Sea Cucumber

If you look into the marine ecosystem, you will often find cucumber-shaped living organisms moving or swimming around in the ocean. These organisms are called sea cucumbers as they resemble the cucumber fruit. These organisms are echinoderms and belong to the class Holothuroidea. They are mostly found in all oceans with the maximum population in the Asia Pacific region. 

Humans also consume sea cucumbers. These sea cucumber foods are cultured in aquaculture systems. They also play a major role in the recycling process of detritus, nutrients, and other organic products in the marine ecosystem. In addition, they provide the perfect platform for bacteria to continue the decay process. Let us learn some important points about these sea cucumbers.

What is a Sea Cucumber?

Many people often get confused when they encounter the question- what is a sea cucumber? They often refer to it as some cucumber plants growing in the sea. However, they are actually animals. The sea cucumber phylum is Echinodermata. The sea cucumber scientific names are many depending on the species. For example, a pelagic sea cucumber scientific name is  Pelagothuria natatrix. It belongs to the family Pelagothuriidae and order Elasipodida. 

Sea cucumbers mostly live on the seafloor. You can find them to be sedentary or mildly moving. However, some species have the ability to swim in the water.

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The Body Plan of a Sea Cucumber

Sea cucumbers follow radial symmetry when considered along the longitudinal axis. However, they are also weakly bilaterally symmetrical when considered transversely. Their body consists of five ambulacra, interspersed by five interambulacral or ambulacral grooves. The first two dorsal ambulacra are called the bivium, and the last three central ambulacra are called the trivium.

The body wall is differentiated into the epidermis and the dermis. It also contains several calcareous ossicles, which is the characteristic feature to differentiate between different species of sea cucumbers. The coelom is located inside the body wall. The coelom is divided into three mesenteries located longitudinally. These mesenteries support and protect the internal organs.

The Body Structure of Sea Cucumber

Sea cucumbers are strictly marine, although they are now cultured in aquaculture systems for human usage. They have an elongated cylindrical body covered with leathery skin. Like other echinoderms, they lack a proper vertebra. Their soft body varies between 2 to 200 cm in length and 1 to 20 cm in thickness. 

The skin has a dull, dark hue with a warty texture. Therefore, the body resembles the skin of a cucumber. In addition, they have an internal skeleton comprising tiny calcareous structures of distinct shapes in the skin. These structures are called ossicles. Some genera, such as Sphaerothuria, contain large ossicles, which give them an armor-like cover.

Like other echinoderms, sea cucumbers have five rows of tube feet that extend from the mouth to the anus. The anus is used both for excretion and respiration. In addition, there is another set of ten retractile tentacles that surround the mouth. They are modified tube feet. Different species have simple, arborescent, or branched tentacles. These tentacles help in burrowing in the ground and intake of food. Large calcareous ossicles are located posterior to them. The sea cucumbers use these organs to feed on mud or small planktons. 

Their gonad is single and branched. 

Organ Systems in Sea Cucumber

Like other advanced echinoderms, sea cucumbers have distinct organ systems. Some of them are as follows:

1. Digestive System 

Sea cucumbers are mostly bottom or gravel feeders. The pharynx is located behind the mouth. It is surrounded by ten calcareous plates forming a ring. In most cucumbers, this ring is attached to the muscles that retract tentacles. Some sea cucumber species possess stomach and esophagus, while in others, the pharynx opens directly into the intestine. The intestine of the sea cucumber is long and looping three times before it opens into the cloacal chamber or the anus.

2. Nervous System

Although sea cucumbers do not possess a true brain, they have a neural tissue ring surrounding the oral cavity. From this neural tissue ring, nerves extend to the pharynx and the tentacles. Additionally, five major nerves run beneath the ambulacral rings along the length of the body.

Sea cucumbers also do not possess any distinct sensory organs. However, the skin is scattered with nerve endings enabling the animal with a sense of touch and light. In addition, certain members of Apodida possess statocysts or small eye-spots near the tentacle bases.

3. Respiratory system

Sea cucumbers take oxygen from water with the help of their respiratory tree pair that branches into the cloaca inside the anus. Therefore, they breathe through the anus and expels water from it. The respiratory trees have a common duct branching into a series of narrow tubules that lie on both sides of the digestive tract. The gaseous exchange takes place through the thin walls of these tubules. They also take part in excretion by allowing the exchange of nitrogenous waste.

A. Circulatory System

Sea cucumbers follow the same circulatory system as other echinoderms. In addition, they have a water vascular system that generates the hydraulic pressure for the tube feet and tentacles. It also contains a complex haemal system comprising of open sinuses and well-developed vasculature. 

Along with water, the blood and coelomic fluid play a major role in circulation. They also have phagocytic coelomocytes that have a role akin to white blood cells in vertebrates. Another coelomocyte containing hemoglobin is unique for sea cucumbers.

B. Locomotive Organs

The sea cucumber phylum follows other echinoderms in having a pentaradial symmetry. That is, their bodies consist of five identical parts with a central axis. Due to their posture, they have also developed secondary bilateral symmetry. Except for Apodida, there are subtle differences between the top and bottom surfaces of the body.

The three trivia have several tube feet with suckers allowing the animal to crawl along the ground. However, the bivium has vestigial tube feet or no tube feet at all. Some sea cucumbers also have papillae. These are fleshy projections from the body wall and conical in shape. These projections also bear sensory tube feet at the apices. 

They can also form long structures resembling antennae. 

Feeding Habits and Mode of Escape From Predators 

Sea Cucumbers are found in all oceans at varying depths. You can encounter them in shallow water and sometimes even at great depth. For example, the genus Holothuria, represented by warty, large sea cucumbers (around 80 to 100 species), is mostly found on coral reefs. They are mainly deposit feeders, extracting nutrition by ingesting sediments.

Several sea cucumbers can expel their internal organs and grow new ones. They do so through their anus. Such a mechanism is generally attributed to physiological reasons. However, it can be an escape mechanism from predators. In addition, some species of sea cucumbers release special sticky filaments that can confuse or entangle an enemy. Sea cucumbers are also found to give shelter to pearl fishes from Carapus species in their anal cavity.

Other species release a toxin that can prove lethal to small fishes. However, they are not lethal to humans. Therefore, South sea islanders prefer to use sea cucumber juice in water to kill or give shocks to fishes.

Cultivating Sea Cucumbers For Several Reasons

Nowadays, humans cultivate sea cucumbers for various reasons. They extract medicines from some species of sea cucumbers. Shampoos and toothpaste are also made from them. Some sea cucumbers have nutritional values. The most important part of the sea cucumber body is the outer body wall. This part is dried and sold as beche-de-mer. 

This sea cucumber food is considered a famous delicacy in China as well as in the rest of Asia. Due to the high demand for this delicacy, many populations of sea cucumber subspecies face tremendous pressure in the ecosystem.

Population Changes in Sea Cucumber Species and Approaches to Mitigate it

Some of the highly valued sea cucumber species are H. whitmaei, H. fuscogilva, Isostichopus fuscus, and H. scabra. However, their population has faced tremendous pressure due to overfishing and has been decimated to a large extent since 1980. Therefore, harvesting targets have been shifted to lower-valued species like Cucumaria frondose, abundant in temperate and polar waters. 

Population studies on some of the species are being carried out. Attempts are also made to cultivate some species like H. scabra, and I. fuscus, in aquaculture ponds as sea cucumber food. However, most scientists believe that the ever-rising increase in demand for beche-de-mer is one of the reasons for the sea cucumber population getting under a lot of pressure.