Sea Anemones

Sea Anemones

A clownfish keeping watch in it's anemone partner.

Sea anemones are a group of predatory marine animals, somewhat resembling fleshy flowers. These ornately colored creatures are named after the terrestrial anemone flower and are close relatives of coral and jellyfish. There are more than 1,000 sea anemone species found throughout the world's oceans. Some have adapted to live in cold water at depths more than 32,000 feet, but most species prefer warmer tropical waters. They run the full spectrum of colors and can be as small as half an inch or as large as six feet across.

A sea anemone is a (generally) sessile polyp with a column shaped body (ending in the familiar flowery oral disc) and either a pedal disk (that affixes the anemone to the substrate) or a modified bulb-like base (used by burrowing anemones to anchor in soft substrate). The oral disc, which operates as both the mouth and the anus, is surrounded by tens to hundreds of tentacles, all armed with cnidocytes, stinging cells which are both offensive and defensive in nature and give the phylum Cnidaria its name. Each cnidocyte contains nematocysts, and each nematocyst contains a coiled venom-filled tubule. When prey, predator, or competitor encounters the tentacles, hundreds of these nematocysts fire their barbed tubules like harpoons. This gives the anemone its characteristic sticky feeling. The venom is a mix of toxins that paralyze the victim. If the victim is prey, this lets the anemone safely transport it to the mouth. The venom is deadly to prey species of fish and crustaceans, but most anemones are harmless to humans. In southern Italy and southwestern Spain, one anemone species is even consumed as a delicacy. Apparently, they are similar in appearance and texture to croquettes (a small breadcrumbed fried food roll), but have an intense seafood taste. A few highly toxic species have caused severe injuries and are potentially lethal to humans, but they're not in our stomping grounds.

A primitive nervous system, without centralization, coordinates all actions. No specialized sense organs are present. Unlike the closely related corals, anemones do not have a hard skeleton. Instead, muscles surrounding a fluid-filled cavity (the coelenteron) maintain the anemone's relative shape. This is known as a hydrostatic skeleton. By closing its mouth, the anemone can keep the coelenteron at a constant volume, making it more rigid. To move around, muscles found in the endoderm (the inner layer of the body "wall," as opposed to the ectoderm, the outer layer) are used to contract and bend the column in various directions. Longitudinal muscles, which run perpendicular to the base, contract the column vertically. Circular muscles, which run around the column parallel to the base, expand or contract the diameter of the column. To achieve these movements, these muscles basically squeeze water to different parts of the column, which subsequently extend and become more rigid. And that's how you use a hydrostatic skeleton!

*Fun Fact: sea anemones detect passing creatures with vibration-sensitive hair cells. We use similar hair cells in our inner ears to detect sound. Unlike humans, who are often stuck with hearing loss after damage to the hairs in our ears, anemones can repair their hair cells using certain proteins. However, new findings, published in the Journal of Experimental Biology, suggest that we might one day be able to co-opt these proteins to restore hearing loss in people, too.

Carnivorous and opportunistic, sea anemones are silent slow-motion predators that will devour any small animal. They accept zooplankton, mussels, fish, crabs and other small crustaceans, marine larvae, worms, and any other "meat" careless enough to stray within reach of their deadly tentacles. The tentacles are triggered by the slightest touch, instantly firing the venom-laded harpoon. The tubule remains attached to the tentacle, so the prey is slowly reeled in. The anemone's digestive enzymes are so strong that they can digest the flesh of a small animal in 15 minutes. Afterwards, it spits out any non-digestible parts, such as bone and shell. Though they have no visible sense organs, anemones can distinguish between food and debris. If you drop a piece of paper onto its tentacles, a sea anemone will grasp but then discard it.

With their stinging tentacles, anemones would seem like an unlikely meal choice. However, there are several potential predators that either aren't affected by the venom or have developed sneaky feeding strategies to cope with the dangerous tentacles. Snails and slugs typically slide up and take chunks out of the anemone. Larger anemones can detach and float away, but smaller ones are usually out of luck. A sea star can wrap itself around the anemone and deploy its stomach, but some luck must be involved because anemones also eat sea stars. Many fish species munch on anemone tentacles, including the butterfly fish and the mosshead sculpin. These fish feed by nipping off the tentacles, rather than eating the anemone whole. The loggerhead sea turtle also counts the anemone among its extensive list of prey, and they do eat them whole.

Some anemones form symbiotic alliances with other organisms to prevent predation. The most famous of these alliances is probably the clownfish and its various anemone hosts. Clownfish are covered by a mucus layer that makes them immune to anemone stings. The fish both receive protection and dole it out, chasing away butterfly fish and other nuisances. The anemone gets the added bonus of food scraps from the clownfish's meals. Some anemones, like their coral cousins, host green algae. In exchange for providing safe harbor and exposure to sunlight, the anemone receives oxygen and sugar, byproducts of the algae's photosynthesis. Still other anemones attach themselves to hermit crab shells. The hermits receive protection and camouflage; the anemones receive a traveling lifestyle, hopefully with more feeding opportunities.

