooves. Along these radial canals are numerous bulb-like structures called ampullae, each connected to a tube foot. When the ampulla contracts, it forces water into the connected tube foot, causing it to extend and stiffen. Conversely, when the tube foot muscles contract, water is pushed back into the ampulla, causing the tube foot to retract. This controlled hydraulic pressure allows the tube feet to extend, adhere, and pull the starfish along.
Subtle Movements and Starfish Larvae
Adult Movement Variations
While adult starfish are known for their slow, crawling movements, some species can exhibit a more rapid “scooting” or “crawling-swimming” motion, particularly when attempting to escape from predators. This accelerated movement, however, still relies on their tube feet and the water vascular system, rather than true swimming.
Larval Swimming
In contrast to adults, starfish larvae are free-swimming. The larval forms, such as bipinnaria and brachiolaria, are planktonic and move actively through the water column. These microscopic larvae are equipped with bands of cilia, tiny hair-like structures, which beat rhythmically to create currents for both propulsion and feeding. This ciliary action allows them to navigate and drift within the plankton, searching for food and suitable environments before they undergo metamorphosis into their familiar adult form. While adult starfish do not swim, their early life stages are capable swimmers.