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Sea Stars
Sea stars are also called Asteroidea. Asteroidea are the type of star fish that is most often associated with the idea of the star fish, they are slightly spiky and usually have five arms. Most Asteroidea live in the northern Pacific ocean but can survive all over the world.
Brittle Stars
Brittle stars are also called Ophiuroidea. These starfish are found as deep as almost four miles down on the ocean floor. These starfish are unique because they can survive in brackish water, which is a rare quality among echinoderms. Many brittle stars are identifiable because they have small, round bodies and long, spindly arms like an octopus, while other starfish are generally slow-moving with short arms.
Five-Armed Starfish
The majority of starfish have five arms, such as the beaded star, the brown star, the common North Atlantic starfish, the netted star, the spiny beaded star, the sugar star and perhaps the most iconic starfish, the cushion or Bahamas star.
Starfish With Six Arms Or More
There are starfish species with more than five arms, species with ten arms, species with 20 arms and even species with up to 50 arms. These arms are the points of the star shape. Examples of species of starfish with more arms than usual include the sunflower star and the millipede starfish. Species of starfish in both the Asteroidea family and the Ophiuroidea family can have only have five or more arms.
Similarities
While there are different types of starfish, all starfish share several similarities with each other. For example, while all starfish are docile and slow-moving, they are still ocean predators. Starfish are carnivorous and will eat any animal that is slow-moving or stationary. All starfish eat other animals like clams, mollusks, snails or even larger prey like injured fish.
All types of starfish also have the same biological makeup. For example, starfish don't have any blood but have a vascular system that is filled with filtered sea water, and they breathe by taking in oxygen from the water.