1. Starfish are echinoderms (spiny skinned sea urchins). They are also known as sea stars and are not really fish despite the name been given to them. Starfish cannot swim, and they do not use gills to breathe.
2. There are over 2,000 species of starfish. All echinoderms have five-point radial symmetry, which means that their body plan has five sections arranged around a central disk.
3. Starfish are found in the deep blue sea of the ocean and shallow water as well. They are found in every ocean of the world. They are never found in fresh water.
4. Most starfish have a spiky shell which offers them protection. Depending on the species, a sea star's skin may feel leathery, or slightly prickly. This tough covering on their upper side is made up of plates of calcium carbonate with tiny spines on their surface. A sea star's spines are used for protection from predators, which include fish, sea otters and birds. Starfish come in a variety of colors and have many different types of patterns.
5. While the five-armed varieties of sea star are the most well known, not all sea stars have 5 arms. Some have many more. Take the sun star for instance, which has up to 40 arms. The Coscinasterias Calamaria also known as the eleven-armed sea star has an armspread that can go up to 30 cm. As the name suggests, it does have eleven arms but there are times when the number can go up to 14.
6. Amazingly, sea stars can regenerate lost arms. This is useful if the sea star is threatened by a predator. The starfish can drop an arm to get away. Sea stars house most of their vital organs in their arms, so some can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc. It takes almost a year for this to happen.
7. Starfish have hundreds of tiny projections known as tube feet on the underside of their body. The tube feet allow the starfish to move along the ocean bottom and open upon the scallops and clams they hunt for food.
8. The starfish has two stomachs. The cardiac stomach eats the food outside the starfish’s body. When the cardiac stomach comes back into the body, the food in it is transferred to the pyloric stomach. After the tube feet open the shell of its prey, the cardiac stomach is extended into the shell to pull the food inside. The tube feet play an important role in helping the starfish to procure its food. The tube feet are used to open up the oysters or clams. Then the stomach is extended into the shell to pull the food inside. They prey on bivalves like mussels and clams, as well as small fish, snails, and barnacles.
9. Instead of blood, sea stars have a water vascular system, in which the sea star pumps sea water through its sieve plate, or madreporite, into its tube feet to extend them. Muscles within the tube feet retract them.
10. The starfish have microscopic eyes at the end of each arm; this enables the starfish to view movement and differentiate between light and dark. They don’t, however, see much detail.