Starfish (sea stars) are beautiful animals that can be a variety of colors, shapes and sizes, although all resemble a star. While some appear smooth, they all have spines covering their upper surface and a soft underside. If you gently turn over a live sea star, you'll see its tube feet wiggling back at you. These iconic marine animals are fascinating creatures. Learn more about them below.
Sea stars are not fish.
Although sea stars live underwater and are commonly called "starfish," they are not fish. They do not have gills, scales, or fins like fish do and they move quite differently from fish. While fish propel themselves with their tails, sea stars have tiny tube feet to help them move along (see more on that below).
Sea stars belong to the Phylum Echinodermata. That means they are related to sand dollars (yes, they are a real animal), sea urchins, and sea cucumbers. All echinoderms have five-point radial symmetry, which means that their body plan has five sections (or multiples thereof) arranged around a central disk. Next time you're in a beach-themed store, see if you can find a dried sea star, sand dollar and sea urchin and find the 5 sections in each.
There are thousands of sea star species.
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Not all sea stars have 5 arms.
While the five-armed varieties of sea star are the most well known, not all sea stars have 5 arms. Some have many more. Take the sun star for instance, which has up to 40 arms!
Amazingly, sea stars can regenerate lost arms. This is useful if the sea star is threatened by a predator - it can drop an arm, get away and grow a new arm. Sea stars house most of their vital organs in their arms, so some can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc. It won't happen too quickly, though. It takes about a year for an arm to grow back.
Depending on the species, a sea star's skin may feel leathery, or slightly prickly. Sea stars have a tough covering on their upper side, which is made up of plates of calcium carbonate with tiny spines on their surface. A sea star's spines are used for protection from predators, which include birds, fish and sea otters.
Sea stars do not have blood.
Instead of blood, sea stars have a water vascular system, in which the sea star pumps sea water through its sieve plate, or madreporite, into its tube feet to extend them. Muscles within the tube feet retract them.
Sea stars move using hundreds of tube feet, which are located on their underside. The tube feet are filled with sea water, which the sea star brings in through the sieve plate, or madreporite, on its top side. Sea stars can move more quickly than you might expect. If you ever get a chance, try visiting a tide pool or aquarium and take a moment to watch a sea star moving around. The sea star's tube feet also help the sea star hold its prey, which includes bivalves like clams and mussels.