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sea-anemone-like Cerianthids
Sea anemones are small marine animals with a tubular body and circles of tentacles. Members of the coelenterate phylum, they are different from sea urchins which have a hard shell and long spines. Armed with stinging cells, the tentacles allow the anemone to paralyze small swimming animals which are then pushed into its mouth.

Sea anemones were one of the first creatures to appear on the earth. They are like jellyfish that have remained attached to a surface. Sea anemones are somewhat like coral expect they are bigger and live as live solitary somewhat mobile polyps rather than as part of a fixed colony. Like coral, sea anemones receive energy from single-celled plants within their tissues called zooxanthellae. The plants need nitrogen to survive.

Sea anemones take in oxygen and expel and carbon dioxide. They reproduce by dividing into two pieces (fission), by budding and by eggs. Eggs and sperms are formed in partitions in the body cavity and ejected through the mouth. The eggs are fertilized by sperm in the water. The fertilized egg develops into free-swimming larvae which develops into an anemone. Sometimes a piece of sea anemone can grow into a new animal.

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Sea anemones attached themselves to rocks by means of suction-cup-like oral discs. They can walk and even jump. When anemones feel threatened they retreat into stumps anchored on rocks, coral or the sea bottom. Some species can survive out of water for several hours during low tide by retaining water in their body cavity. Sea anemones are often found in tidal pools. When exposed at low tide they pucker up to prevent desiccation and look like lumps of jelly. Some spurt out water when you touch their center.

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 Anemone Tentacles and Poison

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The tentacles of a sea anemones are coated with stinging cells called nematocysts. The tentacles have millions of microscopic harpoons coiled like springs that inject venom when fired according to cues from touch or chemicals. The tentacles and venom are used to capture and stun plankton and swimming prey.

When something brushes up against a tentacle, a mechanical trigger opens the cell and the nematocyt springs out, embedding itself in the flesh of the victim. The victim is paralyzed by the venom and pushed towards the mouth of the sea anemone by the tentacles. The prey is then digested by cells in a primitive stomach.

Sea anemone venom immobilizes prey by disrupting the transmission of information between synapses of the prey's nerve cells. Humans generally only feel the venom if the nematocyst penetrates their skin and even then it is very weak. Most people who are stung fell a prickly sensation and little more.

Clownfish and Sea Anemones

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Clownfish hang around and are able to survive among the venomous tentacles of sea anemones. They are able to do this because the mucous on their skin is different from that found on the skin of most fish, which stimulates the discharge of toxins by sea anemones. If a clownfish strays from the anemone for too long it must establish immunity after returning through a series of brief encounters with the anemone’s stinging tentacles. Scientists are examining the mucous coating on clownfish that protects it from sea anemone toxins.

Clownfish are almost always found near an anemone. They may venture away from the anemone to feed on zooplankton but when threatened they quickly return to the safety of the tentacles. About 10 species of anemone are known to host clownfish. Some will accept various species of clownfish. Others are species specific. The same is true with clownfish. Some are associated with a single species of anemone while other chose different species to host them.

Often times a breeding pair or a half dozen clownfish will live among a single host anemone. When several fish live at a single host there is a definitive pecking order with the larger fish having dominance over the smaller ones. The breeding females lays here eggs at the base of the anemone and her mate watches over them until they hatch, when the larvae of the bony fish that emerge drift in the currents and search for hosts of their own.

Young clownfish approaching a sea anemone for the first time do so very carefully but once they are used to their environment they actively move around the poisonous tentacles with few worries. Sometimes the clownfish will even crawl among the tentacles when the anemone closes up at night.

Clownfish receive protection from predators from the sea anemone. In return for this protection and scraps of food provided by the anemone, the clownfish keep's the anemone clean and occasionally feeds on the small parasites that torment it. Clownfish may also help attract fish and other creatures the sea anemone can eat and scare off some fish such as butterflyfish that are not affected by sea anemone poison and eat their tentacles.

Some shrimp and sea anemones have a symbiotic relationship. The shrimp feed on mucus excreted by the anemone and get protection from large fish. The anemone feeds on nitrogen in excrement from the shrimp.

Sea anemones are often fed by sea slugs.


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Echinoderms (meaning "spiny skinned") are a species 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 6,100 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.


