Sea Cucumbers: Characteristics, reproduction, habitats and more

Sea cucumbers are echinoderms, from the Holothuroidea class, like starfish and sea urchins. Join us to learn everything about them

There are about 1,250 known species, and most of them  have  soft-bodies.  All of them inhabit the ocean, although some inhabit shallow waters and others live in the depths of the ocean. They live on or near the bottom of the ocean, sometimes partially buried below it.

Main Characteristics

Most sea cucumbers, have a soft and cylindrical body, more or less elongated and occasionally fat on their limbs, and generally without solid appendages. Their most outstanding trait relies in their leather-like texture with a gelatinous appearance, so they can be easily confused with slugs. Their skin´s color ranges from brown, black or olive green.

As for their size, it varies according to the species, but the average one is around 20 centimeters in length, but there are also very small sea cucumbers, which only measure only 1 centimeter, as well as very large specimens reaching up to 5 meters.  Their body´s diameter varies between 24 and 25 centimeters.

Like all echinoderms, sea cucumbers have an endoskeleton just below their skin and calcified structures that are usually reduced to isolated microscopic ossicles (or sclerietas) joined by connective tissue.  Their mouth and anus are located at each extremity of their bodies.

As in other echinozoa, sea cucumbers have five ambulacra separated by five ambulacral  grooves, which have four rows of tubular legs, but their size decreases or is absent in some holothurians, especially on the dorsal surface. The two dorsal ambulacra form the bivium, while the three ventral ones are known as the trivium.

Sea cucumbers are mainly sedentary and very slow walkers, since they seldom move more than twelve meters a day. Some species are able to swim although very awkwardly. They can live between 5 to 10 years.

They communicate with each other by hormonal signals that expel water. Many species of sea cucumbers have respiratory trees that are necessary for exchanging gases.

They are paired tubes that are very branched and attached to the intestine, located very close to the anus. This type of breathing is called cloacal. Moreover, sea cucumbers have no brain, only a ring of neuronal tissue that sends the nerves to the tentacles, as well as to the pharynx.

They also lack circulatory system, since they have a vascular system of water, which exerts a hydraulic pressure on the tentacles and feet allowing them to move.

Here they are

Habitat and Distribution

Sea cucumbers have a very wide distribution. They are found in all the oceans of the world, although there are a lot of species that live in the Asian portion of the Pacific Ocean. Although they can live in any marine environment, they are more common in salty waters, which have a shallow depth, notably preferring coral reefs.

Their home varies from the interstitial environment, being able to be exposed to the moment when the tide is low, to settle in the deepest ocean trenches without problem.

According to the species to which the sea cucumber corresponds, it can be benthic living buried in soft sediments, can swim or be members of the plankton employing the water currents as a mean of mobilization.

¿How do Sea Cucumbers Reproduce?

Like starfish, sea cucumbers can reproduce sexually or asexually, since they can regenerate. They have 1 single branched gonad consisting of a series of tubules that end in a duct located on the upper surface of their body, very close to the tentacles. The sexes are separated, but there are some species that are protruding hermaphrodites.

In sexual reproduction, the male and female release the sperm and eggs, respectively into the oceanic water, so fertilization is external. From the eggs, the larvae are released swimming freely up to 3 days after hatching.

However, the tentacles won´t apperar until the third larval stage, called «pentacularia». In few species the fertilization and the development are internal, but in these cases, the eggs are incubated internally until the hatchlings are born, through an opening near the anus.

Let´s watch

Sea Cucumbers´ Alimentation Habits.

Sea cucumbers can feed on detritus, algae or in some cases also plankton or waste materials. They collect their food from the superficial sediments through their tentacles that remain extended on the surface. To be able to eat their food they use their tube feet to dig into the substrate.

The mouth´s tentacles are covered with a layer of mucus that helps them catch the plankton that floats in the water. Although the most common is that the sea cucumber opens its mouth to consume the sediment along with the organic detritus using the tentacles.

The sediments pass through the mouth and enter the body, where they are conducted to the anterior intestine and the small intestine for their respective digestion and absorption.

Once they have processed all the necessary food, they dispose of the mud and waste unusable for them.

