Starfish, correctly classified as sea stars, belong to the phylum Echinodermata, a group of marine invertebrates that includes sea urchins and sea cucumbers. Starfish do have hundreds of structures called tube feet that operate much like powerful suction cups, though the answer to whether they possess true suction cups is complex. Their ability to grip surfaces, move, and even pry open prey is entirely dependent on these flexible, tubular appendages on their underside. These tube feet are the external components of a unique internal hydraulic system that allows for remarkable control over attachment and movement across the seafloor.
The Anatomy of Tube Feet
Rows of tube feet, technically known as podia, line the ambulacral grooves that extend along the underside of each arm. These appendages feature a small, disc-shaped tip resembling a suction cup. While many species utilize this terminal disc to create a vacuum-like seal, the primary force for attachment is temporary chemical adhesion. The starfish secretes a dual-component adhesive that acts like a temporary, biological glue to secure its grip on rocks or shells.
This adhesive system allows the tube feet to maintain a strong hold even on irregular surfaces where a vacuum seal would fail. Each individual tube foot is a slender, muscular cylinder that can be extended or retracted independently. The sheer number of these podia, which can number in the thousands on a single large starfish, provides the collective force needed for locomotion and powerful anchorage.
The Water Vascular System and Hydraulic Power
The movement and gripping power of the tube feet are driven by the starfish’s unique water vascular system (WVS), which acts as a hydraulic network. Seawater is drawn into this system through a small, sieve-like plate on the animal’s upper surface called the madreporite. The water then travels down a stone canal to a central ring canal that encircles the mouth. From the ring canal, a radial canal extends outward into the ambulacral groove of each arm, delivering water to the tube feet.
Each tube foot connects to a small, muscular, bulb-like sac inside the arm, known as the ampulla. Extension begins when the ampulla’s muscles contract, forcing fluid into the connected tube foot. This increase in hydrostatic pressure causes the tube foot to elongate and push against the substrate. To retract the tube foot, muscles within the podium contract, forcing the fluid back into the ampulla and pulling the starfish forward or releasing the grip. This coordinated hydraulic action allows the animal to move slowly but with immense force.
Functions of Tube Feet Beyond Locomotion
The functions of the tube feet extend far beyond simple walking or clinging to a surface; they are important for the starfish’s feeding habits, particularly when preying on bivalves like clams and mussels. The starfish attaches its tube feet to both halves of the shell and maintains a steady, unrelenting pull, eventually fatiguing the mollusk’s adductor muscles. Once a minute gap is created, the starfish can evert its stomach out through its mouth and into the shell cavity to digest the prey externally.
Tube feet also serve as sensory organs, providing the starfish with information about its immediate environment. Sensory cells within the podia allow the animal to detect chemicals in the water, which helps it locate food sources and potential mates. Furthermore, the thin walls of the tube feet facilitate gas exchange, acting as respiratory surfaces. Oxygen diffuses from the seawater directly into the fluid of the water vascular system, while carbon dioxide moves out.