Starfish (properly know as sea stars because they are not fish) are echinoderms and multi-armed invertebrates. There are about 2,000 species of starfish and the live in almost every ocean habitat. Most starfish have five arms but some have 20 or more. They come in a variety of shapes. Some are thick and have short stumpy arms. Others are thin and have long tentacle-like arms. The largest ones are about two feet across.[Source: Fred Bavendam, Smithsonian magazine]

Starfish play an important ecological role but also can be a scourge. In some places where they have been removed there has been an overpopulation of mussels which has prevented other sea creatures from thriving. Overpopulations of starfish can also cause problems. Crown of Thorn starfish have ravaged coral in the Great Barrier Reef and other places.

Sunflower stars can reach two feet across and have more than 20 arms. Not only is it one of the largest starfish it is also quite fast, reaching speeds of five feet a minute.

Starfish Characteristics

Starfish bodies are covered by thousands of stony plates, called ossicles, that are imbedded in the skin but not connected. This means they have surprising flexibility. You can jam a star fish into almost any space and given enough time it can usually extricate itself.

The bottom of the starfish is covered by hundreds of tube feet with a mouth at the center. The mouth opens into a primitive saclike stomach, whose folds extend out into the arms. Around the mouth is a circulatory system of water tubes and blood vessels, reproductive organs and other organs. Starfish have primitive eyes positioned on the end of each arm that allows them to react to light and chemical sensors that help them locate prey.

Starfish tube feet have suckers that allow them to cling to objects. The feet are operated using a unique vascular system that relies on hydraulic pressure to create or release a vacuum. One species travels at 30 inches a minute. Lost starfish arms grow back and sometimes the lost arms gives birth to a new starfish. If a starfish is cut in half each side will grow into a new starfish.

Starfish Feeding Behavior

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Some starfish feed on detritus on the ocean floor. Others feed on sea urchins, sea anemones, clams, other mollusks, other starfish and other invertebrates. Some catch prey, swallow it hole and digest it in their bodies. Some catch their prey with tiny tentacles that pass the meal toward the starfish's mouth.

Most starfish have the ability to push their stomachs out through their mouths and digest whatever it touches, often a mollusk. Many star fish feed on corals by producing digestive fluids which they squirt into the polyp compartments and extract the polyp as a soupy liquid. Many starfish are venomous. They use their toxin to stun prey Others trap their prey with mucus.

Some starfish envelope mollusks with their tube feed and slowly wrench the shell open and feed on the flesh inside or find a small crack in the shell, as small as a tenth of millimeter, and inject their stomachs into the shell and eat the animal. Their habit of feeding on mollusks has made starfish unpopular with fishermen who raise and collect clams, scallops and oysters.

Starfish generally reproduce by laying a great number of small eggs. The egg are surrounded by a protective covering, which in turn are covered by a layer of jelly. In the early stages of fertilization, the sperm attaches itself to the jelly. Chemical signals cause the protective covering to rupture and allow the sperm to fertilize the egg.

Starfish lay around a million eggs in their lifetimes. Maybe one of these will survive until adulthood. Only a few animals feed on adult starfish: large tritons, some birds and crabs and sea otters.


Brittlestars are echidorms along with starfish, sea urchins and sea cucumbers. There are over 2,000 species of brittlestars. Unlike other echidorms they can move quickly and escape quickly to crevices and rocks where they hide. They are different from starfish.

Brittlestars catch live fish, squid and crabs with their highly flexible arms. They are so named because their arms break off so easily. The limbs quickly grow back. Some species change their colors from night to day.

Brittlestars have skeletal plates that function as microscopic lenses and serve as primitive eyes by focusing light unto nerve cells below. Scientist are study the lens for insights into improving switching stations for optical fibre networks.

Sea Urchins

Sea urchins are small round sea creatures with a hard shell surrounded by spines. They are echinoderms like starfish, sea anemones and sea cucumbers but are different from sea anemones, which are small marine animals with a tubular body and circles of tentacles. The word "urchin is derived from the French word for "hedgehog."

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Most species of sea urchins have long spike-like spines that protect them from fish, crabs, turtles and other sea creatures that like to feed on their soft undersides. Sea urchin spines are mounted on hinges and can be moved around. Scattered among the spines are finger-like appendages that pick and remove dirt and other foreign material.

Fire urchin spines pack a particularly nasty sting that produces a searing pain that can linger for weeks. They advertise their toxicity with bright yellow and orange colors. The toxins are released by tiny blue balls at the end of the their tentacles.

Sand dollars are kinds of sea urchins. They spend their lives buried in the sand and collect oxygen and food with one of kind tube feet and excrete waste with another kind. Sand dollars have a five-pointed star pattern on their surface and very short spines that together have a velvety feel. What you see on the beach is the hard material left when the sea creature dies.

