sea cucumber Sea cucumbers are primitive, squishy invertebrates (animals without backbones) found only in salt water. They live in a variety of environments, including reefs and sea grass beds in all the world’s oceans. They are are common on both shallow and deep water ocean floors. Most are benthic (bottom-dwelling) but a few are pelagic (live in the open ocean). They are typically found in coastal areas as well as intertidal (littoral) zones on the shores. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
Sea cucumbers (Scientific name: Holothuroidea) are echinoderms, making them distant relatives to starfish and urchins, but they differ from sea urchins and starfish in that their bodies are covered with soft, leathery skin instead of hard spines. The spines of of sea cucumbers have been reduced to wart-like bumps embedded in the animal’s leathery skin. Five strips of muscle inside the body wall, running from front to back, show they are echinoderms. [Source: NOAA]
More than a thousand species of sea cucumbers exist around the world. They vary in size from two centimeters to two meters. In some places they are quite plentiful, making up 90 percent of the biomass found there. Small tentacles around the mouth are used to probe around in the sand and mud for food which is sucked from the muck with fleshy lips.
Sea cucumbers are eaten by a wide range of sea creatures, namely fish and crabs. In some places, especially Asia, and particularly China and Korea, sea cucumbers are considered a delicacy and and an aphrodisiac and consumed by many people.
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Global Coral Reef Monitoring Network gcrmn.net
Echinoderms (meaning "spiny skinned") are a phylum of animals that includes sea urchins, starfish and sea cucumbers. They are invertebrates with no head. Their internal organs — and often their outer organs — are arranged in five symmetrical parts around a central stomach. The creatures have no front or back. Humans and other mammals are bilaterally symmetrical with nearly identical left and right sides and distinct front and backs.
There are around 7,000 different species of echinoderm. Most echinoderms have tough skins and flexible spines and/or tube feet. They also have unique groups of hydraulic organs that serve several functions and work in conjunction with muscles to power the tube feet. The tube feet have three main functions: 1) they help they animal move with powerful suction that grasps onto things and muscles that expand and retract; 2) they can take in oxygen from the seawater and serve as gills; and 3) they are packed with sensory neurons and help the animal sense its surroundings.
Echinoderms “do not have a ‘heart’ or anything analogous to it,” Chris Mah, a marine invertebrate zoologist at the Smithsonian National Museum of Natural History in Washington, told National Geographic. They don’t have blood either. Instead, they have millions of tiny, hairlike structures called cilia that beat constantly, pumping seawater via “a system of internal pipes and bags,” Mah says. Their internal cavity also has “all the various cells needed for transporting nutrients, immune cells, and so forth.” [Source: Liz Langley, National Geographic, February 13, 2016]
Sea Cucumber Characteristics
Although they vary in color, most sea cucumbers are black, brown, or olive green. They range in size from three centimeters (1.2 inches) to one meter long (three feet). The largest sea cucumbers have a diameter of 24 centimeters (10 inches). Sea cucumbers are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (have a body temperature that fluctuates with the surrounding environment), have bilateral symmetry (both sides of the animal are the same) and have radial symmetry (symmetry around a central axis), [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
According to Animal Diversity Web: Sea cucumbers generally look long and worm-like, but retain the pentaradial symmetry (the body is divisible into five equal parts, with each part being identical in terms of structure), characteristic of the Echinodermata. Some may be spherical in body shape. The mouth and anus are located on opposite poles, and five rows of tube feet run from the mouth to the anus along the cylindrical body. Ten to 30 branching tentacles surround the mouth. The tentacles are actually part of the water vascular system.
The water vascular system, found in all echinoderms, accommodates the elongated body of the sea cucumbers. Coelomic fluid (fluid within the body cavity), rather than sea water, circulates through the water vascular system. The ring canal around the gut has 1-50 polian vessicles, which may function for hydraulic regulation. Each radial canal has rows of ampullae. Podia, which are the external portion of the tube feet, may, be suckered, reduced, or lost. Podia are more randomly scattered along the body than in other echinoderms. The esophagus, foregut and radial canal of the water vascular system are supported by calcareous plates. /=\
Respiratory trees, which branch out near the rectum of the animal are used for gas exchange as water is pumped through the anus. The respiratory trees are part of the organs that are expelled occasionally by the sea cucumber. While support in most echinoderms is from the skeletal structure, in sea cucumbers, thick sheets of body wall muscles provide support. Sea cucumbers have the most developed hemal (circulatory) system of echinoderms, having well developed vessels and several single chambered hearts along the intestinal system. The hemal system provided both functions for gas and food transport. Most species live from five to ten years. Microscopic ossicles (or sclerietes) are on the dermal layer and are used in taxonomic identification. /=\
Most sea cucumber species have an internal calcareous ring. This ring is made up of a series of plates joined together around the esophagus. The shapes of these plates vary from species to species and can be used in classification. It is also the only part of a sea cucumber to fossilize. /=\
Sea Cucumber Ability to Change from Solid to Liquid.
