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Be A Host To Your Anemone

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Be A Host To Your Anemone
By: Ronald L. Shimek, Ph. D.

Why Not Keep An Anemone?
It would seem that keeping a host sea anemone should be the easiest thing in the world for an aquarist. After all, such animals are offered for sale in every local fish store and, “Hey, what could be so hard about keeping one?” Well, the answer to the last question is, “Just about everything.” To forestall the worst possible outcomes, in this short article, I hope to provide you with some information about how to care for one species of these fascinating and beautiful animals.

Host sea anemones present a real quandary for most aquarists. They are ecologically very important and with only one exception they effectively cannot be propagated in captivity. Furthermore, in natural situations, populations do not recover at all well after harvesting. Generally, when a host anemone is harvested for sale in the aquarium trade all of the associated animals: symbiotic fishes, shrimp, worms, and other crustaceans, die. They can’t migrate to some other anemone, as these are either full to capacity with their own symbionts or the nearby anemones have already been harvested by a collector. Once an anemone is harvested, the reef where it was changes drastically. Research indicates that it may decades or longer before a replacement anemone is likely to be found there. Today’s coral reefs are severely threatened by global warming. In the past 30 years, over 20 percent of coral reefs have disappeared and most of the rest are damaged. Removal of such ecologically important animals as host anemones accelerates such change. Nonetheless, the economic law of supply and demand ensures that the harvesting of host anemones will continue until we either run out of anemones (which has happened in numerous localities), or such collection is outlawed.

The image of a sea anemone with its symbiotic clownfish is one of enduring icons of coral reefs. It appears that aquarists will be purchasing these animals for their aquaria as long as there are anemones and boxes of water to put them into. This means anemones will be purchased, and within short order, most of them will die. Given that they are effectively immortal in nature, this is inexcusable; especially as it is relatively easy to keep at least one species, the bulb-tipped anemone, known to scientists as Entacmaea quadricolor.

Some Generalalities

Sea anemones are animals, not some weird type of marine plant. As a result their care is more demanding and involved than is the care of a daisy in a windowsill planter. To care for them properly, one needs to know a bit about them, and a bit about how they live. Biologists classify sea anemones as being in the Class Anthozoa of the Phylum Cnidaria. There are several thousand species of sea anemones, but in this article I will concentrate on Indo-Pacific Host sea anemones.

A generalized sea anemone is polyp; a baglike animal having tentacles and a mouth at one end. he opposite end, “the pedal disk,” is specialized for attachment. Muscular and flattened, it has a glandular epidermis that secretes sticky mucus, which helps the animal grip the substrate. Anemone body walls consist of three layers: a non-cellular middle layer of material called “mesoglea” sandwiched between two tissue layers. The outer tissue layer is called the “epidermis,” while the internal one is called the “gastrodermis.” In most sea anemones the mesoglea is thick and fibrous and it forms a rugged and durable sheet of proteinaceous material to which both the gastrodermis and epidermis connect (Koehl, 1976). The interior of the bag is the gut, also known by the jargon terms of the coelenteron or gastrovascular cavity. Gut works equally well, and is a lot easier to type, so I will use it here.

Figure 1. Sea Anemone Anatomy (Modified from Fig. 4.35, Kozloff, 1990). Incomplete septa do not reach the pharynx, and septal perforations allow water to move between the gut compartments during contraction or deformation.

The simple anemone body plan, however, has been significantly altered by natural selection. No longer is the mouth just a simple “hole” opening to the inside, rather the inner edges of the mouth fold down and continue internally as a tubular projection, called the pharynx. Additionally, thin tissue sheets, “septa,” extend from the outer body wall toward the center of the cavity. Many of these fasten to the pharynx. These septa divide the gut into a great many compartments at the top of the animal but all of them are open to the central core at the bottom. If you were to cut across an anemone parallel to the substrate, the inside would be seen to be subdivided into many narrow “pie slice-shaped” sections.

