The water vascular system is a hydraulic network found in echinoderms, a group of marine invertebrates that includes starfish. This specialized system uses water pressure to facilitate various bodily functions. It plays a foundational role in the starfish’s ability to navigate its environment, acquire food, and perform other daily activities.
Key Components of the System
The water vascular system begins with the madreporite, a small, sieve-like, calcareous plate on the starfish’s upper (aboral) surface. Beneath the madreporite lies a small sac, or ampulla, which connects to the stone canal.
The stone canal is a tube that extends downwards from the madreporite, leading to the ring canal. This ring canal forms a circular tube around the central disc of the starfish, near its mouth. Extending from the ring canal are five radial canals, with one running down each arm within the ambulacral groove on the underside.
Along each radial canal, numerous lateral canals branch off, each connecting to a tube foot. A valve at the junction of the lateral canal and the tube foot prevents the backward flow of fluid. Each tube foot consists of a bulb-shaped ampulla, located internally, and a flexible, external stalk called the podium, which culminates in a sucker.
How the System Operates
The water vascular system functions as a hydraulic mechanism, powered by seawater. Water enters the system through the madreporite, a porous plate on the aboral surface, and then travels through fine pore-canals that unite to form collecting canals, eventually opening into an ampulla beneath the madreporite. From this ampulla, water flows into the S-shaped stone canal, connecting with the pentagonal ring canal around the mouth.
From the ring canal, water is distributed into the five radial canals, each extending along an arm. Along these radial canals, water enters numerous lateral canals, each leading to an ampulla connected to a tube foot. When the starfish extends a tube foot, muscles in the ampulla contract, forcing water from the ampulla into the podium.
The influx of water causes the podium to elongate and extend. The suckers at the end of the tube feet then attach to a surface. To retract the tube foot, the longitudinal muscles within the podium contract, pushing the water back into the ampulla, which then relaxes and refills. This coordinated extension and retraction of thousands of tube feet allows the starfish to move across surfaces and cling to objects.
Vital Functions of the Water Vascular System
The water vascular system is central to several functions for starfish, beginning with locomotion. The coordinated extension and retraction of the tube feet, driven by hydraulic pressure, allows the starfish to crawl and move across various surfaces. The suckers on the tube feet provide adhesion, enabling the starfish to grip onto substrates and pull itself forward. This mechanism also allows starfish to climb vertically on rocks.
Beyond movement, the system is also involved in feeding. Starfish use their tube feet to grasp and manipulate prey, such as bivalves. The tube feet can generate significant suction, allowing the starfish to pry open the shells of its prey, often just enough for the starfish to evert its stomach into the shell to digest the prey externally.
The water vascular system also facilitates respiration and waste excretion. The thin walls of the tube feet allow for gas exchange, where oxygen from the surrounding seawater diffuses into the starfish’s body, and carbon dioxide is released. Similarly, metabolic wastes can be expelled through the surface of the tube feet. The fluid within the water vascular system, which is largely seawater with some proteins and potassium salts, also contains amoeboid cells called coelomocytes, which aid in internal transport and defense processes.