WITHOUT A BACKBONE		BY RONALD SHIMEK Ph.D.

Some Hints About the Diets and Habitats of Host Anemones

RON SHIMEK

A skunk clownfish (Amphiprion perideraion in an anemone tentatively identified as Heteractis crispa. Note the anemone is living in a crevice in the reef, a characteristic habitat of this species.

There is a considerable diversity of form and biology “hidden” in the name of “sea anemone.” This is really a group in which “one size does not fit all,” and, unfortunately, aquarists — and scientists — tend to forget that fact.

Although other species are sometimes seen for sale, there are basically three different types of anemones kept by aquarists: Pacific host anemones, Caribbean Condylactis anemones and various corallimorph anemones, also called “mushroom” polyps. I will concentrate this discussion on some of the Pacific host anemones, as these seem to be among the hardest of anemones for aquarists to keep. The basic lessons about the host anemones are often applicable to the other types as well. Most sea anemones are not particularly difficult to keep if the aquarist is aware of the necessary dietary and habitat requirements.

Basic Physical Requirements

All members of the family Cnidaria are dependent upon the water quality of their environment. For coral reef animals, such as these sea anemones, this means water conditions mimicking those on coral reefs. This is water that has full-strength salinity (of 35 to 36 parts per thousand) and appropriate temperatures, generally from 27 to 29 degrees Celsius (about 81 to 85 degrees Fahrenheit).

Low nitrates are nice, but not essential. Additions of strontium and other similar pollutants are not necessary for anemone health. Water movement appears to be a necessity, but the force, direction and duration of currents vary within each system and should be experimented with.

Dietary Requirements

Probably the most important thing for the hobbyist to do is to provide the proper diet. All sea anemones are predatory, and as such have been subjected to natural selection to “optimize” their physiology. As slow-moving predators, sea anemones have to wait until their prey makes contact with them, and then they must kill it rapidly.

RON SHIMEK

S. mertensii with clowns. Stichodactyla mertensii living in a deep crevice in coral rubble.

The nematocyst venoms that are used to capture or immobilize their various prey and the enzymes necessary for the digestion of these prey items are metabolically expensive. Natural selection acts to eliminate the production of those enzymes not necessary for capture and digestion of the predator’s normal prey. As a consequence, anemones that eat fish in nature may not be able to eat shrimp in an aquarium, and will likely not even perceive shrimp as food.

Consequently, sea anemones are neither indiscriminate predators nor omnivorous, catching and eating whatever falls on them. Rather, they are active, discriminatory predators that 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.

Stichodactylid, or “carpet,” anemones are often found in areas where small bottom-dwelling or larval fish are found, and are thus well designed to catch such “bottom-hopping” prey. Species of Heteractis and Entacmaea quadricolor, or the “bulb-tipped anemone,” project their relatively large tentacles up off the bottom where they might encounter swimming or planktonic prey.

The appropriate diets for individual host anemones should be determined by systematic testing of various potential foods, such as diced fish and whole shrimp, clams or squid. It is broadly possible to predict beforehand what the diet of any given animal will be. However, geographical and habitat differences within subpopulations of a given species may have led to specialization for alternative prey. Stichodactylids tend to be fish predators. However, because hobbyists can’t provide the natural food of reef fishes or reef fish larvae, experimentation must be done to provide an acceptable substitute. The other host anemones will eat other categories of food.

The entire prey item should be fed. Avoid feeding just muscle tissue. Predators get important and essential nutrition from all parts of their prey, including the guts and internal organs. Stichodactyla haddoni, the “saddle carpet anemone,” eats whole fish and almost never regurgitates any remains, indicating total use of the food. Entacmaea quadricolor, on the other hand, appears to thrive on crustacean prey. Large whole krill, about the size of cocktail shrimp, fed regularly will lead to growth and a healthy appearance, and relatively little will be regurgitated here as well.

Many anemones, including the Pacific host anemones, have zooxanthellae. People tend to believe that animals with zooxanthellae don’t need to be fed or fed very much. This is definitely not the case. Zooxanthellae may provide nutrition to a sea anemone, but the zooxanthellae, in turn, need many chemicals, such as nitrogenous compounds, available only from their hosts and which the hosts get only from their food. Additionally, the anemones acquire necessary mineral and proteinaceous materials from their diets, not from zooxanthellae.

Once an acceptable food has been found, the appropriate volume and schedule must be determined. Try gradually increasing the amount of food to find out how much will be taken at one time. Note the time to regurgitation of undigested material, if any is regurgitated. With only one opening to the gut, anything that is undigested has to come out the mouth, too. Then feed again the day following the regurgitation. If you don’t see any obvious expelling of food residue, then feed again after two or three days. Probably, at a minimum, the animals will need to be fed two or three times a week to remain healthy.

Not only do these species need to be fed, but for good health they often need to be fed a lot. As an example, my female S. haddoni eats the equivalent of a tablespoon of fish every other day. Any less food and she starts to shrink. Much more and she grows, and with her oral disk at about 50 centimeters (almost 20 inches) in diameter, I don’t want her to get too much larger. This feeding regimen kept her in good enough condition that she spawned in mid March and April of this year.

Habitat Requirements

The zooxanthellate anemones need light, but not exceptionally bright light. Most will do well in a moderately lit aquarium. Much more critical is providing an acceptable substrate. The natural microhabitat conditions of the host anemones are known (Dunn 1981), and these are illustrated quite well by Fautin and Green (1991) in their book on host anemones and anemone fishes. As an example, Stichodactyla haddoni lives buried in sand, so these animals will do best with at least 15 to 25 centimeters (6 to 10 inches) of sand around their columns.

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. For an anemone that lives buried in the sediments, the contact of sediment particles on the sides of the column is an essential requirement. Put another way, without the nervous stimulation resulting from the contact with sediments, the animal will be stressed and prone to move to try 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.