For this reason sea cucumbers have great importance for their ecosystem, since they are a great devourer of the substrate; and their depositions have huge value for the soil´s enrichment. They are capable of generating physical, chemical and chemico-physical changes, making a sort of cleaning service for the world´s oceans, and recycling and decomposing matter and aerating the seafloor.

In addition, the sea cucumbers are able to break down the particles into much smaller pieces that become the food of smaller animals such as bacteria. A relatively small specimen of 30 centimeters in length is very laborious, being able to process approximately 120 grams of sediment per day, which translates into 44 kilograms per year.

In addition to this, the sea cucumbers are constantly digging into the seabed, so they spread the substrate around, giving a continuous mobility to the sea´s nutrients.

When the organic matter reaches the bottom of the sea, it usually begins a decomposition process in which the oxygen that is in the water is consumed, so if the sea cucumber didn´t exist it would remain anoxic, that is, without oxygen.

This condition would generate adverse conditions for the survival of any species that inhabits the sea, especially those that are directly on the ground.

The fact that the ocean floor remains oxygenated creates a nutrient cycle that benefits primary productivity. Since the sea cucumbers are constantly removing the sediment to feed, it makes oxygenation possible while avoiding stratification and recycling remains of organic matter.

These recycled wastes are returned to the water column, where they will be used again by other animals taking advantage of the nutrients again. This creates an important trophic chain for marine life.

Let´s watch

Main Types of Sea Cucumber

As we have mentioned along this article, there are around 1500 species of sea cucumbers. Let´s meet the most oustanding

Three-Rowed Sea Cucumber Isostichopus badionotus.

The chocolate-chip sea cucumber inhabits mainly the warm waters of the Atlantic Ocean. It is common to find them in the Caribbean, Bahamas and Florida, equally; in Bermuda, South Carolina and Brazil. Its size is approximately 20 centimeters in length, although there are specimens  of even 45 centimeters.

They have some characteristic dark warts, distributed in three rows on the dorsal surface, while the base color of the body can vary between white to pale orange being even brown, with many brown spots around.

Apostichopus japonicus.

This species mainly inhabits the superficial temperate waters that surround the southeast coast of Asia. It is commonly known as Japanese spiky sea cucumber, due to its innumerable spiny protuberances that are found throughout its body.

They are usually of three different colors that can vary between red, green and black. The most outstanding feature of this sea cucumber species is its ability to enter a state of lethargy when the sea temperature rises too high during in the summer season. While in this state, it feeds on its limbs which it can regenerate later, but loses weight considerably.

 

Holothuria Scabra.

This species is characterized  by its very neutral coloration,  with some small black wrinkles on the upper part becoming paler at the bottom. It can grow up to 40 centimeters, although it is usually wider than it is long.

The Holothuria Scabra is commonly found on the soft sediments of the Indo-Pacific region, and it is one of the most consumed sea cucumber species throughout history, dating back to about 1000 years ago. It reached its highest peak In the 90s, when it was seen in a dry presentation for a price of 100 dollars per kilogram.

Holothuria Nobilis Black Teatfish

This is a black sea cucumber, which is often covered with a thin layer of sand to go unnoticed by predators. It is a kind of firm and rigid body with prominent lateral papillae and anal papillae, with translucent cuvierian tubules in the lower part.

It is usually found on reefs but also exposed on debris or sea grasses. In several countries the cultivation of this species takes place in tanks suitable for its growth and development.

Let´s watch them

Main Predators

They have few predators, among which starfish, crustaceans, turtles and some fish that feed mainly juvenile forms or different larval stages of sea cucumbers, could be mentioned.

To defend themselves from predators the sea cucumbers are able to bury in the sand, leaving out only their mouth tentacles so that they are harder to catch.

In the same way, they are able to eviscerate some of their internal organs, thus the predator will eat these parts while the sea cucumber escapes and then regenerates its limbs to be complete again. They can also produce some toxic substances through their skin, which make their predators to move away. Other species can project mucous and sticky threads on their aggressors that make them irritate or disable.

To protect themselves against parasites, they synthesize a protein called  lectin which inhibits the development of such individuals.

Other threats

The sea cucumber is considered in many countries of Southeast Asia as a very precious food, with a huge gastronomic and economic value. For this reason some populations have been overexploited causing it to disappear in some countries such as Russia, Mexico and the Galapagos Islands. In addition to eating, sea cucumber has been linked to the cure of many diseases such as cancer or tumors, as well as having sanitary properties, which has contributed to its exaggerated demand.

