Urticina piscivora, a large, fish-eating temperate-water anemone from the Pacific Northwest of the US offers clues to the feeding of tropical sea anemones. Photo by Linda Snook/NOAA.

What sea anemones eat and how to keep them nourished in the aquarium

By Ronald L. Shimek, Ph.D.

It seems a simple question: what should I feed my new anemone?

Unfortunately, the diet of tropical sea anemones is not exactly known. But research on sea anemones that thrive in temperate waters may give us clues as to what to feed the new creature in your reef tank.

In the shallow waters off the west coast of North America is the Oregonian Biome—one of the most biologically diverse marine ecosystems on the planet; it is arguably as diverse as some coral reef areas. The region is home to well over 50 species of large sea anemones, including one I believe is the largest known, Urticina columbiana—more massive even than large Indo-Pacific host anemones.

Researchers working in the Oregonian Biome have discovered the feeding preferences for a number of local anemones: Urticina columbiana eats jellyfishes while its cousin, U. piscivora, consumes fish and U. grebelnyi prefers sea urchins and dislodged compound tunicates.

Another large anemone in the area, Cribrinopsis fernaldi, eats swimming scallops, while the sizable Metridium farcimen consumes small plankton; the intertidal species, Anthopleura elegantissima, specializes on settling barnacle larvae in the spring, but also will eat other plankton. Its larger congener, A. xanthogrammica, eats mussels dislodged from the intertidal areas above it.

Most species, with some exceptions including Metridium farcimen, will often eat other things if they can catch something else in the wild or are offered an alternative in the aquarium.

Mouth of a large sea anemone in Lembeh Strait, North Sulawesi.
Image by Larry Tackett.

In contrast, virtually nothing is known about the natural diets of tropical anemones. This may be partially based on two misconceptions: the clear waters of the coral reef must mean that food items are rare or the presence of zooxanthellae in tissue means that food is not necessary. But a major reason for the lack of data is the relative scarcity of anemones.

Many coral reef areas have (or had, depending on whether they have been harvested for the aquarium trade) host anemones which are typically not as abundant nor as diverse as anemones in temperate regions so researchers may have not opted to examine them. Additionally, host anemones are often very large and have an array of symbionts surrounding them which make laboratory and field work difficult. For whatever reason, there has been relatively little work done on them.

Trio of Pink Skunk Anemonefish, Amphiprion perideraion, wallow in the nematocyst-covered feeding tentacles of a large Heteractactis magnifica.
Image by Larry Tackett.

New Window on Tropical Species
The recent use of a small zooxanthellate anemone has opened a new window into tropical sea anemone biology. Scientists have discovered that an anemone that seems to found in many, perhaps all, coral reef aquaria is a species (or perhaps two) of Aiptasia. Called variously A. pallida, or A. pulchella all these familiar animals are actually great research species. They are small to moderately sized, so they can be kept in laboratory situations and harbor no fish or crustacean symbionts like host anemones, so it is not an environmental problem to collect a few hundred thousand. And since they are almost impossible to kill and are extremely fecund are also advantages to the experimentalist. Finally, and much more validly, it appears to be a good physiological model for all tropical anemones. It is likely we will have a lot more anemone information in the future.

Food Basics
Two major, and distinctly different, food types are necessary for sea anemones—“energy foods” and “structural foods”. We know this because ALL cells (animal, plant, algal, bacterial or fungal etc.) require them.  Energy foods are necessary to perform tasks, while structural foods are the raw materials of growth. Both types of foods are processed very differently by cells and are essential for the survival of the organism.

Energy Foods

Energy foods, carbohydrates (starches, sugars) and lipids (fats), in humans and sea anemones are essentially identical. Research has shown that sea anemones have the necessary digestive enzymes to break down both fats and carbohydrates to their final sugar components. Although the most commonly discussed sugar is glucose, or “grape sugar,” there are a lot of other sugars found in any given cell which are generally convertible to glucose. Reactions involving glucose, then, are the standard to discuss.