These septa also are associated with the tentacles; septal walls are continuous with the sides of the tentacle base. Consequently, the number of tentacles normally equals the number of septa. Continuing with the inside story, the center edges of the septa, below the level of the pharynx, often are extended out into long, threadlike, internal strands called septal filaments. These are often heavily laden with nematocysts and may be used to finish killing any ingested prey.

Natural Born Killers

Nematocysts, used by all cnidarians to capture their prey, are tiny proteinaceous capsules secreted by some of the animals’ epidermal cells. The capsules’ thick walls enclose fluid-filled cavities under immense pressure, about 176 kg/cm2 or about 2500 lb/in2. Capsular contents may be toxic venoms. A coiled thread extends into the cavity. There are about 30 different types of nematocysts, and some of them have hollow threads, which allow the capsular contents to exit through them and enter the prey. Others contain simple threads or glues to entangle and immobilize prey. (Hesslinger and Lenhoff 1988; Russell and Watson 1995; Thorington and Hesslinger 1996). Additionally, nematocysts are often “tuned” to the type of prey – so that if the anemone eats fish, its nematocyst venom is specific to vertebrates and won’t particularly affect shrimps or squids. Such vertebrate-specific venom will,however, affect aquarists.

Figure 2. Diagram of a nematocyst. Left. Showing the position in a epidermal cell with a nerve cell process entering the epidermal cell to contact it. Right. Internal structure of the nematocyst.

Figure 3. Nematocyst Action (Modified from Ruppert et al., 2003). The sequenced of events, from top to bottom, occurring in nematocyst discharge. This whole sequence can take place within 1 millisecond.

Normally anemones capture their prey using tentacular nematocysts. The septal filament nematocysts may have another function. Once a prey item is ingested, it is wrapped in septal filaments, and their nematocysts likely hold the filaments tightly to the prey, creating small volumes between the filament and the prey where enzymes secreted by nearby cells could be confined for the digestion of the prey (Baumann, 1995). Additionally, the cells of the gastrodermis ingest small fragments of the prey and much of the digestion occurs in those cells (Ruppert et al. 2003).

Figure 4. Nematocyst aggregations are visible as the dark pink colored lines in these tentacles of a temperate anemone, Cribrinopsis fernaldi.

Sea anemones lack muscles, instead they contain many epidermal and gastrodermal cells which contain contractile fibrils and, like muscles, are able to contract. The contractile elements occupy much of the cell and these cells approach the capabilities of true muscles in their abilities. Under extreme conditions, bands of these muscle-like cells in the septa can contract and flatten even the largest sea anemone into a thin pancake of tissue. Contractile tissue bands are also found around the column’s base and as sphincters around the mouth. Other smaller ones are found on the tentacles and along the sides of the column.

Sea anemones are covered, both internally and externally, by small, beating, hair-like cellular projections called cilia. Externally, they generate small water currents that move particles off the animal. In the gut, the currents they create circulate fluid through the gut. There are also one or two densely ciliated tracts called “siphonglyphs” that run from the outside to the inside down the corners of the mouth. These tracts pump water into the animal ensuring that the internal contents don’t become stagnant. These ciliary currents are also the only way fluid can get into a contracted animal to re-inflate it. Such inflation may take over 24 hours.

Anemones possess simple networks of nervous tissue running through the epidermis. They lack true sensory organs, but their surfaces are covered with small sensory cells. They have nothing resembling a brain, and cannot learn. Nonetheless, many sea anemones have a few stereotyped behavioral responses, probably based on a series of simple reflex arcs.


Sexes are generally separate in sea anemones. The typical reproductive pattern is one where both sexes spawn into the sea where fertilization occurs. Within a few days of fertilization, a swimming motile larva called a planula is formed. Depending on the species, the larva may or may not feed. Eventually the larvae will metamorphose into a small sea anemone and take up existence in the benthic environment. Asexual reproduction is common in some anemones, including the bulb tipped anemone, Enatacmaea quadricolor, which may reproduce by splitting into two or more clonal descendants. Such cloning is absent or very rare in other host sea anemones.

