Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea. However, common usage frequently finds "starfish" and "sea star" also applied to ophiuroids which are correctly referred to as "brittle stars" or "basket stars".
Australian southern sand star (Luidia australiae): The mottled coloring of this species helps tp camouflage it in the sediment of seagrass beds of the Pacific Ocean around Australia and New Zealand. Typically sporting seven arms, it grows to be around 16 inches in diameter. It is sometimes found washed up on the beach after storms.
Sunflower star (Pycnopodia helianthoides): The sunflower star is the largest sea star in the world, reaching an armspan of 3.3 feet. That space is taken up by 16-24 arms. They're found along the coast of North America, from Alaska to California, but they're largest in the northern areas. They dine on sea urchins, clams and snails and are usually found in subtidal areas where there is always water, since they can't support their bodies out of water.
Nine-armed Sea Star (Luidia senegalensis): the only species to be named for the fact that it has nine arms. Found in the western Atlantic ocean, this starfish, like many species, everts its stomach to engulf its prey, and essentially "swallows" with its stomach. The nine-arm sea star dines on mollusks, small crustaceans, and sea worms, as well as filters stomachfuls of sediment to feast on tiny organisms.
Pink short spined star (Pisaster brevispinus): Even starfish look pretty in pink. This sea star can reach a whopping two feet in diameter, and can weigh up to two pounds. It dines on clams and sand dollars, so is usually found on sand or mud, but its soft texture allows it to also grip on coral and rocks where it can feast on mussels, tube worms and barnacles.
Crown-of-thorns starfish (Acanthaster planci): The name of this species is fairly clear. The spines covering its upper surface make it look like, well, you know. Those spines are also venomous, which aid it in its quest for world domination. Found over a wide range in subtropical waters, from the Red Sea to across the Indian Ocean, and across the Pacific Ocean to the western coast of Central America, this species preys on coral polyps. As one of the largest starfish in the world