Sea Anemones: Characteristics, Feeding and Toxins

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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.

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: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Coral Reef Alliance ; Global Coral reef Alliance ; Global Coral Reef Monitoring Network


20120518-sea anemone 800px-Zeeanemoon-1.jpg Sea anemones, jellyfish and corals belong to the family of colonizing organisms called coelenterates (Greek for cavity) and the 9,000-species phylum Cnidarians (meaning "stringing thread"), a group of tentacled creatures which also includes anemones, jellyfish and corals and hydriods. Most reproduce asexually without mating by producing buds from their own bodies.

All coelenterates are simply a hollow sac or shallow cup of cells with a mouth at one end surrounded by tentacles. Armed with stinging cells, the tentacles help them to paralyze small swimming animals which are then pushed into its mouth. Coelenterates have a primitive gut for digestion and their mouth also serves as an anus.

Coelenterates take in food through their mouthes and ingest it in the stomach, with the indigestible parts being expelled back out the mouth. They are almost exclusively carnivores but have no teeth. Instead they have tentacles lined with whiplike structures called nematocysts that release poison barbs that are strong enough to paralyze prey and allow it to be pulled into the coelenterate’s mouth and gut. Jellyfish have their tentacles pointed downward while anemones and corals have theirs pointed upwards.


Cnidarians are essentially the same as coelenterates — cup-like animals — but looked at in a slightly different way. The tube can either be a medusa, flattened into a bell shape, or a polyp, with the closed end attached to a hard surface. Corals and sea anemones are polyps. Medusas are mostly jellyfish. Some hydriods and jellyfish exist in both medusa and polyp forms in their lifetimes.

Cnidarians include corals, sea anemones, jellyfish, and relatives. Phil Myers wrote in Animal Diversity Web: The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry regard as primitive. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions. [Source: Phil Myers, Animal Diversity Web (ADW) /=]

20110307-NOAA sea anemone like Cerianthids_1077.jpg
sea-anemone-like Cerianthids
Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella. Polyps, in contrast, are usually sessile (fixed in one place). They have tubular bodies; one end is attached to the substrate, and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense. /=\

Reproduction in polyps is by asexual budding (polyps) or sexual formation of eggs and sperm (medusae, some polyps). Cnidarian individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and free-swimming. /=\

If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. These tiny organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned. A nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death. /=\

Sea Anemone Characteristics

Sea anemones are venomous, 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) and have radial symmetry (symmetry around a central axis). [Source: Kevin Ashley, Animal Diversity Web (ADW) /=]

Sea anemone structure
1) Tentacles
2) Mouth
3) Retracting muscles
4) Gonads
5) Acontial filaments
6) Pedal disk
7) Ostium
8) Coelenteron
9) Sphincter muscle
10) Mesentery
11) Column
12) Pharynx

Sea anemones have tube-like, columnar bodies, topped with a round cap bearing a crown of numerous tentacles. The external structure consists of the tentacles, the oral disk, and the pedal disk. can be found.The tentacles, which are covered in nematocysts, capture and transport prey to the oral disk. The oral disk serves as both the mouth and the anus. The mouth is the opening to the coelenteron, a single sac like cavity that performs all digestive functions. The pedal disk attaches the sea anemone to hard surfaces. The internal structure of a sea anemone consists of the contracting muscles, the gonads, the acontial filaments, and the ostium. The retracting muscles consist of simple longitudinal fibers that contract to move the anemone vertically. The sphincter muscles allow the tentacles to close over the oral disk. The gonads can be found in the mesentery. The ostium are where water is let in and out of the anemone. The acontial filament are found in the bottom sac section of the coelenteron. The acontial filaments are laden with nematocysts. Acontia filaments are used for protection from predators.

The tentacular crown has at least six rings of tentacles with a mouth in the center. Sea anemones have a basal pedal disk, used to attach the animal to substrate (usually a rock or coral). Once attached to the substrate, they typically do not move; however, an anemone can use its foot to move to a new location if conditions are unfit for survival. The column is dark green to brown in color, with irregular tubercles on the surface.

According to Animal Diversity Web: The nervous system of sea anemones has been described as elementary. Instead of a brain and central nervous system, they have a nerve net, which allows transmission of a signal in all directions instead of following a single pathway. This is beneficial because it permits response to stimuli from all sides of an anemone's radial body. Giant green anemones have shown response to electromagnetic radiation in experimental studies. Exposure to X-ray or ultraviolet light induces tentacle retraction and muscle contractions, both of which reduce height. These anemones also respond to mechanical and electrical stimuli, and their tentacles contain receptors that detect anthopleurin, a pheromone produced by wounded anemones. When anthopleurin is detected, an anemone shows an alarm response by retracting its oral disc and tentacles. If wounded or eaten, the anemone releases anthopleurin to warn its neighbors.

