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They are all marine creatures, although some live in the intertidal zone, some in deep water, some in tropical areas and some in cold water. They typically have a central disc body and five arms (or multiples thereof). Some species have many more arms, such as the 40 arms of the sun star. They are often brightly coloured in shades of red or orange, or blue/grey/brown.
The surface of their skin can be spiny or smooth, depending on the species. A starfish's skin may feel leathery, or slightly prickly. Starfish have a tough covering on their upper side, which is made up of plates of calcium carbonate with tiny spines on their surface. A starfish's spines are used for protection from predators.
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.
Instead of blood, starfish have a water vascular system, in which the sea star pumps sea water through its sieve plate. Starfish have hundreds of little feet along their arms called tube-feet. These tubes are connected by small canals and allow them to operate a water vascular system which enables their respiration, locomotion, food and waste transpiration. They move across both land and sea by alternatively contracting muscles that allow water into the tube feet, causing them to extend and contract in order to push them against the ground.
Their mouth and gut is located at the bottom of their central disc. They have a unique way of eating their prey. They eat bivalves like mussels and clams, as well as small fish, snails, and barnacles. Starfish have two stomachs and they can push their first stomach out through their mouth and into its preys shell. This means they can eat something like a clam by pushing their stomach into its shell, rather than trying to get it into their mouth. It starts to digest the animal, then pulls its stomach back in and passes it to the second stomach. This allows the sea star to eat larger prey than it would otherwise be able to fit into its tiny mouth.
Starfish can reproduce both sexually and asexually, although they commonly reproduce by spawning. To increase their chances of fertilization, starfish probably gather in groups when they are ready to spawn, use environmental signals to coordinate timing (day length to indicate the correct time of the year, dawn or dusk to indicate the correct time of day), and may use chemical signals to indicate their readiness to each other. Fertilized eggs live as plankton, suspended in the water and swimming by using beating cilia. Unlike adults, the larvae are bilaterally symmetrical and have a distinct left and right side. Eventually, they undergo a complete metamorphosis, settle to the bottom, and grow into adults.
Some, but not all, species of starfish can regrow a new limb given time. Some species can even regrow a new central disc from a single arm, therefore creating a whole new starfish. However, often part of the central body is required to be joined to the arm. This fragmentation is often used to evade predators or as an escape response. It often takes a starfish several months or even years to carry out this regeneration, and they are vulnerable to infections during this stage.
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