Starfish do not “walk” in the traditional sense, unlike animals with legs or fins. Instead, these marine invertebrates employ a unique method of movement across the ocean floor. Their locomotion relies on an intricate hydraulic system, allowing them to glide and adhere to various surfaces with remarkable precision. This process enables them to navigate their underwater environment, climb, and even right themselves if overturned.
The Water Vascular System
At the core of a starfish’s mobility is its water vascular system, a network of fluid-filled canals and reservoirs that operates on hydraulic principles. Water enters this system through a sieve-like plate on the starfish’s upper surface, called the madreporite. This porous structure acts as an intake filter.
From the madreporite, water travels through the stone canal, which connects to a circular ring canal around the starfish’s central disc. Branching off from this ring canal are radial canals, with one extending down each arm. Along these radial canals, thousands of small, muscular sacs called ampullae are situated, each connected to a tube foot. These components regulate water pressure, providing power for the tube feet.
How Tube Feet Enable Movement
Starfish movement involves a coordinated action of these tube feet, powered by the water vascular system. When a starfish needs to move, the ampullae, which are bulb-like structures located inside the arm above each tube foot, contract. This contraction forces seawater from the ampulla into the associated tube foot, causing it to extend and elongate.
The tip of each tube foot features an adhesive disc, which can then attach to the surface. Once attached, muscles within the tube foot contract, shortening the foot and pulling the starfish’s body forward. This process is repeated by thousands of tube feet, working in waves, creating a smooth gliding motion. To detach, the tube foot’s muscles relax, and adhesive properties are released, allowing the foot to prepare for the next step.
Beyond Locomotion: Other Functions of Tube Feet
While their primary role is movement, starfish tube feet serve several other functions. These appendages are instrumental in feeding, allowing many starfish species to capture and manipulate prey. For example, some starfish use the suction of their tube feet to pry open the shells of bivalves like clams and mussels, exerting a continuous pull until the prey’s adductor muscles fatigue.
Tube feet also play a role in respiration, facilitating gas exchange between the starfish and its aquatic environment. Their thin walls allow for the absorption of oxygen and the release of carbon dioxide directly into the surrounding water. Beyond these roles, tube feet contribute to sensory perception, enabling the starfish to detect chemical cues and physical contact.