Starfish in the seashell shop on Llandudno Pier
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Ten facts about Starfish
They are not fish. Starfish (or Sea Stars) belong to the Phylum Echinodermata. That means they are related to sand dollars, 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 about 2,000 species of starfish. Some live in the intertidal zone, some in deep water, some in tropical areas, some in cold water.
While the five-armed varieties of starfish are the most well known, not all have 5 arms. Some have many more. Take the sun star for instance, which has up to 40 arms!
Amazingly, starfish can regenerate lost arms. This is useful if it is threatened by a predator – it can drop an arm, get away and grow a new arm. starfish house most of their vital organs in their arms, so some can even regenerate an entirely new animal 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, starfish skin may feel leathery, or slightly prickly. They have a tough covering on their upper side, which is made up of plates of calcium carbonate with tiny spines on their surface. The spines are used for protection from predators, which include birds, fish and sea otters.
Instead of blood, starfish 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.
Starfish move using hundreds of tube feet, which are located on their underside. The tube feet are filled with sea water, which the it brings in through the sieve plate, or madreporite, on its top side. They 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 starfish moving around. The tube feet also help to hold its prey, which includes bivalves like clams and mussels.
Speaking of prey, starfish have a rather unique way of eating theirs. A starfishes mouth is on its underside. They prey on bivalves like mussels and clams, as well as small fish, snails, and barnacles. If you’ve ever tried to pry the shell of a clam or mussel open, you know how difficult it is. Starfish wrap their arms around the animal’s shell and pull it open just enough. And then it does something we could never imagine – it pushes its stomach through its mouth and into the bivalve’s shell. It then digests the animal and slides its stomach back into its own body. This unique feeding mechanism allows it to eat larger prey than it would otherwise be able to fit into its tiny mouth.
While they can’t see as well as we do, starfish have an eye spot at the end of each arm. This is a very simple eye that looks like a red spot. The eye doesn’t see much detail, but can sense light and dark.