Ampullae are bulb-shaped structures located above the tube feet of starfish and other echinoderms. Ampullae are an important part of the water vascular system of starfish, which is the responsible for the animal's locomotion. The only function of the ampullae is to store the water that enters the vascular system and send it to the minuscule tube feet, located in the ventral side of the starfish's arms.
Each arm of a starfish has several hundred ampullae. These diminutive sac-like organs have a mesh of muscle fibers and can expand when water from the radial canal is pumped into them. The muscle fibers of the ampullae contract, preventing the water to flow back to the radial canal. The contraction of the muscles also causes the water stored in the ampullae to move to the tube feet, which are the locomotion organs of the starfish. Nerve cells connected to the ampullae and other parts of the vascular water system control the relaxation and contraction of the ampullae's muscle fibers.
Water Vascular System
Water enters the vascular system of a starfish through the madreporite, an orifice located on the upper part of the animal. The madreporite is linked to the radial or ring canal, which branches out to each arm of the starfish, forming the lateral canals. Specialized cells with cillia help move the water inside the vascular system. Each lateral canal is connected to the ampullae, which are linked to tube feet. The minuscule tube feet expand with water, being able to move by contraction and even attach the starfish to rock or coral surfaces. Starfish can only move slowly through this water vascular system.
Geographic Distribution of Starfish
Starfish live only in marine ecosystems, near the bottom of oceans. The majority of starfish species live in the northern Pacific. The bat star (Asterina miniata) is a common species in North America, at waters deeper than 900 feet. Although most species prefer shallower waters, Albatrossaster richardi is found at 19,800 feet.
In addition to feet, a starfish also has its mouth on the underside, while the anus is on the upper side. Starfish are carnivores, and most species can eject their stomachs out of their mouths to capture prey too large to pass through their mouths. Later, they withdraw their stomachs back into their bodies and finish digestion. Some species also feed on algae.