A good home for S. haddoni is a section of 6- to 10-inch diameter PVC sewer pipe filled with sand. The pipe should be filled with enough fine aragonitic sand to simulate the natural depth of the sandy habitat. The animal can be placed in a hole in the sand and allowed to dig to the bottom of the aquarium. Alternatively, the pipe can be placed on the top of the aquarium’s bottom substrate and the anemone placed in it and allowed to fasten down. Then sand can be added to fill the pipe. Anemones placed with enough sand can, and will, occasionally retract beneath the sand surface. This is normal and is no cause for alarm. The PVC pipe can be camouflaged by piling rocks around it.

Other anemones, such as Stichodactyla mertensii and Entacmaea quadricolor, are found naturally in crevices or holes, with only their tentacle crowns or oral disks visible. An acceptable home for these species is a small depression that has rocks on all sides where the animal can contact them with its sides. With these species, the sides of the animal appear to need to have contact with rocks, and probably calcareous rocks, for the animal to remain immobile.

Fautin and Green (1991) indicate that Heteractis crispa, “the sebae anemone,” and Heteractis malu are found in sediments with only their oral disks exposed, while Heteractis magnifica, the “Ritteri anemone,” are often found fully exposed on reefs. Thus, within this group of three rather closely related species are distinctly different habitat requirements. One should not expect a sebae anemone to behave like a Ritteri anemone.

Similarly, these species may have different dietary preferences. One of the commonly stated rules in ecological science is that two animals with the same dietary or habitat requirements cannot coexist. Heteractis crispa and H. malu overlap to a great extent in their ranges, and tend to occupy similar habitats. It is unlikely they would have the same diets.

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. This is true only to the extent that 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, and the aquarist had better do his or her best to come as close to this as possible or the animal will not get sufficient nutrition. Habitat variations seem to be a lot less acceptable.

Recreating appropriate habitat and food is not particularly difficult, although it may mean that some anemones are not suited for some aquaria. For example, a Stichodactyla haddoni, which requires enough sand substrate to bury into, will not thrive in a bare reef aquarium with little sand. Likewise, that animal will not do well on a continuous diet of shrimp or squid, but will thrive on fish. A Heteractis magnifica, on the other hand, can do well with a bare rock to sit on. Keep in mind, of course, that such an animal may also perceive a hobbyist’s newly purchased piece of expensive coral as nothing more than suitable substrate and decide that it is the best place to live. Such is the price you must pay for a happy sea anemone.

Good habitat and good food will result in well-maintained animals, which may result in reproduction. Some anemones asexually divide by binary fission to form clones. Such animals may also reproduce sexually. Evidence from Fautin and Green (1991) and many aquarists indicates that there are two “types” of Entacmaea quadricolor, one that is primarily clonal and one that is not. These may well be two different species — much work remains to be done before this can be confirmed. Additionally, sometimes even non-clonal anemones will split in response to injury or other odd stimuli.

Other anemones, such as Stichodactyla species, appear to lack the capability to undergo fission and may reproduce only by spawning. Spawning in wild populations of these species has not been documented. The only documented spawning of Stichodactyla has occurred in aquaria and resulted in significant production of gametes, which can result in significant problems for a reef system.

When my Stichodactyla haddoni female first spawned she produced about 200,000 greenish eggs, each about 0.6 millimeter (about 1/50th of an inch) in diameter. Between those and the egg-folicle cells shed immediately after the eggs, the water in that aquarium (a 60-gallon hex) was significantly fouled — it had the appearance of “green milk” within five minutes of the initiation of spawning. Rapid and significant water changes (80 percent of the tank volume) and subsequent mechanical filtration were necessary to prevent loss of the fish and other animals in the system. Either the biological oxygen demand caused by the spawning or chemicals released by the female resulted in disorientation of some of the fish in the tank, and they swam into and were killed and eaten by the anemone. Interestingly enough, the resident pair of Amphiprion clarkii did not seem to be disoriented or otherwise effected by the spawning event.

No spawning of any kind has been documented for any of the other host anemones or for any other individual of S. haddoni. If a hobbyist is successful in observing a spawning event in his or her home aquaria, it can (and should) be documented. The database maintained by The Breeders Registry (P. O. Box 255373, Sacramento, CA 95865-5373, U.S.A.) is an appropriate place for such documentation. Perhaps enough information can be gathered to allow the captive breeding and propagation of these species.

REFERENCES Dunn, D. F. 1981. The clownfish sea anemones: Stichodactylidae (Coelenterata: Actinaria) and other sea anemones symbiotic with pomacentrid fishes. Trans Am Phil Soc 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. Pp. 160. Gosselin, L. A. and P-Y. Qian. 1997. Juvenile mortality in benthic marine invertebrates. Mar Ecol Prog Series 146:265-282. Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. Pp. 866. Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. Pp. 1056.

Most anemones have separate sexes, and embryonic development will not proceed further without sperm from a male. Anemone embryos develop relatively rapidly into a larva called a planula. Such larvae typically don’t feed, and will settle out of the plankton within a few days and metamorphose into small anemones (Kozloff 1990, Ruppert and Barnes 1994).

For well-fed small anemones, growth may be rapid, but don’t count on it. The growth rates of small anemones in general, and of small host anemones in particular, has simply never been measured. For many marine animals, the period just from after larval settlement until sufficient juvenile or subadult size is reached is exceptionally hazardous.

The basic principle of attempting to duplicate or provide a habitat that approximates the natural environment, along with providing a natural diet, will generally work well for all anemones maintained by hobbyists. Indeed, such an underlying principle 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. If you have comments or questions I would welcome them by e-mail at roxielf@sunrise.alpinet.net.