Sea cucumbers are soft-bodied invertebrates from the Holothuroidea class of thorns. Although they are usually a long cylindrical structure reminiscent of worms, some of them are also spherical. They are so-named because their shape will remind an observer of a cucumber. Though they may look strange, they are of great importance to the marine ecosystem, as they form an important part of the base of the marine biomass. More than 1700 species of sea cucumbers have been identified, and they are common in almost all seas around the world. The longest of all (Synapta maculata) is three meters, while the smallest has a length of one centimeter. However, in general they are 3–27 cm long.
Sea cucumbers consist of three main parts: the body fluid, internal organs, and the body wall. Each part has separate, biologically important compounds. The most important distinguishing feature of the sea cucumber is the calcareous ring surrounding the pharynx or throat. This ring is a connection point for the muscles of oral tentacles and the endpoints of longitudinal muscles.
One characteristic that distinguishes the sea cucumber from the other echinoderms is its crown, which consists of 10-30 tentacles around the mouth. The tentacles can be shaped like fingers, feathers, umbrellas, or shields. These structures help to sense, touch, and hunt. Sea cucumbers move slowly and carry plankton, mud, and other organic material, especially diatoms, which they catch in the environment. 90% of sea cucumber species have an inner skeleton of calcium carbonate platelets right under the skin. This structure consists of reduced calcareous microscopic ossicles and connective tissue. In some species, the ossicles expand to form a flattened, plate-like structure.
There are two important reasons why the economic value of these creatures is high. One of these is pharmacological properties, especially in America and Asia: tablets acquired from the animal’s body wall are used in various treatments. In Australia, compounds derived from these living organisms are used in the production of anti-inflammatory medicines.
A large industry has been formed due to the sea cucumber’s popularity as food. People catch them in the wild, but they’re also bred in captivity.
Some sea cucumbers have a group of organs that are not present in other vertebrates. These are the “respiratory trees” that are present in the body cavity. Thanks to the hydrostatic nature of the respiratory organs, water can be drained into the body. The sea cucumbers “breathe” by removing the oxygen from the water via the respiratory trees.
Sea cucumbers do not have a real brain and have no obvious sensory organs, either. With the help of various nerve endings connected to the skin, they gain a sense of touch and can feel the presence of light. Some sea cucumbers have a large number of adhesive disc tube “feet.” Since they cannot swim, they move only with their strong longitudinal muscles or with these tube feet. They also use these slowly moving feet as sensory organs.
1) Some marine species living in coral reefs have great defense mechanisms in order to protect themselves. The sea cucumber has a unique one: by squeezing its muscles, it first stretches some of its internal organs, makes them sticky, and then blows them out of its anus, onto a potential attacker. The predator, covered with the sticky organ of the sea cucumber, is perplexed and may even be temporarily blinded. The sea cucumber is not adversely affected by such an attack: the internal organs are recreated within a few weeks. It can continue breathing thanks to the sea water taken into the body cavity until the organs are renewed.
2) The Holothuria sanctuary sea cucumber has a unique defense mechanism: the “cuvier organ.” When the sea cucumber feels itself in danger, the cuvier leaves the tubules like a fishnet. These tubules, used as munitions in defense, have been created to be highly adhesive. Thanks to this mechanism, a fish or crab can easily be made ineffective.
3) Another effective method of protection for sea cucumbers is a form of chemical defense. A toxic chemical soup, secreted by some species, can pose a serious danger to other organisms. Toxic substances in the skin of some have also been created as a deterrent to living creatures who want to hunt them down.
4) However, the most original feature of sea cucumbers is their ability to pass through narrow passages by literally liquefying their bodies. Thanks to the magnificent neurological control mechanism given to it, a sea cucumber can convert solid tissue into a fluid, and then go back to a solid state again. The special collagen fibers in the tissues make this possible: 70 percent of the protein in a sea cucumber’s body wall consists of collagen. By “liquefying,” a sea cucumber can flow through a small crack, and then it can return to its original state. This trick can also be used to hide from predators.
Like so many organisms, sea cucumbers have been perfectly created to thrive in their specific environments. Their unique, awe-inspiring traits are a testament to the incredible design visible in the natural world.