Starfish or Sea Stars
Despite most people knowing these creatures as Starfish (scientific name Asteroidea), they are actually classed as animals and not fish. Scientists and marine life experts prefer to call them Sea Stars.
There are about 2,000 known species of sea stars and they come in a variety of shapes, colours and sizes. Each and every one of them resembles the shape of a star, which is how they were commonly names Starfish.
We have seen a lot of scuba divers pass by the beautiful sea stars without giving them a second glance. However, you can learn a lot from these mesmerising sea creatures. Besides being interesting in themselves, they also can have tiny creatures living on them, such as shrimps and crabs, great for macro photographers.
All sea stars have spines on their upper surfaces, even though some sea stars can appear to be smooth. Their upper side is quite tough, made up of plates of calcium carbonate. Their spines offer protection from predators. They have tiny tube like feet on their undersides. Unlike fish, they do not have gills, fins or scales, they use their tube like feet to help them move along and they are able to move quickly too.
Due to the symmetry of sea stars, they don’t have a left side or right side, only a top and bottom. The amazing thing about sea stars is their ability to regenerate a lost arm if a predator threatens them. Most of the sea star’s vital organs are in their arms. This means that some sea star species are able to regenerate an entirely new sea star from simply one arm and a portion of the central disc.
Sea stars don’t have blood. Instead, their cardiovascular system is made up mostly of seawater. The sea star has a sieve plate, which is like a trapdoor called a madreporite. It is often noticed as a light coloured spot on the topside of the sea star. The seawater moves from the madreporte into the sea star’s tube feet and that is how it extends an arm.
The sea star uses its tube like feet to hold its prey, which includes small fish, clams and mussels. They have developed their own unique way eating the insides of these shellfish. They manage to prise open the shell, just enough to push its stomach through its mouth and into the shell. It then digests the animal before sliding its stomach back into its own body.
If you thought that a sea star didn’t have eyes, then you are mistaken. Unfortunately, they cannot sea as well as humans can, but they do have one eye on each of their arms. Their eyes are simple and resemble a red spot and are located at the end of each arm.
It is difficult to distinguish between male and female sea stars because they appear identical. They release sperm and eggs into the water. When the sperm fertilizes the eggs, they produce swimming larvae. These larvae settle on the seabed, which later develop into adult sea stars.
One of the notably spiny sea stars is the crown of thorns, hence the name. Crown of thorns starfish have an enormous appetite for coral and each adult can get through up to 10 square metres of precious coral tissue every year.
Next time you are on a fun dive, don’t swim past those lovely sea stars. They are more fascinating than what you may think.