There are some 2,000 species of sea star living in all the world’s oceans, from tropical habitats to the cold seafloor.

The lifespan of a starfish varies considerably between species. It ranges from 5 to 35 years.

The five-arm varieties are the most common, hence their name, but species with 10, 20, and even 40 arms exist.

They have bony, calcified skin, which protects them from most predators, and many wear striking colorsthat camouflage them or scare off potential attackers.

They can vary widely in size, with the paddle-spined starfish as small as 1 centimeter (0.4 inch) wide and the Midgardia xandros topping 1.38 meters (4.5 feet) across.

Starfish have no brains and no blood. Their nervous system is spread through their arms and their “blood” is actually filtered sea water.

They are found from the intertidal zone down to abyssal depths, 6,000 meters (20,000 feet) below the surface.

Although they are named “starfish“, they are not related to fish at all. Starfish, like sea urchins and sand dollars, do not have backbones, which makes them part of a group called invertebrates.

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.

The underside of the starfish is covered with hundreds of tube feet, which it uses for walking around, for attaching tightly to rocks, and for holding on to prey. To move, each tube foot swings like a leg, lifting up and swinging forward, then planting itself on the ground and pushing back.

Starfish eat in a very strange way. The starfish’s mouth is on the bottom of its body. The mouth is at the middle of the star. A starfish can push its stomach out through its mouth.

The starfish has two stomachs. The cardiac stomach eats the food outside the starfish’s body. When the cardiac stomach comes back into the body, the food in it is transferred to the pyloric stomach. After the tube feet open the shell of its prey, the cardiac stomach is extended into the shell to pull the food inside. The tube feet play an important role in helping the starfish to procure its food. The tube feet are used to open up the oysters or clams. Then the stomach is extended into the shell to pull the food inside.

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They prey on bivalves like mussels and clams, as well as small fish, snails, and barnacles.

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.

Although they are able to regenerate detached arms and some detached arms can grow into new starfish, most species reproduce by spreading and fertilizing eggs.

The female starfish is capable is releasing over 2 million eggs at any one time, although the average amount of eggs that the female starfish releases is closer to 1 million. This is important because small portion of the eggs survive to become adult starfish.

The eggs released by the female starfish are then fertilized by the male starfish and the fertilized eggs develop into larvae which are able to swim around. Starfish larvae [pic. below] swim for about three weeks before settling and beginning metamorphosis into the more common appearance of the starfish.

Starfish reproduction typically is heterosexual, but hermaphroditism (reproductive organs of both sexes in one animal) occurs.

Starfish can change their gender when it’s convenient for them and can easily change back to their original sex.

Starfish, despite their crusty exteriors, are vulnerable to predators like crabs, sea otters, sharks, manta rays and other starfish.

Purely marine animals, there are no freshwater sea stars, and only a few live in brackish water (salt water and fresh water mixed together).

Starfish aren’t social creatures, but some species will congregate in large groups during certain times of the year to feed.

Common feature for all starfish is that their body is radially symmetrical (regular arrangement of body parts around a central axis) .

With their appealing symmetrical shape, starfish have played a part in literature, legend, design andpopular culture.

With its multiple arms, the starfish provides a popular metaphor for computer networks, companies and software tools.

Starfish are sometimes eaten in China, Japan and in Micronesia.

The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death.

The starfish is today considered to be a threatened species of animal mainly due to habitat loss and pollution Swhich are drastically reducing the starfish populations.

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