The Echinodermata are a phylum of the Deuterostomes and comprise about 6300 species. Among the most famous classes of Echinodermata are the starfish, sea urchins and the sea cucumbers. They are usually quiet colourful and vary in diameter from 5mm to 140cm.
Although some brittle stars have light-sensitive cells on their arms, Echinodermata generally have neither heart, brain, or eyes. Apart from feather stars, sea cucumbers and some sea urchins, their mouth opening is at the bottom while the anus sits on the top.
Furthermore, the Echinodermata are separated sexually and reproduce by direct release of sperm cells and eggs into the water. In addition, some brittle stars and starfish can reproduce asexually by dividing an arm or simply dividing their bodies. If they are in danger, they can consciously do so.
Habitat and Distribution
Echinoderms live worldwide in all seas and oceans and few of them even in the brackish waters. In particular, Echinodermata make up to 90% of the biomass in some habitats. Most species are adapted to specific environmental conditions and are therefore regionally limited.
The majority of the echinoderms are bentic and some of them dig in sand. While coastal species mostly live up to a maximum depth of 100m, others live up to a depth of 10000m.
While some echinoderms live on organic waste and plant parts, others feed on predators. On the one hand, they take care of the degradation of organic material and, on the other hand, the regulation of small animal populations.
In addition, mainly sea cucumbers and specialized sea urchins dig the seabed as earthworms do on land. The seabed is freed from organic substances and changed both physically and chemically.
However, Echinodermata are becoming a plague in some regions. Because of the lack of predators, large populations of echinoderm are falling over vast areas and eating them completely bald.
The Starfish are a class of the Echinodermata and contain about 1600 species. When diving in the Canary Islands, we can often see different species. Usually they have five arms, though there are exceptions. Moreover, they are known for their exceptional regenerative capacity.
The class of the Sea Lilies and Feather Stars contains about 600 species, the majority being the Feather Stars. Usually, they are anchored in the ground with a stem. To Feather Stars, however, this applies only to young animals.
The Sea Urchins are a class of the Echinodermata and contain about 950 species. When diving in the Canaries, they are ubiquitous, sometimes even a plague. Especially in the case of night dives, one should be careful not to get stung by a thorn.
The class of Sea Cucumbers includes about 1200 species. In contrast to the other classes of Echinodermata, they are not radial-symmetric. When diving in the Canary Islands it is inevitable to see them. Especially at night they come out of their holes.
The Brittle Stars are a class of the Echinodermata with more than 2000 species. Of this, however, more than 1200 live in depths over 200m. They can occur in masses. In the Mediterranean, for example, there are places with more than 10000 individuals per m²!