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Sea Cucumbers are an essential animal in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter.
They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian species worldwide is about 1,717.
On the Picture you can see the longest species Synapta maculata which can grow up to 3m in length, but these are extremes and most measure generally between 10 and 30 cm in length.
Sea cucumbers are named for their resemblance to the fruit of the cucumber plant. Many have a cylindrical shaped forms while others are sausage shaped like.
The mouth is surrounded by tentacles, which can be pulled back inside the animal and are differ on the species and food.
Most possess five rows of tube feet called podia, similar to the ones on a Sea Urchin to move along.
A remarkable feature
of these animals is the “catch” collagen that forms their body wall. This can be loosened and tightened at will, and if the animal wants to squeeze through a small gap, it can essentially liquefy its body and pour into the space. To keep itself safe in these crevices and cracks, the sea cucumber will hook up all its collagen fibers to make its body firm again.
The most common way to separate the subclasses is by looking at their oral tentacles. The shape of the tentacles is generally adapted to the diet, and to the size of the particles to be ingested: the filter-feeding species mostly have complex arborescent tentacles, intended to maximize the surface area available for filtering, while the species feeding on the substratum will more often need digitate tentacles to sort out the nutritional material.
In this Video you can see a Filter feeding Sea Cucumber
A single specimen can swallow more than 45 kg of sediment a year, and their excellent digestive capacities allow them to reject a finer, purer and homogeneous sediment. Therefore, sea cucumbers play a major role in the biological processing of the sea bed
Most sea cucumbers reproduce by releasing sperm and ova into the ocean water. Depending on conditions, one organism can produce thousands of gametes. Sea cucumbers are typically dioecious, with separate male and female individuals, but some species are protandric.
At least 30 species fertilize their eggs internally and then pick up the fertilized zygote with one of their feeding tentacles. The egg is then inserted into a pouch on the adult’s body, where it develops and eventually hatches from the pouch as a juvenile sea cucumber. A few species are known to brood their young inside the body cavity, giving birth through a small rupture in the body wall close to the anus.
Symbiosis and commensalism
Numerous small animals can live in symbiosis or commensalism with sea cucumbers, as well as some parasites.
Some cleaner shrimps can live on the tegument of holothurians, in particular several species of the genus Periclimenes (genus which is specialized in echinoderms), in particular Periclimenes imperator. A variety of fish, most commonly pearl fish, have evolved a commensalistic symbiotic relationship with sea cucumbers in which the pearl fish will live in sea cucumber’s cloaca using it for protection from predation, and a source of food, the nutrients passing in and out of the anus from the water.
On the Picture you can see a Polynoid worm symbiosis.
Sea cucumbers are often ignored by most of the marine predators because of the toxins they contain (in particular holothurin) and because of their often spectacular defensive systems. When startled, these cucumbers may expel some of their sticky cuvierian tubules (enlargements of the respiratory tree that float freely in the coelom) to entangle potential predators, through a tear in the wall of the cloaca in an autotomic process known as evisceration.
Replacement tubules grow back in one and a half to five weeks, depending on the species. So I don’t think I have to mention that it is not a good idea to play with sea cucumbers due to the problem of not being able to defend themselves for several weeks and because of the toxins which may cause rashes or even anaphylactic shocks kin some persons.
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