Asteroidea – Starfish or Sea Stars
The Starfish are a class in the Phylum of the Echinodermata. Asteroidea comprise about 1600 species. The scientific name is derived from the Greek and is composed of astḗr “star” and eídos “form”.
The basic shape of the body is a five-armed star, with the arms tapering. In the middle is the central disc. However, there are many different varieties. Thus, the number of the arms in the Solasteridae is dependent on the age, species of the genus Heliaster have over 40 arms and in the genus Labidiaster it is between 25 and 40. There are also variations within a species. If some of the starfishes are bitten off an arm, two new ones grow.
In addition, there are Asteroidea with an extremely flat body, while others resemble a prickly sea urchin. It also happens that the area between the arms is so pronounced, whereby the body forms a pentagon.
The size of the Starfish ranges from one centimetre to over one meter. On average, however, it is about 20cm.
The skin skeleton of the Starfish consists of limestone plates that can be moved against each other. They retain their flexibility.
The mouth is at the centre of the body. The anus lies between the two arms. However, this is not present in all species.
Also at the bottom are small feet. A group of feet extends into the corresponding direction for the movement, adheres firmly, and then contracts again, with which the body is retracted. This can be seen very good in the following video.
Asteroidea do not have eyes. For this, they have light-sense cells with which they perceive changes in brightness.
Habitat and Distribution
Starfish live exclusively in the sea, being detected in depths down to 10000m. They live benthic. Asteroidea are found on rocky and sandy or muddy ground, as well as in coral reefs, sea grass meadows and kelp forests.
Their distribution ranges from the tropical waters to the polar zones, with the greatest diversity of species being found near the coast.
In the temperate climates most of the asteroids live predatorily or of carrion; in the tropical areas there are more algae, detritus and mudfeeders.
Predators prefer feeding on slow or stationary animals. The range of possible prey is great. For example Mussels, snails, crustaceans, polychaete worms, sea anemones, corals, sponges, moss animals and sea squirts are on the menu. Other echinoderms such as sea urchins, brittle stars, sea cucumbers and starfish are also eaten.
Some species of asteroids can evert their stomach. The prey is then pre-digested outside the body. When this has happened, the stomach and the food are put back into the body. Some starfish are also able to eat animals as a whole.
Although there are exceptions, most species of starfish have separated sexes and fertilization takes place externally, i.e. in the open water.
After fertilization, they live planktonic. After two larval stages a metamorphosis takes place to the juvenile starfish.
An asexual reproduction is also possible. Thus, a severed arm, in some species, can again become a complete starfish. There are also species of asteroids that can actively divide their body.