Sugars have one important special property: they can be made by photosynthesis by plants or plant-like organisms. Using the electromagnetic energy found in sunlight, these organisms can take a few molecules of carbon dioxide and water and, by a process that seems almost magical, bind them together to form a glucose molecule. In the process oxygen molecules are liberated as waste. The important thing to remember is that the light energy used to make the glucose molecule remains in that molecule as the chemical energy necessary to hold the molecule together. Making glucose takes a LOT of energy, so if glucose is later disassembled in some manner, all of that energy is released, and may be recovered and used by the animal to perform a task.

While other metabolic reactions use sugars to make mucus, fats, starches, or, in some animals, a structural material called chitin, the fundamental use for sugar is to provide energy through its breakdown to carbon dioxide and water. When energy is needed, the synthetic process is reversed; sugars are combined with oxygen and, in a complex set of reactions, are broken down to carbon dioxide and water, in the process liberating the stored energy. In the cell, this energy is not released as light. Instead, the solar energy used to bind the glucose together has been transformed, and is chemically “harvested.” It is used to build copies of a special small chemical product which, in turn, can be utilized to release its energy on demand. This process of breaking down sugar to produce energy is referred to as “respiration,” and as oxygen is necessary for the process to go to fruition, the respiration is called “aerobic” respiration. Exactly the same amount of energy and the same types and amounts of byproducts are liberated in the cell as would be liberated in a fire burning the same amount of sugar. Aerobic respiration provides the sea anemone with the energy it uses for everything it does, and is one reason sea anemones need to have a good supply of well aerated water flowing around them.

Structural Foods

Animals also need to acquire and process structural foods, the raw materials used to construct tissues, repair damage, and make gametes. The primary types of structural foods are proteins, or the protein sub-units called amino acids, and minerals, such as calcium carbonate. Although many marine animals can absorb dissolved minerals out of the water, most also obtain required minerals from digestion of prey organisms. Proteins and protein components are primarily obtained by eating and digesting prey. Some, but by no means all, marine animals can also absorb some nitrogen-containing products, often ammonium ions, from the water.

Referring to all foods by these two categories is a bit of a simplification, and additionally, when animals eat, few foods are either purely structural or energy. Nonetheless, the two food categories are fundamentally different and foods of one category cannot be converted to the other. For example, the only chemical elements comprising sugars and fats are carbon, hydrogen and oxygen while the critical chemical element found in proteins is nitrogen. Although many other chemicals may be found in proteins, the essential backbone of every protein molecule is comprised of nitrogen.

For all animals, then, there must be at least two distinct types of food.
With the exception of the plankton-eating species, it appears at first that all of these anemones are eating foods that are almost wholly “structural foods.” But that is not the case. Those animals that are eating mussels and scallops, for example, receive a sizable component of energy foods from the plankton that the mussels and scallops have eaten as well as mucus which can be broken down into sugars and protein. Additionally, all predators will break down any fats in their prey, such as egg yolk in the gonads, directly into energy foods, so they are getting a well balanced diet.
Bleached tentacles of a wild Bubbletip Anemone, Entacmaea quadricolor. Image by Denise Nielsen Tackett.

Practical Aspects Of Sea Anemone Nutrition
As with most animals that are kept in aquaria, sea anemones can tell the aquarist a lot about their care by their behavior and responses to their environment.

Unfortunately, since these are very simply constructed animals, their repertoire of behaviors is small—it can be hard to determine what behavioral responses are to any particular problem. The same response may be given to vastly different problems. In most cases, time is of the essence so it is necessary to quickly deduce and correct any problems before the animal dies.

Unfortunately, the most severe responses, such as refusal to attach or eversion of the gut generally means that the animal is already dying or the conditions in the aquarium are sufficiently bad that the animal will die in a short time. Other responses that indicate improper conditions including inadequate feeding are continual movement, periodic detachment, or retreat into the rockwork.

Newly purchased sea anemones are often stressed, almost to the point of death, by the transport and distribution chain. They often don’t have much in the way of reserves to sustain them through their acclimation to any new situation. However, even animals that have been in an aquarium for some time may be in a precarious state because of inadequacies in their care.

What are the conditions of proper care of the tropical sea anemone? Optimum physical conditions are a temperature of 82°F to 84°F (27°C–29°C) and a salinity is 35 PSU to 36 PSU. Colder temperatures stress the animals as much as warmer ones, and an animal kept at 77°F (25°C) has a metabolic rate that is only about half of optimum and will not be able to metabolize fast enough to correct problems before it is too late. Additionally, the aquarium system’s water has to be well filtered and well aerated, and the animal is properly illuminated.

