Sea cucumbers are primitive animals that are found in a variety of environments, including reefs and sea grass beds in seas all over the world at various depths. They are echinoderms but they differ from sea urchins and starfish in that their spines 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.

Sea cucumbers vary in size from two centimeters to two meters. In some places they are quite plentiful, making up 90 percent of the 90 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.

Websites and Resources: National Oceanic and Atmospheric Administration ; Smithsonian Oceans Portal ; Ocean World ; Woods Hole Oceanographic Institute ; Cousteau Society ; Montery Bay Aquarium

Websites and Resources on Fish and Marine Life: MarineBio ; Census of Marine Life ; Marine Life Images ; Marine Species Gallery

Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ; Coral Reef Pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.

Sea Cucumber Ability to Change from Solid to Liquid.

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sea cucumber
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 abilty 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

Sea cucumbers are often found on the sandy sea floor in areas around coral reefs. 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.

When threatened, sea cumbers extrude their external organs and eject a thread-like material that entangles intruders while the sea cucumber slowly makes its escape and regrows it entrails over the following weeks. When picked up or threatened they often also squirt out water.

Sea cucumbers feed on detritus and help to recycle nutrients on the sea floor as earthworms do in topsoil. 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.

Sea cucumbers generally don't lie face up or face down but in 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.

Pearlfish and 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 Products

spotted Nudibranch
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. 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 made from strips of muscle inside the sea cucumber body. It used to be commonly eaten in the West. 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 ensnarl prey are used by islaners to protect their feet from sharp coral.

Overharvesting of sea cucumbers has threatened the sea cucumbers themselves and the marine ecosystem in which the play an important role as recycler of marine nutrients. 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.

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sea slug
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.

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 a few years ago but now sell for about ¥900 a kilogram. 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.

Sea Slugs and Opisthobranchs

Sea slugs are strange creatures that have changed little in a 100 million years. Some species have sensory horns and most have exterior gills on their tails. Some of those found in coral reefs are quite colorful. The term sea slug can mean a nudibranch (See Below) or various other opisthobranchs. The term is also used to describe other kinds of reduced-shell gastropods and even cucumbers. [Source: Fred Bavendam, Smithsonian magazine, August 1989]

Opisthobranchs are a subclass of mollusks and gastropod that includes sea hares, nudibranchs, bubble shells and pteropods. They are essentially snails that dispense with their shells when they become adults. Some can swim but most move along the bottom of the ocean. Most are less than 10 inches in size. There are about 4,000 species of opisthobranchs of which 3,500 are nudibranchs and sea slug species.

Columbia University's Eric Kandel won a Nobel Prize in medicine for his studies of dopamine and the brains of sea slugs. Blind and deaf, sea slugs can sense light and dark but mostly they locate prey through touch and by following their chemical trail. Using a hooklike-tongue called a radula, it places the prey in its mouth and swallows it hole.

Their lack of a shell makes sea slugs more mobile. They move around with a fleshy "foot" similar to that on seas shells, which advances them forward through muscular contraction or the movement of hairlike cilia on the bottom of the foot. The upperside of their long soft bodies is covered with waving delicate tentacles that come in variety of brilliant colors.

Sea Slug Feeding

Their lack of a shell also makes more vulnerable to attacks from predators. Some bright colored sea slugs feed on poisonous sea anemones, ascidians, hydroids and sponges and direct the poison through their digestive system into special sacs at the tips of their brightly-colored "fingers" called cerata. The bright colors are believed to be a means of informing other creatures they are poisonous. Sometimes the toxins kill, but mostly they taste really bad or are extremely irritating. Cerata grow back.

Sea slugs are slow-moving carnivorous creatures that feed on corals, sponges and anemones. Some attacking sea slugs rear up like striking cobras. Some of these grab sea anemones with their relatively large jaws and are sucked into the anemone's tube, where they take their time consuming its tentacles.

Many sea slugs feed on coral polyps. The color of sea slugs often closely resembles the color of the coral they feed on. This give them camouflage against potential predators. One species found in the Great Barrier Reef sprouts legs that look like coral branches and feeds on algae living in its tissues much like coral does.

Opisthobranchs Reproduction

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All adult opisthobranchs are hermaphroditic. Many mate by formulating copulating chains with a number of individuals lined up head to tail. The first in line is a female and the last is a male with those in the middle acting as both male and female. Eggs hatch free-swimming larvae that disperse and later metamorphose into juveniles without shells and eventually adults.

Among one species, the Navanax, mating couples take turns being male and female. David Attenborough wrote: "First one behaves as a male, extracting a long tentacle-like penis from a pore in its head and pursuing the other by following its trail of mucus. When it eventually catches up, the pursued lifts its hind end, allowing the pursuer to insert its penis in genital pouch in the rear. The two move along in tandem. After about ten minutes, they separate and change roles.”

“ Hermissenda “ seas slugs are hermaphroditic cannibals that can mate as a female, mate as a male, eat its own species or be eaten by its own kind.


