Although they live underwater and are commonly called “starfish,” they are not true fish. Unlike fish, they do not have gills, scales, or fins.
Also, they move quite differently from fish, which have tails to propel themselves. Instead, sea stars use tiny tube feet for moving along.
Thus, since they are not classified as fish, scientists prefer to call them “sea stars.”
Now, for ten more points.
- They belong to the phylum Echinodermata, relating them to sand dollars, sea urchins, sea cucumbers, and sea lilies – about 7,000 species in all.
- Like many echinoderms, they have radial symmetry – their body parts are arranged around a central axis. Since many sea stars have five-point radial symmetry, ruling out an obvious left and right half, only a top side and a bottom side.
- Sea stars come in roughly 2,000 species ranging from tropical waters to the polar regions. Some dwell in the intertidal zone, while others live in the ocean depths.
- Not all sea stars have just five arms. Some species have many more, such as the sun star, which can have up to 40 arms.
- Most of their vital organs are housed in their arms. Some species can regenerate an entirely new sea star from just one arm and a portion of the star’s central disc, which is useful if a sea star is injured by a predator. It does, though, take about a year for an arm to grow back.
- Instead of blood, sea stars have a circulatory system made up primarily of seawater.
- Their appetite is bigger than their mouth, but no problem. A sea star’s mouth is on its underside, so when it catches food, the sea star will wrap its arms around the animal’s shell and pull it open just slightly. Then the sea star pushes its stomach through its mouth and into the bivalve’s shell, digests the animal, and slides its stomach back into its own body. This allows the sea star to eat larger prey than it would otherwise be able to fit into its tiny mouth.
- Sea stars do have eyes as a spot at the end of each arm. While a five-armed sea star has five eyes, the 40-armed sun star has 40. A sea star eye looks like a red spot. It doesn’t see much detail but it can sense light and dark.
- Rather than a brain, a sea star has a decentralized nervous system, with nerves extending into each arm, something that allows each arm to operate independently while coordinating with the others.
- Predators include birds, fish, and sea otters.