Most anemones don't actually need a hermit crab for the traveling lifestyle. Though they seem to prefer to stay "planted" in one place, many species have the ability to move. At the bottom of the anemone's columnar body is an adhesive, muscular foot, which they can use to slide along the sea floor. They can also simply detach and float away, or even "swim" by flexing their bodies. A few species don't attach themselves at all and instead float upside down in the water with the help of a gas chamber in the foot. For the majority that do tend to stick in one place, having the option of moving becomes extremely handy if the environment becomes unlivable. Intertidal species are capable of retaining sufficient water to endure exposure to air and sun during low tide, but during times of drought, when even the high tide recedes, the ability to move house is the difference between life and death, and those organisms that are truly sessile, such as oysters and barnacles, are doomed. Some species of anemones move for another purpose: war. These anemones live in large "armies" near each other. "Scouts" look for new land to claim, and if one army gets too close to another, "warriors" will slap the offending invaders with special club-like tentacles that leave patches of stinging cells stuck to the enemy. Territorial fights often result in serious injury and even death to those on the front lines.

Unlike other cnidarians, anemones lack the free-swimming medusal stage of their lifecycle; they are exclusively polypoid. A fertilized egg develops into a planula that develops directly into another polyp. Anemone species vary in their reproductive abilities. Some have separate sexes; others are hermaphrodites (possessing both male and female reproductive organs); some can switch between the two. In sexual reproduction, males (or hermaphrodites) release sperm and females (or hermaphrodites) release eggs into the water column, and fertilization occurs. (They eject the sperm and eggs through the mouth/anus.) Some species can reproduce asexually, a useful trait when you're stuck on a crab who's indifferent to your love life. They have three types of asexual reproduction at their disposal. First, budding: a new anemone develops from an outgrowth, or "bud"; the clone remains attached as it grows, separating from the parent when it is mature. Second, pedal laceration: a ring of flesh breaks off (from the foot) and fragments, each fragment becoming a new individual. Third, lateral fission: the anemone seems to crawl in two opposite directions at once and slowly tears itself in half lengthwise (don't try this at home!); each half becomes a new whole. Each of these three methods results in genetically identical copies.

Texas waters host a few anemone species, including the warty anemone, the tricolor anemone, and the onion anemone. The warty anemone, Bunodosoma cavernata, is a very common anemone and is easily recognized by its warty appearance. Color is usually brownish or tannish with vertical rows of pale, dusky, or blue bumps on the column. Tentacles are numerous, short, and sometimes marked with red stripes near mouth or with white bumps at the base. It can be found in Port Aransas and Galveston (and maybe other areas along the coast and in the Gulf), attached to jetties or buried in the sand on the beach at low tide. It is occasionally found on the backs of hermit crab shells. Average measurements: 3.5 inches in height and 2 inches in diameter of the oral disk (mouth).

The tricolor anemone, Calliactis tricolor, varies in color but will always have dark spots around the base. Usually beige/dark brown with beige streaks; dark orange and red streaks; or purple. Tentacles are short, usually whitish, but may be dark orange or pink. Mouth is tricolored, usually with a darker outer ring and orange or pinkish center. It is often found on hermit crabs or the calico box crab. Averages 2 inches in disk width. Releases orange stringy filaments from both the mouth and dark pores at the base when disturbed.

The onion anemone, Paranthus rapiformis, is distinguished by its "cocktail onion" shape with longitudinal stripes. Usually burrows in the sediment with tentacles above the surface of the sediment. Medium size anemone, extending, on average, up to 13.75 inches. Color is pink to cream or reddish-brown to greenish-gray, with light longitudinal stripes. Tentacles are colorless to light brown and translucent. When the mouth is closed, the body is round, resembling a cocktail onion. However, the body can elongate, sometimes looking more worm-like.

Though sea anemones are not endangered, they are susceptible to overexploitation and habitat destruction due to their long lifespans (up to 50 years), slower relative growth rates, and lower reproductive rates than their resident fish, which can also be adversely affected. Pollution, over-collection, trampling, and other human activities can negatively affect anemones. So, tread lightly when you're in their neighborhoods!

Where I learned about anemones, and you can too!

Texas Marine Species

Texas Parks & Wildlife

World Register of Marine Species

A Functional Biology of Sea Anemones
By J. Malcom Schick

‪Beachcomber's Guide to Gulf Coast Marine Life
By Susan B. Rothschild

National Geographic

National Wildlife Federation

Audubon Nature Institute

Kansas University Biodiversity Institute & Natural History Museum

Seattle Aquarium

Electronic Atlas of the Wildlife of British Columbia



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