Host Anemone’s

There is a considerable diversity of form and biology “hidden” in the common name of “sea anemone.” This is really a group where “one size does not fit all” and, unfortunately, aquarists tend to forget that fact. The several types of Indo-Pacific host anemones are amongst the hardest of anemones to keep. The difficulties of care stem from different reasons in the different species. There are 10 species of anemones that will act as hosts to clownfishes. These are the adhesive anemone, (Cryptodendrum adhaesivum), the bulb-tipped anemone, (Entacmaea quadricolor), the beaded anemone (Heteractis aurora), the Sebae anemone, (Heteractis crispa), the Ritteri anemone (Heteractis magnifica), the malu anemone (Heteractis malu), the long-tentacled anemone (Macrodactyla doreensis), the gigantic carpet anemone (Stichodactyla gigantea), the haddoni or green carpet anemone (Stichodactyla haddoni), and Merten’s carpet anemone (Stichodactyla mertensii). Common names change with the wind direction, and scientific names are applied almost haphazardly by dealers, so don’t trust any identification unless you can verify it yourself.

The long-term survival rate (where long-term means a year or more) in captivity for most of these species is essentially zero. Experienced hobbyists may be able to maintain sebae anemones, long-tentacled anemones, and some of the carpet anemones for extended periods; however, about the only anemone a novice reef keeper stands a chance of keeping alive is a bulb-tipped anemone. Consequently, I will focus on the care of this species.

Basic Physical Requirements

Sea anemones are simple animals; they lack any way to significantly vary or regulate their internal environment. All such animals are dependent upon the water quality of their environment for their well-being. They must have more-or-less perfect coral-reef water conditions. The water must have oceanic salinity of 35 to 36 parts per thousand and temperatures from 27°C to 29°C (about 81°F to about 85°F). The addition of strontium and other similar pollutants should be avoided. Water movement is a necessity; however, the acceptable force, direction, and duration of currents may vary and should be experimented with.

Dietary Requirements

Host, and many other, anemones have zooxanthellae. People tend to believe that animals with zooxanthellae either don’t need to be fed or fed very much. This is definitely NOT the case. Zooxanthellae may provide nutrition to their host, but in turn, they need many chemicals, such as nitrogenous compounds, available only from their host’s digestion of food. Additionally, the anemones acquire significant and necessary mineral and proteinaceous materials from their diets rather than from their zooxanthellae. Host anemones need to be fed, and for good health often need to be fed a LOT.

After good water, the most important thing hobbyists need to provide is the proper diet. Sea anemones are slow-moving predators that wait until they contact their prey contacts them, but then they must kill it rapidly. Sea anemones are active discriminatory predators which maximize the likelihood of predatory success by choosing an appropriate microhabitat to encounter their mobile prey and by having the appropriate toxin to kill it. As you might expect, there is a correlation between the type or behavior of their prey and the morphology of the predator. As an example, Entacmaea quadricolor, projects its relatively large tentacles up off the bottom where they might encounter swimming or planktonic prey. Additional factors in dietary specificity include the nematocyst venoms used to capture the prey and the enzymes necessary for digestion. These are metabolically expensive. Natural selection acts to eliminate the production of enzymes not specifically necessary for capture and digestion of a predator’s normal prey. As a consequence, anemones that eat fish in nature may not be able to eat shrimp in an aquarium, and may not even perceive of shrimp as food.

The appropriate diets for individual anemones should be determined by systematic testing of various potential foods, such as diced fish or whole shrimp. While it is broadly possible to predict what the diet will be differences within subpopulations may lead to specialization for alternative prey. Fortunately, Entacmaea quadricolor appears to be a bit of a dietary generalist, thriving on fish and crustacean prey, thus many foods are acceptable to it. When feeding, the whole prey item should be fed. Predators get important and essential nutrition from all parts of their prey including the guts and internal organs.