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Nudibranchs, sometimes called sea slugs, are among the world’s more colorful creatures, with species boasting bright yellow, pink and orange features and patterns. Most are finger-sized with gills-forming tufts on their backs (Nudibranch means “naked gill,” a feature that separates them from other kinds of sea slugs). They don’t have any shell — or skeleton — so when they perish after living for about a year they leave behind no evidence of their existence. [Source: David Doubilet, National Geographic, June 2008]

Jennifer Holland wrote in National Geographic, “Nudibranchs crawl through life as sick and naked as newborn. Snail kin whose ancestors shrugged off their shell millions of years ago, they are just skin, muscle and muscles and organs sliding on trails of slime across ocean floors and coral heads the world over...Found from sandy shallows and reefs to the murky seabed nearly a mile down, nudicranchs thrive in water both warm and cold and even around billowing deep-sea vents.”

There are more than known 3,000 species of nudibranch (many scientists believe only about half of all nudibranch species have been found). They feed on coral, sponges, barnacles, eggs, small fish and one another. Some feed only on a single species such as sea pen. Some have teeth and jaws. Others rely on enzymes to break down prey.

Hermaphrodites, with both male and female organs, they pair up and lay in coils, ribbons or tangled clumps of eggs, up to 2 million at a time. Two-centimeter-long "Hypselodoris whitei” mate head to tail through a genital aperture. Each fertilizes the other and both produce eggs. Hatchling emerges and float away in the current.

Nudibranchs have a muscular foot that enables them to crawl across the hard surfaces. They can release their grasp and tumble in the current. Some can even swim freely. Sensing is done with oral tentacles that taste and feel surrounding and antenna-like appendages called rhinophores that act like a nose to detect chemical signals. The eyes can not make out their fantastic colors, discerning little more than light and dark.

Nudibranch Toxins and Colors

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Nudibranchs, wrote Holland “are well equipped to take care of themselves. Not only can they be tough-skinned, bumpy and abrasive, but they’ve also traded the familiar shell for less burdensome weaponry: toxic secretions and stinging cells. A few make their own poisons, but most pilfer from the foods they eat. Species that dine on toxic sponges, for example, alter and store the irritating compounds in their bodies and secrete them from skin cells or glands when disturbed. Other nudibranchs hoard capsules of tightly coiled stingers, call nematocysts, ingested from corals, anemones and hydroids. Immune to the sting, the slugs deploy the stolen artillery along their own extremities.”

Many nudibranchs announce they carry toxins with garish colors that contrast markedly against the green and brown rocks and reef they crawl over. Creatures that mimic the designer colors, including non-toxic nudibranchs, are also left alone. Some species, especially those that are most active at night and limit themselves to a small range, deploy camouflage rather than warning signs.

Some species of fish, sea spider, turtle, sea star and crab can eat toxic nudibranchs and suffer no ill effects, Some people eat them too after removing their toxic parts. Chileans and islanders off of Russia and Alaska roast or boil sea slugs or eat them raw. Photographer David Doubilet compared the experience to “chewing on an eraser.”

Kinds of Sea Slugs and Nudibranchs

Clown nudibranch
Spanish dancers are a kind of sea slug that undulates through the water looking like multicolored ray wing with no body. They are often bright red in color and the fluttering movements have been compared with famous flamenco or belly dancers. Their eggs are laid together in a delicate mass, resembling a chiffon scarf. Giants of the sea slug family, the can reach a length of 40 centimeters, feeding exclusively on sponges.

Some species of sea slug and nudibanch float in the water and consume jellyfish. When they eat jellyfish they take the stinging cells into their gut unmolested. The toxin eventually migrates through the seas slug's tissue into its back and provided protection against predators. The “Glaucus” sea slug, which is camouflaged in blue and silver, prey on toxic Portuguese men-of-war, appropriating their stinging cells of its own defense.

The four-centimeter-long “Phyliodesmium iriomotense” feed exclusively on corals. It’s translucent body houses a branching digestive gland with tentacle-like certa-outgrowths the creature can shed if attacked. The “bornella angilla”, a swimming nudibranch, flees dangers by folding its appendages and swimming like an eel.

Sea Hares and Sea Hare Orgies

Mating Nudibranchs
Sea hares look like shapeless blobs. Related to sea slugs and nudibranchs, they are mollusks and gastropod with a soft body. What little remains of their ancestral shell is imbedded in their skin. They live on the sea bottom and feel their way around with two long antennae that project out the animal's head, sort of like rabbit ears, the source of the animal’s name. Sea hares have a large foot and inhabit sea grass and seaweed beds and feed on fleshy algae, which they crop using a hard mouthpiece called a radula. .

The black sea hare is the world's largest gastropod. It can reach lengths of 39 inches and weigh as much as 30 pounds. Dolastatin is a drug taken from an Indian ocean sea hare that shows promise in treating skin cancer and has made it as far as clinical trials.

Sea hares have chemical sensory antenna which they use to analyze scents and chemical substances in the water. They help the animals find food and locate mates. Their main defense against predators is a foul taste that potential predators go out of their way to avoid. They can also eject a noxious purplish ink-like liquid that predators also despise.

Adult sea hares carry both eggs and sperm. They can mate as a male or female or both, and often have mass orgies in which sea hares form long conga-line-like chains of individuals fertilizing and being fertilized.

After the orange and yellow eggs are fertilized they are secreted in a thin noodlelike mass. A single sea hare may produce a mass of eggs several hundred meters long with as many as a million eggs. Sea hares die soon after they mate and spawn. After they die they turn to tough rubbery blobs.

Image Source: National Oceanic and Atmospheric Administration (NOAA); Wikimedia Commons

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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