Sea cucumbers have the extraordinary ability to change their tissue structure from a liquid to solid and back again by using plasticizing proteins which act as looseners and other proteins that act as stiffening agents. This ability has made them of keen interest to scientists who are studying things like making artificial muscles and repairing torn ligaments and tendons.
Sea cucumber tissues contain a compound called collagen. Triggered by neurological impulses it can changes to liquid or solid and is used by the animals as a means of defense. If you hold a sea cucumber in your hand it feels soft at first and then stiffens and gets hard. If a sea cucumber is threatened it liquefies its tissues so it can squeeze into the crack of a rock and then changes to solid so it can become wedged there and the predator can not pull it out.
The sea cucumber's ability to turn their skin quickly changes from soft to hard has a number of applications that scientists and companies are studying. In experimental treatments for people with Parkinson’s disease, for example, electrodes are inserted into the brain to restore function. To be implanted properly the electrodes have to be hard and stiff when implanted but when they are in the brain soft is more advantageous. According to research by Chistoph Weder of Case Western Reserve University on the nano-level sea cucumber tissue is a soft material impeded with tiny fibers of collagen. When certain chemical are secreted the fibers form bonds, stiffening the matrix of the skin. Eder and other researchers are duplicating the structure using cellulose nanofibers in a polymer matrix.
Sea Cucumber Behavior, Senses and Movement
Sea cucumbers are often found on the sandy sea floor in areas around coral reefs and sea grass bed. They can attach themselves to rocks with small tubular feet but they move around with muscular movements in their bodies. They spend most of their lives lying in the ocean bottom or crawling lethargically. Sea cucumbers generally don't lie face up or face down but on their sides, using an opening called an anus for excreting waste, breathing and sucking in water for tubules insides its body. During the mating season they rear up and twist and turn like charmed snakes.
Generally, sea cucumbers are slow-moving and sedentary and remain in the same area. They usually burrow into soft sediments or are lodged in cracks or crevices under rocks or in reefs. Sea cucumbers crawl using podia or by moving their body's wall muscles. Some deep sea species have elongated podia used for walking. In other species trivium (podia suckers) are modified for creeping. A few open water species can swim (although not well) with webbed papillae. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
Sea cucumbers sense using touch and chemicals usually detected with smell and communicate with chemicals usually detected by smelling. They also employ pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species). They are generally solitary and do not communicate with members of their own species, although they can detect chemicals released during spawning, and synchronizing egg and sperm release between animals within the same general area. The podia (tube feet) covering the ventral surface perceive sediment texture as the animal crawls over surfaces. Adult pheromones may attract larvae, which tend to settle near adults of the same species.
Sea Cucumber Feeding and Their Importance to Marine Ecosystems
Echinoderms in general have a decentralized nervous system. Sea cucumbers can right themselves when turned over, suggesting they have tactile and light receptors. In addition, some evidence suggest their tentacles may be chemically sensitive. The non-centralized nervous system of echinoderms allows them to sense their environment from all sides. Sea cucumbers have a nerve ring near the base of the tentacles.
Most sea cucumbers are scavengers, moving along the seafloor and feeding on detritus and tiny particles of algae or microscopic marine animals collected with tube feet that surround their mouths. Their mouth is surrounded by 10 to 20 retractable tentacles which suck in mud, sand and plankton like a vacuum clean and eject processed sand. They help to recycle nutrients on the sea floor as earthworms do in topsoil. The particles they grind down to smaller pieces are further broken down by bacteria and become part of the ocean’s nutrient cycle. [Source: NOAA]
According to to Animal Diversity Web: As suspension or deposit feeders sea cucumbers trap particles and plankton on mucus-covered tentacles. The tentacles are pushed into the mouth to ingest food. Secretory cells from papillae of the tentacles and gland cells of the foregut secrete mucus.[Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
Sedentary sea cucumbers hold out extended tentacles to trap particles and plankton. Motile ones crawl across the ocean floor and use tentacles to capture sediment and organic detritus. Sediment feeders are highly selective deposit feeders, generally consuming highly organic sediments. Members of the subclass Apodacea ingest sediments as they burrow through the substrate. Branched buccal tentacles surround the mouth. From the mouth, the esophogus leads to the foregut and then intestine, where digestion and absorbtion occur. /=\
Sea cucumbers are important ecologically as they ingest and process food particles and discharge newly fixed and enriched matter that has a nutrient content greater than its surrounding environment. According to Animal Diversity Web: Once ingested, sediment particles stay inside the animal about 3 hours to ensure complete nitrogen-fixation. This highly active process of filtering and enriching sediments is extremely important to marine conditions. Reworking modifies the physical and chemical stability of the marine environment by continually producing "new" ground and causing pulses in biodiversity. Through destroying both stratification of sediment layers and ridding its community of infestations, the Chocolate chip sea cucumber plays a key role in structuring the micocommunities that exist. [Source: Celia Rangel, Animal Diversity Web (ADW)]
Sea Cucumbers Spew Their Guts to Deter Predators
The main known predators of sea cucumbers sea stars, gastropods, crustaceans, fish and humans, They are most vulnerable in their larval stage. Some sea cucumbers discharge sticky tubules, known as Cuvierian tubules, at a potential predators. The tubules are sticky clusters found at the base of the respiratory tree. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
When threatened, some sea cucumbers extrude their external organs and eject a thread-like material that entangles intruders while the sea cucumber slowly makes its escape.To do this the sea cucumbers violently contract their muscles and shoot some of their internal organs out of their rear ends. When picked up or threatened sea cucumbers often also squirt out water. [Source: NOAA]
Sea cucumbers also expel their organs as a seasonal event, When expelling organs, sea cucumbers usually release the gut, their sex organs and one or both their branching respiratory organs. The missing body parts and entrails are quickly regenerated, regrowing in a couple of weeks. Some are toxic. See Sea Cucumber Species.