Once an acceptable food has been determined, the appropriate volume and schedule must be determined. The aquarist should gradually increase the amount of food to find out how much will be taken at one time. With only one opening to the gut, anything that is undigested has to come out the mouth, too. The time to regurgitation of undigested material; if any is regurgitated, should be eaten. Then feed again on the day following the regurgitation. If no obvious expelling of food residue occurs, feed again after two or three days. Probably as minimum, the animals will need to be fed two or three times a week to remain healthy.

Habitat Requirements

Being zooxanthellate, bulb-tipped anemones need light, but as long as they are well fed they don’t need exceptionally bright light and will do well in a moderately-lit aquarium. Much more critical is providing an acceptable substrate. The behavior responsible for habitat selection is hard-wired into these animals, and they have little plasticity in such behavior. Simply put, they do not have a nervous system capable of learning to live in a new type of environment. Without the appropriate habitat, the animal will be stressed and prone to move to find an appropriate habitat. A moving anemone is an unhappy anemone, and will eventually die unless it can find a place that it can call home. Entacmaea quadricolor are naturally found in crevices or holes, with only their tentacle crowns or oral disks visible. An acceptable home for this species is a small depression that has rocks on all sides where the animal can contact them with its sides. With this species, the sides of the animal appear need to have contact with rocks, and probably calcareous rocks, for the animal to remain immobile.

Figure 5. Entacmaea in a crevice in a home aquarium. Such a habitat seems to satisfy the animal’s need for a hole or “crack in the rocks” within which it will live.

One might be excused for thinking it would be possible to simply train the animals to live in a different place, or to possibly accept alternative foods. Only to small extent is this true; it is unlikely an aquarist can provide the same diet the animal would have in nature. If the animal is to survive, it will need an acceptable, if not identical, food. However, the aquarist had better do their best to come as close to it as they can or the animal will not get sufficient nutrition. Habitat variations seem to be a lot less acceptable. For the anemone’s good health, aquarists must attempt to recreate the appropriate habitat, and must provide an appropriate food. Such is the price you must pay for a happy sea anemone. Good habitat and good food will result in well-maintained animals. In turn this commonly results in asexually produced clonal individuals. br />

The basic principle of attempting to duplicate or provide a habitat that approximates the natural environment while providing a natural diet is fundamental to all good captive animal husbandry and must be practiced for successful maintenance of most of the animals in the marine aquarium hobby.

Ronald L. Shimek, Ph. D.

References Cited:

Bumann, D. 1995. Localization of digestion activities in the sea anemone Haliplanella luciae. Biological Bulletin. 189: 236-237.

Dunn, D. F. 1981. The clownfish sea anemones: Stichodactylidae (Coelenterata: Actinaria) and other sea anemones symbiotic with pomacentrid fishes. Transactions of the American Philosophical Society. 71: 1-115.

Fautin, D. G. and G. R. Allen. 1992. Field guide to Anemonefishes and their host sea anemones. Western Australian Museum, Perth, Australia. 160 pp.

Gosselin, L. A. and P-Y. Qian. 1997. Juvenile mortality in benthic marine invertebrates. Marine Ecology Progress Series. 146: 265-282.

Hesslinger, D. A. and H. M. Lenhoff. (Eds.): 1988. The biology of nematocysts. Academic Press. San Diego. 600 pp.

Koehl, M. A. R. 1976. Mechanical design in sea anemones. pp.23-31. In: Mackie, G. O. (ed.) 1976. Coelenterate ecology and biology. Plenum. New York. 774pp.

Ruppert, E. E, R. S. Fox, and R. D. Barnes. 2003. Invertebrate Zoology, A Functional Evolutionary Approach. 7th Ed. Brooks/Cole-Thomson Learning. Belmont, CA. xvii +963 pp.+ I1-I26pp

Russell, T. J. and G. M. Watson. 1995. Evidence for intracellular stores of calcium ions involved in regulating nematocyst discharge. Journal of Experimental Zoology 273: 175-185.

Thorington, G. U. and D.A. Hessinger. 1996. Efferent mechanisms of discharging cnidae: I. Measurements of intrinsic adherence of cnidae discharged from tentacles of the sea anemone, Aiptasia pallida. Biological Bulletin. 190: 125-138.