Sea Anemone Tentacles and Poison

20110307-NOAA sea anemone_100.jpg
sea anemone
The tentacles of sea anemones contain stinging cells called cnidocytes, which hold venomous organelles called nematocysts, used to paralyze and capture prey, as well as to defend against attackers. 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.

Sea Anemone Behavior and Senses

Sea anemones are nocturnal (active at night), sessile (fixed in one place) sedentary (remain in the same area), solitary and colonial (living together in groups or in close proximity to each other). They typically do not move far from the spot in which they settle after their larval stage; their home ranges do not extend beyond their bodies. [Source: Kevin Ashley, Animal Diversity Web (ADW) /=]

Although they are solitary, sea anemones are commonly found in groups at densities of up to 14 individuals per square meters. Individuals located in the same area will often maintain physical contact through their tentacle tips; they are not typically aggressive toward each other. When an anemone is removed from a rock in a densely populated colony, neighboring anemones do not move to the new empty space. Individuals that have been transplanted from one colony to another may induce aggressive behavior, including body inflation and use of acrorhagi (tentacles specialized for attacking) in surrounding anemones. These animals may retract their tentacles during low tide in order to avoid drying out.

Sea anemones sense using ultraviolet light, touch, electric signals and chemicals usually detected with smelling or smelling-like senses. They 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).

Sea Anemone Reproduction and Development

giant green anemone

Sea anemones are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They engage in external reproduction in which sperm from the male fertilizes the female’s egg outside her body, employing broadcast (group) spawning, the main mode of reproduction in the sea. It involves the release of both eggs and sperm into the water and contact between sperm and egg and fertilization occur externally. [Source: Kevin Ashley, Animal Diversity Web (ADW) /=]

Sea anemones engage in seasonal breeding. Giant green anemones spawn once yearly, although they may release eggs and sperm in multiple waves during one spawning event. They release their eggs and sperm during warmer months (typically late summer-early fall). The number of offspring ranges from 3000 to 9000. As these anemones are broadcast spawners; there is no parental involvement in the raising of offspring.

Giant green anemones are divided into females and males and only reproduce sexually.. There are no noticeable differences in appearance between the sexes. Females and males reach sexual maturity at five to 10 years, at which time they develops sex organs. These sea anemones are polygynandrous (promiscuous), with both males and females having multiple partners. Spawning in this species seems to be triggered by warmer water temperatures. In one study, animals in captivity released eggs and sperm at night. Females have been observed releasing thousands of eggs at a time and doing so multiple times within a short period. Males release sperm, which disperses rapidly.

The life cycle of sea anemones is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. A giant green anemone begins its life when an egg is externally fertilized in the water. Ova are spherical, 175- 225 micrometers in diameter, purple in color, and covered with spines, while sperm are 2-3 micrometers long and two micrometers in diameter, with tails 50 micrometers in length.

Cell cleavage begins within three hours after fertilization and development continues to a planula stage, in which the larvae swim or float freely and have the ability to disperse long distances, potentially inhabiting new locations away from their parent organisms. During this stage, planulae eat zooplankton, phytoplankton, and even other larvae. Each planula secretes a mucus thread; food particles adhere to this thread and are drawn to the mouth where they are ingested. Settlement occurs at least three weeks after fertilization. Once larvae find suitable locations, they attach to substrate and develop their pedal disks, completing metamorphosis into adults.

Clownfish, Other Sea Creatures and Sea Anemones

20120518-790px-Clownfish_(Amphiprion_ocellaris).jpg 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.

