Oct 29, 2009 – Sea Star starfish in the Indo-Pacific Ocean – Brandon Cole / Visuals Unlimited, Inc

• Wake up, eat fresh fruit for breakfast, go snorkeling, take a nap…
  Visit the tropics and dream a while
• The biggest starfish in the world has at least three times as many arms as this little guy.
  How big is it?
• If you cut a starfish in half, both pieces will regenerate into a whole new starfish.
  What else do echinoderms do?
• Everyone knows the ocean is full of wonder and mystery.
  But who knew there was so much singing?

Sea stars (also often called “starfish”) are echinoderms belonging to the class Asteroidea. The names “sea star” and “starfish” essentially refer to members of the Class Asteroidea. However, common usage frequently finds “sea star” and “starfish” also applied to ophiuroids which are correctly referred to as “brittle stars or “basket stars”.

Sea stars exhibit a superficially radial symmetry. They typically have five “arms” which radiate from a central disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars do exhibit some superficial remnant of this body structure, evident in their larval pluteus forms.

Sea stars do not rely on a jointed, movable skeleton for support and locomotion (although they are protected by their skeleton), but instead possess a hydraulic water vascular system that aids in locomotion. The water vascular system has many projections called tube feet on the ventral face of the sea star’s arms which function in locomotion and aid with feeding. Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the second stomach can be extended outward to engulf and digest prey. This feature allows the sea star to hunt prey that is much larger than its mouth would otherwise allow. Sea stars are able to regenerate lost arms. A new sea star may be regenerated from a single arm attached to a portion of the central disk.