Asteroideas
True starfish are in the Asteroidea group. These starfish don't have a clear distinction between their central body and arms. In other words, it's hard to tell where their body ends and arms begin. They move with tube feet rather than the motion of their arms. Most starfish are predators, and many feed on mollusks and barnacles. Many starfish can turn a part of their stomachs out through their mouths to digest food outside of their bodies.
Ophiuroideas
There are more than 1,600 species of ophiuroids that include basket stars and brittle stars, according to the "University of California's Museum of Paleontology" website. They are found in most parts of the world, usually in deep water. Ophiuroids usually have five flexible and distinct arms and a disk-shaped body. Brittle stars move quickly by wriggling their arms. Basket stars are usually larger than brittle stars and have branched arms that are even more flexible.
Big Starfishes
Three starfishes vie for the title of biggest starfish. The fat star, or Thromidia catalai lives up to its name as specimens weighing more than 13 lbs. have been found. They are pinkish colored and live along deep reefs in the Pacific. Crown-of-thorns, or Acanthaster planice, is bright orange-red and purple with yellow or hot pink spikes. This beautiful monster has 12 to 19 arms and can grow 20 inches in diameter. It lives in the Indian and Pacific Oceans and preys on coral, threatening the entire reef ecosystem. Midgardia xandaros has the longest arms of any starfish in the world at over 2 feet long, and it can have up to 12 of them. It has an impressive spread of more than 4 feet. Not much is known about Midgardia xandaros as it lives deep in the ocean. It was discovered in the Gulf of Mexico in 1972.
Smallest Starfish
The paddle-spined sea star, discovered in 2007 by Museum Victoria scientists northeast of Portsea, Australia, is less than 1/4 inch, according to the "Museum Victoria" website. It's named after the fringe of plates around each arm, which scientists think help the tiny sea star shelter in seaweed and not be swept away by waves. It can use its sucker feet to pull itself into two pieces. The museum is conducting further research on this small creature.