Holothuroidea – Sea Cucumbers
The sea cucumbers are a class in the phylum of the Echinodermata. Holothuroidea comprise about 1200 species.
The sea cucumbers have an elongated, cylindrical body shape. The abdomen and the backside must be clearly distinguished from one another. The five-beam radial symmetry of the Echinodermata can be seen only on the five rows of the ambulatory feet. At the anterior end, they have an opening, usually surrounded by tentacles.
The size of different species varies between 5mm and 2m.
Compared to other classes of echinoderms, Holothuroidea have only skeletal remains. So-called calcite needles (sclerite). Instead of the skeleton, they have a skin muscle tube and a thick layer of mutable tissue.
A special feature of the sea cucumber is the possession of a water lung.
Habitat and Distribution
Sea cucumbers live exclusively in the sea, down to depths of 10000m. Besides the soil-inhabiting species, there are also pelagic. There are modified body forms among specialized substrate inhabitants as well as in swimming species.
Holothuroidea inhabit both, the polar regions and tropical areas of the oceans.
Among the sea cucumbers, there are substrate and plankton eaters. Substrate feeders creep across the soil and absorb sediment, including organic material. Indigestible is excreted, while organic material, such as detritus and algae, is digested.
Plankton feeders have a more pronounced tentacle rim to better filter the plankton.
Like other echinoderms, sea cucumbers have separated sexes. Populations in a bay simultaneously release egg and sperm cells into the water to increase the chances of successful fertilization. To do so, they align themselves vertically with the front end.
Some species breed their eggs.
In addition to sexual reproduction, Holothuroidea are able to reproduce asexually. This is done by division.
In the deep sea (under 9000m) the sea cucumbers represent 90% of the benthic biomass.
Some species have Cuvierian tubules. When they are in danger the Cuvierian tubules are sprayed in the direction of the attacker and form slimy threads, which can be toxic.
There are Holothuroidea that can eject part of their intestines to distract attackers. These are simply newly formed later.