Benefits of  the Sea Cucumbers.

This animal is considered a delicacy in Chinese cuisine and it is the main ingredient in the New Year’s banquet called «The Eight Immortals Crossing the Sea». It is said that sea cucumbers  provide many health benefits, which is why its market has increased, reaching western countries such as the United States.

Joints Pain

Because of its high levels of chondroitin sulfate, an important component for cartilages, it is said that this animal provides a very prosperous relief for fatigue, sexual impotence and joint pain.

The sea cucumber extract, alleviates the strong pain caused by diseases such as arthritis, as it manages to relax the joints, in addition to having anti-inflammatory properties. These remedies are widely used in traditional Chinese medicine, having a great general acceptance.

Gums.

Although it is not fully proven, there are preliminary studies that say that a paste made from sea cucumber´s extract can improve the healing of patients who had undergone treatments for gum disease. This study was published by the «Journal of Oral Science», in which 28 individuals were divided into two groups.

The people in the first group brushed their teeth daily with sea cucumber´s paste, while the other group used toothpaste that didn´t contain any sea cucumber: it was a common paste.

After three months of study, the group that had brushed their teeth using the sea cucumber´s paste had a significant improvement in terms of bleeding, inflammation and depth in the periodontal pockets, comparing with the results of the other group.

Proteins

According to Subhuti Dharmananda, Director of the Institute for Traditional Medicine in Portland, Oregon, sea cucumbers  provide a lot of proteins to those who need them. Its protein level is only exceeded by white eggs. In addition to this the Harvard School of Public Health states that the intake of sea cucumber can help people to reduce the risk of suffering from cardiovascular diseases, especially if this protein replaces refined carbohydrates.

 

Cultivation of Sea Cucumbers.

In recent years, the cultivation of this marine species has been popularized in order to avoid its marine overexploitation, in addition to not taking away from the oceans so important controller of the balance that the seabed must maintain.

Reproduction in captivity.

The reproduction process being in captivity is basically the same as if they were in their natural habitat. The grower should only be aware that the sea temperature doesn´t  have sudden changes, as it could adversely affect the new individuals. When the eggs are fertilized, they are taken and passed to a set of incubators where the larval development process will take place. The larva remains in a water column during approximately 20 days, where it passes through three stages that are characteristic for its morphology and diet.

The first phase that faces when the larva is in the auricular stage, which is characterized by being phytoplankton, since they only consume the plankton to survive this stage.

The second stage is a transition one, and is called the doliolaria phase. At this moment the sea cucumber goes from being a larva of planktonic life to a benthic life, going through a morphological process, and looking for a suitable substrate where it can settle.

Once settled on the proper substrate, the third phase is carried out, where it is properly settled by the appearance of tentacles that will help in its feeding, as well as the limbs that will help it to eat.

In the fourth phase, called pentactula, the larva ends its development becoming a juvenile individual, almost with the final form that it will have as an adult. When juvenile specimens are found in the sea, they are very difficult to find and their behavior is not well understood. However being in captivity has been observed the main reasons for this behavior.

In the juvenile stage, the sea cucumber usually has a length less than one millimeter, which makes it difficult to observe. In addition to this, it can be noted that at this stage, the specimens constantly seek to be covered with sand or buried between rocks and substrate, which, together with the lack of coloration, allows them to remain camouflaged with the contour.

Feeding Sea Cucumbers in Captivity.

The cultivation of different species of  microalgae that can be provided to adult larvae and sea cucumber is quite recommended. In addition, commercial micro algae are also used in order to complement the diet while helping to reduce the costs associated with the cultivation of live food. We invite you to read our article Cultivation of Alagae to learn more about this process

Tank´s Temperature

In recent years, it has been studying what should be the optimum temperature for the proper growth of the sea cucumbers. It has been determined that each specimen has different ranges of tolerance and temperature, which allows it to take better advantage of the energy it, has obtained from the food.

Although it is still in testing periods to know what are the perfect environmental conditions for the sea cucumber according to its species, tests are being carried out under controlled conditions in laboratories. However, this method is expensive.

In addition, tests are carried out with fluctuating conditions, which take place in external tanks, which in spite of reducing production costs is not as effective because it could have a higher mortality rate.

Let´s watch