Madreporite Definition and Examples

The madreporite is an essential part of the circulation system in echinoderms. Through this plate, which is also called a sieve plate, the echinoderm draws in seawater and expels water to fuel its vascular system. The madreporite functions like a trap door through which water can move in and out in a controlled manner.

Composition of the Madreporite

The name of this structure came from its resemblance to a genus of stony corals called madreporite. These corals have grooves and many small pores. The madreporite is made of calcium carbonate and is covered in pores. It also looks grooved like some stony corals. 

Function of the Madreporite

Echinoderms don't have a circulatory system of blood. Instead, they rely on water for their circulatory system, which is called a water vascular system. But the water doesn't flow freely in and out, it flows in and out through a valve, which is the madreporite. Cilia beating in the pores of the madreporite bring the water in and out. 

Once the water is inside the echinoderm's body, it flows into canals throughout the body.

While water can enter a sea star's body through other pores, the madreporite plays an important part in maintaining the osmotic pressure needed to maintain the sea star's body structure.

The madreporite also may help protect the sea star and keep it functioning properly. Water drawn in through the madreporite passes into Tiedemann's bodies, which are pockets where the water picks up amoebocytes, cells that can move throughout the body and help with different functions. 

Examples of Animals With a Madreporite

Most echinoderms have a madreporite. Animals in this phylum include sea stars, sand dollars, sea urchins and sea cucumbers.

Some animals, like some large species of sea stars, may have multiple madreporites. The madreporite is located on the aboral (top) surface in sea stars, sand dollars, and sea urchins, but in brittle stars, the madreporite is on the oral (bottom) surface. Sea cucumbers have a madreporite, but it's located inside the body.

Madreporite

Exploring a tide pool and find an echinoderm? If you're looking to see the madreporite, it is probably most visible on sea stars. The madreporite on a sea star (starfish) is often visible as a small, smooth spot on the sea star's upper side, located off-center. It is often made up of a color that contrasts with the rest of the sea star (e.g., a bright white, yellow, orange, etc.).

Sources

• Coulombe, D.A. 1984. The Seaside Naturalist. Simon & Schuster. 246pp.
• Ferguson, J.C. 1992. The Function of the Madreporite in Body Fluid Volume Maintenancy by an Intertidal Starfish, Pisaster ochraceus. Biol.Bull. 183:482-489.
• Mah, C.L. 2011. Secrets of the Starfish Sieve Plate & Madreporite Mysteries. The Echinoblog. Accessed September 29, 2015.
• Meinkoth, N.A. 1981. National Audubon Society Field Guide to North American Seashore Creatures. Alfred A. Knopf: New York.