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Class Asteroidea


Knobby Starfish Knobby Starfish
Borut Furlan/WaterFrame/Getty Images Sea Star Regenerating Lost Arms / Blue Ocean Society

A Northern sea star ([i]Asterias vulgaris[/i]) regenerating 3 lost arms.

© Blue Ocean Society Northern Sea Star / Blue Ocean Society

Northern sea star. This image shows the sea star's spines and the madreporite (the white dot), which is used to bring sea water into the star's vascular system.

© Blue Ocean Society

While the classification name, "Asteroidea," may not be familiar, the organisms it contains likely are. Asteroidea includes the sea stars, commonly called starfish. With about 1,800 known species, sea stars are a variety of sizes and color, and are a wide-ranging marine invertebrate.


Organisms in the Class Asteroidea have several arms (usually between 5 and 40) arranged around a central disk.

Asteroidea's Water Vascular System

The central disk contains the madreporite, an opening that lets water into the asteroid's water vascular system. Having a water vascular system means that sea stars have no blood, but bring water in through their madreporite and move it through a series of canals, where it is then used to propel their tube feet.


The Asteroidea are known as the "true stars," and are in a separate class from the brittle stars, which have a more defined separation between their arms and their central disk.

Habitat and Distribution:

Asteroidea can be found in oceans around the world, inhabiting a wide range of water depths, from the intertidal zone to the deep sea.


Asteroids feed on other, usually sessile organisms such as barnacles and mussels. The crown-of-thorns starfish, however, is causing extensive damage by predation on coral reefs.

The mouth of an asteroid is located on its underside. Many asteroids feed by expelling their stomach and digesting their prey outside their body.


Asteroids may reproduce sexually or asexually. There are male and female sea stars, but they are indistinguishable from one another. These animals reproduce sexually by releasing sperm or eggs into the water, which, once fertilized, become free-swimming larvae that later settle to the ocean bottom.

Asteroids reproduce asexually by regeneration. It is possible for a sea star to not only regenerate an arm, but also nearly its entire body if at least a portion of the sea star's central disc remains.

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