Starfish, also known as sea stars, are marine invertebrates belonging to the phylum Echinodermata (“spiny skin”). They are characterized by the water vascular system (WVS), a complex network of fluid-filled canals. The WVS utilizes ambient seawater to power many of the animal’s life functions, and is a defining feature for all echinoderms.
How Starfish Use Water
The primary function of the water vascular system is to provide a hydraulic skeleton for movement. Fluid pressure allows the animal to extend and retract its numerous tube feet, located on the underside of its arms. Muscles surrounding internal water sacs, called ampullae, contract to force water into the tube feet, causing them to elongate. This coordinated process allows the starfish to glide across the ocean floor or cling tightly to rocks.
The hydraulic mechanism is also instrumental in feeding and prey capture. Tube feet manipulate food items and generate suction strong enough to pry open the shells of bivalves like clams and mussels. Water moving through the system also plays a role in internal transport, helping to circulate gases and nutrients.
The Madreporite: Water Intake and Regulation
Seawater enters the water vascular system through the madreporite, a specialized, porous, button-like plate. Located on the aboral (upper) surface, it functions as a sieve, filtering incoming water to prevent larger particles from clogging the internal canals.
The madreporite serves as both an entrance and a pressure-equalizing valve for the hydraulic network. Water travels from the madreporite down the stone canal to the ring canal that encircles the mouth. The structure is also involved in maintaining the internal fluid volume of the starfish.
Where Water Exchanges
Water does not exit the starfish through a single dedicated opening but is continuously exchanged across multiple permeable surfaces. The hydraulic fluid in the water vascular system is constantly being refreshed and exchanged with the external environment through the thin walls of the tube feet. As the tube feet extend and retract for movement, their exposed surfaces facilitate the diffusion of water and gases.
Beyond the mechanical exchange, gas and metabolic waste products are eliminated through two primary structures: the tube feet and small, finger-like projections called papulae. The papulae, also known as dermal branchiae or skin gills, are thin-walled outgrowths of the body cavity that extend through the body wall on the aboral surface. These structures significantly increase the surface area available for gas exchange, allowing the starfish to take in oxygen and expel carbon dioxide.
The main nitrogenous waste product, ammonia, is removed from the body primarily by diffusion across the thin membranes of the papulae and the tube feet. This method of waste removal is highly efficient in the marine environment, as the ammonia simply diffuses directly into the surrounding seawater. The tube feet manage hydraulic pressure and a portion of the waste exchange, while the papulae are specialized for respiration and waste elimination.