Sea cucumbers or holoturids (Holothuroidea), whose name derives from the Greek holythurion meaning 'which is totally agitated', are a class belonging to the phylum of Echinoderms, which has approximately 1,150 species alive today. They are characterized by an elongated and soft tubular body, although somewhat leathery in appearance. At one end of the body we find the mouth, surrounded by tentacles (between 10 and 30). The oral tentacles can be very variable depending on the species, from simple tentacles, with finger-like projections, with a comb or feather shape, or even flattened tentacles. At the opposite end of the body is the anal opening.
Although externally there are no signs of the characteristic pentaradian symmetry of Echinoderms, internally a certain degree of pentaradian symmetry can be observed, with organs and systems present in numbers multiple of 5.
Generally, sea cucumbers are around 20-30 cm long, with extreme cases ranging from species that barely reach 1 cm in length, to species that reach 3 meters. They have an exclusively marine distribution and are practically ubiquitous. We can find them from shallow coastal waters, to the deepest waters of the oceans. They inhabit both rocky bottoms and sandy and muddy regions, where they move with slow crawling movements. There are species that to some degree have developed the ability to swim.
The feeding of the sea cucumbers consists mainly of particles in suspension that they catch by means of mucous secretions in the oral tentacles, or moving and catching with their tentacles particles in the marine bed.
Diverse species of sea cucumbers are able to detach from part of their viscera, by means of a sudden muscular contraction, which they use as a defense mechanism against predators or when feeling irritated. Later, the lost viscera will be completely regenerated by the animal. Another curious fact is the existence of a commensal relationship between certain species of sea cucumbers and fish of the Carapus family, which use the sea cucumber, and more specifically its anal region as a refuge when they feel intimidated.
Generally, sea cucumbers have separate sexes, although there are some hermaphrodite species. The fertilization is external, after the release of eggs and sperm into the water, forming a swimming larva, which is called (auricularia). After a planktonic phase, the larva undergoes a process of metamorphosis, to give rise to an individual with adult characteristics. Asexual reproduction also occurs in certain species.