The sea star, commonly known as the starfish, is a marine invertebrate that relies entirely on surrounding seawater for survival and movement. This organism possesses the Water Vascular System (WVS), a specialized organ system fundamental to almost all physiological functions. The WVS uses hydraulic pressure to power the body and facilitate interaction with the environment. This complex network governs the flow of water, enabling locomotion and other life processes.
The Water Vascular System: An Overview
The Water Vascular System is a closed, fluid-filled network of canals unique to echinoderms, functioning as a hydraulic system. It provides the animal with structural rigidity and the power necessary for movement, feeding, and gas exchange. The central component of the WVS is the ring canal, a circular tube located within the sea star’s central disc, which encircles the mouth. From the ring canal, a radial canal extends outward into the center of each arm, running along the ambulacral groove on the underside. These canals distribute water to hundreds of smaller structures along the arm. The entire internal system is lined with ciliated epithelial cells, which help to circulate the fluid within the canals.
Entry Point: The Madreporite
Seawater enters the Water Vascular System through the madreporite, a specialized structure located on the aboral, or top, surface of the central disc. This structure is a light-colored, porous, button-like plate resembling a sieve. The madreporite acts as a filter, ensuring that seawater entering the internal canals is free of large debris and foreign particles. Water passes through numerous tiny channels before entering the stone canal, which connects to the main ring canal. While some water exchange occurs across the thin membranes of the tube feet, the madreporite is the main inlet for replenishing the system’s fluid.
The Exit Mechanism: Tube Feet and Ampullae
Water pressure is utilized and regulated through the numerous tube feet, which are located on the underside of each arm in the ambulacral grooves. Each tube foot is a small, muscular, tubular projection with a suction-cup-like disc at its tip. Directly above each tube foot, inside the arm, is a bulb-shaped, muscular sac called the ampulla, which acts as the power source. The tube foot and ampulla are connected by a lateral canal, operating on a simple hydraulic principle. When the ampulla contracts, it forces the fluid—largely seawater—downward into the attached tube foot. This sudden increase in internal water pressure causes the tube foot to elongate and extend outward, allowing it to make contact with the substrate. To detach or retract the tube foot, the ampulla relaxes, and the longitudinal muscles within the tube foot contract, pushing the water back into the ampulla. While water is not constantly “exiting” the system, its use is externalized through the extension and retraction of these tube feet.
The Primary Function of Water Flow
The movement of water through the Water Vascular System facilitates three major functions necessary for the sea star’s survival. Locomotion is the most noticeable function, as the coordinated extension and retraction of the hundreds of tube feet allow the animal to crawl across the seabed or up vertical surfaces.
The hydraulic action of the tube feet is also integral to the sea star’s feeding behavior, especially when preying on bivalves like clams or mussels. The animal uses the gripping power of its tube feet to attach to the shells and exert a steady, prolonged pull, eventually tiring the adductor muscles of the prey and prying the shell open. Once the shell is slightly open, the sea star can evert its stomach into the gap to digest the soft tissues inside.
Furthermore, the tube feet and other thin-walled projections called papulae serve as the primary sites for respiration and waste removal. The constant flow of seawater over the thin walls of the tube feet allows dissolved oxygen to diffuse inward and carbon dioxide and nitrogenous waste, primarily ammonia, to diffuse outward into the water.