Sea anemones are often fed on by sea slugs. 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. Sex shrimp are also known as squat anemone shrimp. They get their names from way they shake the rear part of their body and their relationship with sea anemones. [Source: Seona Choi, Animal Diversity Web (ADW) /=]

Giant Green Anemones

Giant green anemones (Scientific name: Anthopleura xanthogrammica) are primarily found along the west coast of North and Central America, from Alaska in north to Panama in the south. They have also been observed in Hudson Bay, Canada and the eastern coast of Russia. They are most often seen in tide pools and intertidal/subtidal zones along rocky shores, at depths up to 15 meters (50 feet). They are typically seen attached to the sea bottom in places with cold waters and high wave activity. They are commonly found in mussel beds. In captivity, they are known to thrive at water temperatures between 15° to 22.2° C (59°- 72°F). There is record of one of these sea urchins being kept in captivity for 80 years. Their longevity in the wild has been estimated at 150 years. [Source: Kevin Ashley, Animal Diversity Web (ADW) /=]

Reporting from Bodega Bay northern California, Mel White wrote in National Geographic: “The giant green sea anemone is a formidable predator, though it waits for unsuspecting prey to wander within reach rather than actively hunting. Resembling a fist-size blob of lime Jell-O out of the water, Anthopleura blooms when submerged, extending delicate tentacles around a sucking maw that swallows prey whole. Sea anemones show their kinship to jellyfish in their use of stinging structures called nematocysts, which they fire like microscopic darts to stun prey. I extend a finger as an anemone taste test and feel only a faint sticky sensation. If I were a crab, I'd be lunch. Once they've set up housekeeping, Anthopleura and many of its neighbors in the intertidal zone show exceptional longevity, both individually and as species. [Source: Mel White, National Geographic, June 2011]

Humans utilize giant green anemones as sources of medicines and drugs. Heart stimulants such as Anthopleurin-A and Anthopleurin-B have been derived from venom in their nematocysts. These drugs strengthen the heart's contractions without altering its rhythm. Protease inhibitors, such as AXPI-I and –II, have also been extracted and are particularly responsive towards trypsin, a potentially damaging enzyme produced in the pancreas.

giant green anemones

Giant green anemones reach lengths of 30 centimeters (12 inches). Column diameter may be up to 17 centimeters (6.6 inches). The tentacular cap crown diameter may be up to 25 centimeters (10 inches) and they may grow to be as tall as 30 centimeters (one foot). Disk and tentacles are green or blue to white, depending on how much sunlight the anenome receives. This is because the anemone has symbiotic algae living inside its tissues. When sunlight is plentiful, the algae grows, producing a bright green color. If the animal is in shade, these algae will be reduced in number or absent. /=\ They have not been evaluated for the International Union for Conservation of Nature (IUCN) Red List. They have no special status according to the Convention on the International Trade in Endangered Species (CITES).

Giant Green Anemone Feeding, Predators and Ecosystem Roles

Giant green anemones are carnivores, feeding mostly on mussels, sea urchins, crabs, and small fish. They often settle in mussel beds to increase food availability. Once prey is in reach, an anemone stretches its tentacles and paralyzes its prey using the nematocysts on its tentacles. It then uses its tentacles to bring food directly to the mouth. Giant green anemones have an incomplete gut, meaning that the mouth functions to take in food as well as to expel waste. Once food is swallowed, it enters the gastrovascular cavity where it is digested, then waste products, including empty shells, travel back up and out of the mouth. Because this is a cold-water anemone, it has a relatively slow metabolism, requiring it to feed only once or twice a month. These anemones have photosynthetic algae (Zoochlorellae sp.) and dinoflagellates (Zooxanthellae sp.) living in their guts, from which they are able to derive additional nutrition. [Source: Kevin Ashley, Animal Diversity Web (ADW) /=]

Their main known predators of giant green anemones are Shaggy mouse nudibranch (Aeolidia papillosa). They are also preyed upon by leather stars (Dermasterias imbricata), tinted wentletrap (Nitidiscala tincta), Chace's wentletrap (Opalia chacei), Scallop-edged wentletrap (Opalia funiculata) and Stearns' sea spider (Pycnogonum sternsi). Sea spiders feed on an anemone’s central column, and sea snails feed on both the central column and tentacles. When threatened, an anemone may react by stinging the attacker with its nematocysts.

Giant green anemones play a role in many parts of the marine food web. They feed on a wide variety of prey including mussels, sea urchins, small fishes, and crabs, and are eaten by a wide variety of predators including sea slugs, sea snails, sea spiders, sea stars, and large crabs. They also serve as a host for symbiotic photosynthetic algae and dinoflagellates, benefiting from the nutrients they produce. Shells ejected by these anemones may serve as shelter for animals such as hermit crabs. Blue-band hermit crabs (Pagurus samuelis), in particular, are often found associated with these anemones. They are even found walking on giant green anemones, unaffected by their nematocysts. It has been suggested that the crabs become so coated in mucous from the anemones that they are not recognized as a threat or prey.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); 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

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