Placement in the Aquarium

Finally, the animal must have a proper habitat; for example, while Heteractis magnifica lives fully exposed in high current conditions, Entacmaea quadricolor does not. It typically lives with the column buried out of sight in a crevice or cave with only the tentacles or tentacle tips showing. The latter species will not do well in buffeting currents, no matter what other care is given to it. Additionally, overly intense illumination will cause it to retreat into the rockwork where it will perish. Consequently, the aquarist must determine the proper microhabitat for the animal prior to obtaining the critter.

Once the proper conditions are met, proper feeding is the most important factor in the care of these animals, but to date the natural diet has never been satisfactorily determined. Fortunately, the animals are not too particular, and many different foods are acceptable. It is often necessary to try several different foods and see which ones the animals will ingest and which are rejected.

Offer whole or diced fish of marine origin (avoid freshwater fish, such as “feeder” goldfish), whole shrimp, squid or clam meat, and artificial foods, such as pellets or well-thawed cubed frozen foods.

If a food is rejected repeatedly, don’t waste more time on it. Place the food on the oral disk; if necessary, reduce the currents in the tank to ensure the animal has a chance to grab the food. If the food is acceptable, the animal will respond by “capturing it” and taking it in. Feed entire prey animals if possible, not just muscle flesh, the anemone will need and can digest the additional nutrients contained in the viscera and skeletons.

Sand Anemone with commensal shrimp. Note ring of dark sand around the anemone's mouth. Image by Denise Nielsen Tackett.

Target Feeding

Unless the anemone is quite small, it is necessary to specifically feed it to ensure that it eats enough food. Although in nature animals can survive on the particulate matter in the water, it is typically far greater than is ever found in aquaria. Aquarium anemones will certainly benefit from brine shrimp, mysids, or extraneous foods from fish, but such foods in aquaria are generally present in insufficient amounts for anything but a periodic snack.

The amount of food to be fed will vary with the species and the size of the animal but is often quite large. As an example, a mature Haddon's Carpet Anemone,  Stichodactyla haddoni, I once had was fed about one or two heaping tablespoons of diced lance fish or cut-up smelt every other day or so.

Anemone Fecal Patrol

Anemones from time to time rid themselves of excess or indigestible food as feces. Defecated material should be removed as soon as it is noticed. It might be a long time between these events. In the seven or eight years I had maintained that S. haddoni, to the best of my knowledge it defecated only twice although it completely consumed everything offered it. As an example, it captured a 4 inch (10 cm) long Raccoon Butterfly fish that I put in to eat Aiptasia. The entire butterfly fish was eaten; no remains were ever egested, and the animal ate its normal mass of food the next day. Burp!

Fecal material is indigestible food, but it is not waste. If it is not removed it will foul the tank since bacteria and other animals will eat it, but it won’t kill them outright. The actual waste material is ammonium ion, often called ammonia by aquarists. As most aquarists know, true biological wastes such as ammonium ion are highly poisonous.

Typically, the amount of ammonium released by a healthy zooxanthellate anemone is unnoticeable and undetectable without special testing apparatus, even if the animal is well-fed. The host’s zooxanthellae will generally absorb all of the nitrogenous waste and convert it into some algal proteins or byproducts and recycle it back to their host. Feeding a healthy zooxanthellate anemone is effectively a waste free process.

The best measure of sufficient feeding is growth or reproduction of fully mature animals. If the aquarist wants his or her anemone to remain small, it should initially be fed enough to grow slightly until it reaches a size where more food is required for it to grow further. Do not increase the food volume at this point, and the animal, although small, will remain healthy. Care needs to be exercised, however, so the animal doesn’t slowly start to shrink, as this may indicate that a problem, such as competition with some other animal, is interfering with the anemone. In such a case, the food will need to be increased. 

Ronald L. Shimek, Ph.D. is the author of A PocketExpert Guide: Marine Invertebrates (Microcosm/TFH Professional Series, 2004).

Excerpt from the March/April 2010 Issue of CORAL, available here.

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