There are over 2,000 species of sea star living in all the world’s oceans, and they come in a variety of striking colors. Though they typically have five arms, some species can have less, or more, with some reaching up to 40 limbs.
These invertebrates have two stomachs--one is used for digestion, and the second one can be everted out to devour and digest its victim. They are carnivores that usually snack on shelled animals such as mussels and clams. Using their tiny, suction-cupped tube feet, most species have the remarkable ability to force open the shells of bivalve mollusks, and their sack-like cardiac stomach comes out from their mouth and flows out inside the shell. The stomach then swallows up the victim to digest it, and finally retracts back into the body.
The sunflower sea star (2nd photo) is the largest sea star in the world. It has 16-24 arms that can grow up to 1 m (3 ft) across. Its color ranges from bright orange, yellow and red to brown and sometimes purple.
Some common sea star species include:
Carpet sea star — is native to southern Australia. They dwell
on rock pools and can be found in four color variations: the cushion sea
star has eight arms and can be any color or combination of colors; the
Patiriella exigua is turquoise with five arms; the P. brevespina is
purple with orange feet and six arms, and the P. gunii can be in any
color and has six arms.
Crown-of-thorns sea star (6th photo here) — is the second
largest sea star in the world. It is a nocturnal sea star that was named
from the venomous thorn-like spines that cover its body. They can be
found in tropical coral reefs in the Red Sea, the Indian Ocean, and the
Pincushion sea star (4th photo) — they live in tropical oceans. The species Culcita schmideliana has patches of different colors and doesn't have any limbs.