Sea Cucumber Mating, Reproduction and Development
Sea cucumber are both oviparous (young are hatched from eggs) and ovoviviparous (young are produced from eggs that hatch within the body of a parent). Most sea cucumber spawn and are fertilized externally, but about thirty species brood their young internally. Most of the time during reproduction, females shoots thousands of eggs into the current. Some capture eggs with tentacles, placing the eggs at the sole or dorsal body surface for incubation. A few have internal fertilization and development, where hatched young are released.
Sea cucumbers are simultaneous hermaphrodites in which individuals have sex organs of both sexes and can produce both sperm and eggs even in the same breeding season. While they are releasing their sperm and eggs, many species of sea cucumbers lift the fronts of their bodies, forming an “s-shape”. This position is thought to increase fertilization efficiency. During the pre-birth stage provisioning and protecting is done by females.
Their life cycle is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. According to Animal Diversity Web: As an echinoderm, members of the Holothuroidea are deuterostomes. The larvae, which are planktotrophic or lecithotrophic, have 3-part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults. /=\
The larvae develop in sea water. After three days the larval stage is called an auricularia and is similar to the bipinnaria larvae of asteroids. The auricularia has a ciliated locomotor band, then further develops into a larval stage called a doliolaria, where the ciliated band is broken up into three to five ciliated "girdles". Many species of sea cucumbers have another non-feeding, barrel shaped larval stage called a vitellaria. Likely a specialized condition, it develops gradually, retaining many of the larval features. As it is metamorphosing it is sometimes called a pentactula larva. After larval metamorphosis, the young sea cucumbers ultimately settle on the substrate and become adults. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
Creatures That Live Inside and On Sea Cucumbers
Transparent eight-inch-long pearlfish live inside some species of a sea cucumber. During the day the pearlfish rest inside a hollow cavity in their hosts and at night they slip out to forage among the reef. Pearlfish enter sea cucumbers by nudging open the sea cucumber's anus with its nose. Inside it is protected from its enemies. For food it eats the sea cucumber's internal organs, which the sea cucumber regrows almost as fast as they are eaten. Young pearlfish can enter the anus easily. When they are older they have insert their sharp-pointed tail into the anus and twist their body and enters the sea cucumber like a corkscrew.
Sea cucumber species used as hosts by pearlfish in the Western Atlantic Ocean and Caribbean Sea include the West Indian sea cucumber (Actinopyga agassizi), furry sea cucumber or the fissured sea cucumber (Astichopus multifidus), chocolate chip sea cucumber or cookie dough sea cucumber (Isostitchopus badionotus), donkey dung sea cucumber (Holothuria mexicana) and brown rock sea cucumber (Holothuria glaberrima).
Entovalva lessonothuriae is a clam species with an an extraordinary life style: it lives inside the esophagus of the sea cucumber, Holothuria pardalis. These sea cucumbers are normally found in sand under boulders on coral-reef flats within the intertidal zone. The clams have been found nowhere else, they seem to occur only in this host species. The relationship between the clam and the sea cucumber is described as endosymbiont rather than parasitic because the clam does not harm its host.
In Fiji, emperor shrimp ride on the backs of slow-moving leopard sea cucumbers eating scum of their host. In addition to providing food and transport the sea cucumber offers defense. When it is disturbed it spills its toxic guts out. The sea cucumber also acts as a gathering place for the shrimp where they sometimes meet mates.
