Even though they are commonly called starfish, these animals are known more scientifically as sea stars, as they're not fish. They do not have gills, fins or a skeleton. Sea stars have a tough, spiny covering and a soft underside. If you turn a live sea star over, you'll likely see its hundreds of tube feet wiggling.
There are over 1,800 species of sea stars, and they come in all sizes, shapes and colors. Their most noticeable characteristic is their arms. Many sea star species have 5 arms, but some, like the sun star, can have up to 40.
Sea stars may reproduce sexually or asexually. There are male and female sea stars, but they are indistinguishable from one another. They reproduce by releasing sperm or eggs into the water, which, one fertilized, become free-swimming larvae that later settle to the ocean bottom.
Sea stars 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.
Sea Star Vascular System:
Sea stars move using their tube feet, and have an advanced water vascular system that they use to fill up their feet with sea water. They do not have blood, but instead take in sea water through the sieve plate, or madreporite, located on top of the sea star, and use that to fill up their feet. They can retract their feet using muscles or use them as suctions to hold on to a substrate or the sea star's prey.
Sea Star Feeding:
Sea stars feed on bivalves like clams and mussels, and other animals such as small fish, barnacles, oysters, snails and limpets. They feed by "grasping" their prey with their arms, and extruding their stomach through their mouth and outside their body, where they digest the prey. They then slide their stomach back into their body.