Starfish, commonly known as sea stars, are marine invertebrates belonging to the phylum Echinodermata. They possess a defining biological feature called the water vascular system (WVS). This unique hydraulic network is a complex series of fluid-filled canals that powers nearly every aspect of the starfish’s life. The system is responsible for several functions, including movement, feeding, and the exchange of gases.
The Essential Anatomy of the System
The water vascular system begins with the madreporite, a small, porous, sieve-like plate located on the starfish’s aboral (upper) surface. This structure acts as an entry point, allowing seawater to filter into the internal canal system. From the madreporite, the water flows through a short, calcareous-lined tube known as the stone canal.
The stone canal descends to the central disc where it connects to the ring canal, which encircles the animal’s mouth. Branching off the ring canal are the radial canals, with one running longitudinally down the ambulacral groove on the underside of each arm. The ring canal also features specialized internal pouches, such as Polian vesicles, which aid in regulating fluid pressure and volume.
Along the length of each radial canal, numerous short lateral canals branch off, connecting to a bulb-shaped muscular sac called an ampulla. The ampulla sits just inside the body wall and is directly attached to a tube foot (podium), which projects externally through the ambulacral groove. This ampulla-tube foot unit is the functional end-point of the system.
The Hydraulic Mechanism of Locomotion
Locomotion is achieved through the coordinated, hydraulic action of thousands of tube feet lining the ambulacral grooves. The process begins when the ampulla, a muscular reservoir, contracts to force its internal fluid into the connected tube foot. This increase in hydrostatic pressure causes the tube foot to stiffen and extend outward.
Once extended, the distal end of the tube foot, which features a suction cup, makes contact with the substrate. The starfish employs a dual-secretory system, first releasing an adhesive chemical to secure its grip, followed by a de-adhesive chemical to release it. Muscles within the tube foot contract after attachment, shortening the podium and pulling the animal forward.
The extension and retraction of the tube feet are not synchronized but occur in a highly coordinated, wave-like fashion. This allows the starfish to move slowly but powerfully in any direction, or to hold fast against strong currents. The number of tube feet working together generates a significant cumulative force, enabling the animal to navigate complex surfaces and overcome obstacles.
Critical Roles in Feeding and Gas Exchange
Beyond movement, the water vascular system is instrumental in the starfish’s unique feeding strategy, particularly when preying on bivalves. The tube feet secure a powerful grip on the two shells of the bivalve. The starfish applies a sustained pulling force, leveraging the hydraulic power of the tube feet to fatigue the bivalve’s adductor muscles.
Once the shell gapes open by a minuscule amount, the starfish employs its specialized digestive method. It everts its cardial stomach through its mouth and inserts it into the opening of the shell. Digestion occurs externally, with the stomach tissue secreting enzymes directly onto the bivalve’s soft tissues. The hydraulic grip of the tube feet is a prerequisite for this unique form of external feeding.
The water vascular system also plays a role in gas exchange and waste elimination. The thin walls of the tube feet provide a large surface area for the passive diffusion of gases with the surrounding seawater. Oxygen diffuses inward, while carbon dioxide diffuses outward. The system assists in the expulsion of nitrogenous waste, primarily ammonia, which diffuses out through the thin membrane surfaces of the tube feet.