Sea Cucumber Food and Products
Humans utilize sea cucumber for food and sources of medicine or chemicals.In Asia sea cucumber is a popular food, with, some say, aphrodisiac qualities. The whole bodies of the animals are dried and used in soups, tonics of medicines. Before drying, the sea cucumbers are boiled and the bodies contract and thicken and organs are expelled.In Taiwan it its served in banquets. It is particularly popular in South Korea, where it is eaten raw and in soups.
“Bêche-de-mer” (French for “sea caterpillar”) is the boiled, dried, and smoked flesh of sea cucumbers. Also knnown as trepang, it used to be commonly eaten in the West and is made from strips of muscle inside the sea cucumber body. Bêche-de-mer is still used to make soups. Most bêche-de-mer comes from the southwestern Pacific, particularly around New Guinea, where the animals (any of a dozen species of the genera Holothuria, Stichopus, and Thelonota) are obtained on coral reefs. Bêche-de-mer is consumed chiefly in China. [Source: Encyclopedia Britannica]
Macerated sea cucumbers that release the toxin holothurin with the Cuvierian tubules have been used by South Pacific Islanders to catch tide pool fish.. People from the small island country of Palau make shoes from the gooey discharge from sea cucumbers. Sticky threads from the discharge that are normally used by the animals to ensnare prey are used by islanders to protect their feet from sharp coral.
Sea cucumbers with long fat projections on the body, like those commonly found in Hokkaido, are especially valued in China. They used to be thrown away and fetched only ¥300 a kilogram in the early 2000s but were selling for about ¥900 a kilogram in the late 2000s.
Sea Cucumber Overfishing and Aquaculture
Some populations of sea cucumbers have been overfished, The brown sea cucumber (Isostichopus fuscus) is listed by Convention on International Trade in Endangered Species (CITES). It occurs on the coasts of Ecuador, Galapagos, Mexico and Peru. Sea cucumbers do not breed easily or quickly. Stocks have already been depleted. Some worry their populations maybe decimated beyond repair in a few years. Such overfishing can effect entire ecosystems. Overfishing has in some places reduced their role in breaking down organics on the ocean floor. Areas without the sea cucumbers have become unihabitable for other organisms.
Large amounts of dried sea cucumbers are traded in Galapagos Islands to Asian markets, mainly Japan, Hong Kong, Taiwan, and Singapore. Stocks have become depleted in these countries, so they have been looking for other sources. Demand from China has cased the price and number of thefts of sea cucumber to rise in Japan. Prices are so high that thieves once broke into a sea cucumber processing plant in Morimach, Hokkaido, tied up factory employees and made off with 160 kilograms of dried sea cucumber.
Soaring prices for sea cucumber from around ¥20,000 for kilogram of dried sea cumber to more than ¥70,000 have resulted in an increase in poaching of the sea creature in Japan , particularly around Hokkaido. The first known case of sea cucumber poaching was recorded in 2005. In 2006 a half dozen cases were reported, including one in which a gang of seven Hokkaido people was caught with 33 tons of sea cucumbers. Sea cucumbers from Hokkaido are preferred because their warts are said to be clearer and less unsightly.
Sea cucumbers in Baja California, eastern Russia, and the Galapagos Archipelago have been the focus of recent attention. In Baja California Isostichopus fuscus has been overharvested. In 1994, the National Institute of Ecology in Mexico declared that I. fuscus was in danger of extinction. In eastern Russia, increasing demand on Cucumaria japonica has led to concern for this species, which is harvested for both food and cosmetic products. Because of commercial exploitation in the Galapagos, Ecuador passed the Galapagos Marine Management Plan in 1999 to regulate conservation of sea cucumbers. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]
The Australian government is trying to seed juveniles of sandfish, Holothuroidea scabra which were reduced by overfishing. Joel K. Bourne, Jr. Wrote in National Geographic: “In a fjord on the British Columbia coast, Cross has devised a polyculture of his own. He feeds only one species — a sleek, hardy native of the North Pacific known as sablefish or black cod. Slightly down current from their pens he has placed hanging baskets full of native cockles, oysters, and scallops as well as mussels that feed on the fine organic excretions of the fish. Next to the baskets he grows long lines of sugar kelp, used in soups and sushi and also to produce bioethanol; these aquatic plants filter the water even further, converting nearly all the remaining nitrates and phosphorus to plant tissue. On the seafloor, 80 feet below the fish pens, sea cucumbers — considered delicacies in China and Japan — vacuum up heavier organic waste that the other species miss. Minus the sablefish, Cross says, his system could be fitted onto existing fish farms to serve as a giant water filter that would produce extra food and profit. [Source: Joel K. Bourne, Jr., National Geographic, June 2014]
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated May 2023