From The World Of Sea Stars

by Hobbies

Sea stars or starfish are not actually fish but related to the sea urchins. There are around 2000 species of sea stars, some of which live in deep water, some in tropical areas.

Sea stars or starfish are iconic marine animals, that are not actually fish. They do not have gills, scales or fins like true fish, but have tiny tube feet to help them move along. They have spines covering their upper surface and a soft underside. Sea stars are related to sea urchins in having a five-point radial symmetry, that is, their body plan has five sections or multiples, thereof arranged around a central disc.

Starfish Are Not True Fish But Related To Sea Urchins


There are around 2000 species of sea stars, some of which live in deep water, some in tropical areas while others in the cold water. They can only thrive in salt water, and are not able to live in freshwater due to the amount of calcium they require.

Not all of them have five arms. Some, such as the sun star have up to 40 arms. 

An amazing fact being, that these fascinating marine creatures can regenerate lost arms. When threatened by a predator, the sea star can drop an arm, get away and grow a new arm. It houses most of its vital organs in its arms, thus an entirely new sea star can rejuvenate from just an arm and a portion of the central disc. It takes about a year for an arm to grow back.

Starfish are protected by an armour. Depending on the species, their skin may feel leathery or slightly prickly. They have a tough covering on the upper side made of plates of calcium carbonate with tiny spines on the surface. 

Instead of blood, they have a water vascular system, in which they pump sea water through a sieve plate into hundreds of tube feet, located on their underside, to extend them. Muscles in the tube feet retract them, making the starfish move around in the water. These tiny projections have suckers at their ends, that help them adhere to bivalves (scallops or clams) and open them up.

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Amazing Facts About Sea Stars

A sea star's mouth is on its underside. It preys on mussels and clams, as well as on small fish and snails. When in contact with its prey,  it would wrap its arm around the animal's shell, and pull it open just enough. Then would push its stomach from its mouth into the bivalve's shell, digest the animal, and slide its stomach back into its own body, then the food is transferred into the next part, the pyloric stomach.

Sea stars have an eyespot at the end of each arm that looks like a red spot and helps them locate their food.

Most of these marine beings cannot move quickly, the typical speed of a leather star being 15 cms in a minute. Some burrowing species have points rather than suckers on their long tube feet and are capable of "gliding" across the ocean floor. A sand star can travel at a speed of 2.8 meters per minute.

Although they do not have well-defined sense organs, they are sensitive to touch, light, temperature, orientation, and the status of water around them.

They lack a proper brain but have a complex network of nerves with a nerve ring around the mouth. The starfish heart beats around six times every minute.

The average lifespan of a sea star is 35 years. According to Robert Paine, they are keystone species in their respective marine communities.

Reference sources:

Updated: 01/05/2016, Hobbies
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candy47 on 10/19/2015

Interesting that their heart beats 6 times a minute and their life span isn't what I would expect. Nice article.

MBC on 10/17/2015

Great photos and information

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