Sea Urchin Characteristics

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Sea urchins come in a variety of shapes. Most are roundish or bun-like and are covered in spines. They move slowly across the sea floor, reefs and boulders, consuming algae on its underside with a mouth like the jaws that grasp a bit in a drill. Around Panama in the 1970s, sea urchins died of so some unknown reason and algae took over reefs, killing large amounts of coral.

A sea urchins five teeth are not only consume prey they also grind away limestone to produce a niche depression for the creature to settle into. How do teeth made of calcium grind down rock also made of calcium without being worn down. Scientists at the University of Wisconsin, using powerful high-resolution X-ray technology to investigate this problem, found the teeth are formed into needle-like crystal-like matrixes and have a bit of magnesium in them that make them harder.

Sea Urchin Behavior and Mating

Sea urchins have tube feet on their undersides like star fish except they are generally longer. They usually stay in one place during the day, often in niche or a a crevice. At night they emerge from their hiding places and slowly roam the reef grazing on blue-green algae that quickly accumulates.

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Sea urchins feed by scraping off the surface of kelps and algae encrusted rocks and feeding it to an elaborate jaw mechanism called Aristotle's lantern, named after its first describer. Sand dollars settle in the sand or mud and sifting out things like worms, algae and plankton for food.

Distinguishing male and female sea urchins is near impossible except when they are spawning. Observations of spawning are quite rare. The male releases a cloud of smoky white sperm and the female releases a cloud of creamy eggs. The releases look like miniature volcanic eruptions.

Sea urchins produce a great number of small eggs. The egg are surrounded by a protective covering, which in turn are covered by a layer of jelly. In the early stages of fertilization, the sperm attaches itself to the jelly. Chemical signals cause the protective covering to rupture and allow the sperm to fertilize the egg.

Most of the eggs and the larvae that emerge from them are eaten by sea anemones and other creatures. A few drift and settle some place long enough to grow into adults.

Sea Urchins and Other Sea Creatures

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Sea urchins that look like they trapped inside a small localized blizzard are in fact surrounded by hundreds of tiny shrimp in the process of laying their eggs.

Pufferfish brave the urchin's spines to get at their soft undersides.

Triggerfish feed on sea urchins by first blowing a jet of water on the urchin to turn it upside down. Avoiding the short spins on the underside the skilled predator pecks away at the urchin's shell with its beak-like mouth to get at the urchin's fleshy interior.

Sea Urchin Spines and Humans

Sea urchin spines break off and are difficult to remove. Some species can cause a severe reaction, producing paralysis and breathing problems. Sometimes you can get an itchy skin rash that can last for a long time,

Sea urchin spines are dangerous and painful to humans not so much because they contain potent toxins but rather because they break off inside the body and become infected.

To remove sea urchin spines apply soft candle wax. Let the wax dry and to set, then pull out the spine. Do not dig them out under the skin. This can cause infection. Left alone the spines will dissolve. Soak the infected area with an antiseptic. Some people have suggested "clobbering the creature with a blunt object" and "peeing on your foot so the embedded spines decompose quickly."

Sea Urchins as Food

Sea urchins are a delicacy in Japan and among the Maori in New Zealand and are eaten by many people in Asia and the Pacific and are becoming popular among lovers of sushi.

Sea urchin divers in southern Australian , who dive in waters filled with great white sharks, earn up to $2,000 a day.

Uni (sea urchin meat) is a popular delicacy in Japan. It is soft, buttery and yellow, red or bright orange in color. To eat it you break open the shell and pick out the sexual organs with your chop stick, dip it in some soy sauce and swallow. In uni-producing areas it is often served in a bowl of rice.

Store-bought uni melts in the mouth. Fresh uni is firm and the texture of each tiny egg can be felt. Uni is best eaten in the summer when the sea creatures fatten themselves up before spawning in the autumn. The best quality stuff is preserved in salt water and is often served with seafood jelly

There are 10 uni-producing region in Japan. The uni from each region being a little different. The west coast of Hokkaido is the main uni-producing area in Japan. Much of the sea urchin sold in Japan comes from California, Oregon and Maine. Sea urchin supplies have been so badly depleted in Japan that uni fishermen are only allowed to fish in Japanese waters two hours a day.

Some domestically-produced sea urchin comes from Miyagi Prefecture. Some measure 15 centimeters across and are said to be at their best when they are harvested in June. Boats set out at 5:00am and fishermen search for the sea creatures using wooden boxes with glass bottoms, Fishermen use poles, which have hook on the front edge, to pick up and drop sea urchins in a basket under the water. The uni harvesting season lasts from June to August.

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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