By Bob Goemans
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Q&A - Clownfish-Anemone Relationship

Authored by: Rob Toonen


I read your article on anemones in the last issue of TFH with great interest, and I feel that I learned something from reading your article. However, I disagree with your recommendation to keep clownfish in tanks without an anemone present. I know from diving that clowns live almost exclusively within the tentacles of an anemone in the wild, and your article points out that this association provides benefits to both clownfish and anemones in nature. I believe that this relationship must have developed for a good reason, but you suggest that even people with clownfish should avoid keeping anemones in aquaria. Having watched my clowns with their anemone, I can tell that they are so much more comfortable and happy in the anemone, and I consider it cruel to keep them without their natural partner. They live in anemones exclusively in the wild so why do you think it is OK to rip them from that environment and keep them in a tank without one?


Well Robert, I certainly appreciate feedback, and am glad that you feel that the article was informative enough to teach you something, even if you disagree with it. I must say that I am still pretty negative about the prospect of keeping any of the clownfish-hosting anemone species in an aquarium successfully – although I should clarify that I don’t feel so negative about all anemones, and some other anemone species like the Curly-que anemone, Bartholomea, appear to thrive in the conditions of most reef, and even some fish only, aquaria.

In terms of the problems with host anemones, I still think that having only 5% of reef aquarists with more than five years of experience being able to keep one alive for any reasonable length of time is a very good reason to advise against their purchase. Despite the attraction of housing clownfish with a host anemone, I have to stand by my advise to avoid the clownfish hosting anemones in an aquarium unless you establish an aquarium specifically around their unique needs (an “anemonarium” as one friend calls it). Reports of success with difficult animals such as anemones is increasing as we learn more about their biology and specific needs in captivity, but until success is widespread, I do not recommend that they be purchased by anyone but an experienced aquarist with a tank designed specifically for housing a clownfish hosting anemone.

I think that the need for clowns to be kept with an anemone that you outline is a common misconception that leads to the unnecessary death of a lot of anemones. Clownfish accept many surrogates for a host anemone in the aquarium, and some of the best choices for a reef aquarium include hardy corals such as hairy mushrooms. Despite the propensity for clowns to find a host of some sort in aquaria, the idea that it is cruel to maintain clownfish without a host anemone is a perception of the aquarist, not the fish. This type of perception is called anthropomorphosis (the projection of human emotions, such as happiness, onto observed behaviors) among animal behaviorists, and it has lead to a great deal of misdirected research and effort over the years. It is now considered one of the cardinal sins of animal behavior research as a result, because it is almost always misleading. While it is true that it makes us happy to watch the clowns bathe in the tentacles of the anemone, it turns out to be better for the clown in many cases to be kept without an anemone, than to provide them with one when kept in captivity. Yes, you read that right - it can actually be better for both the clown and its host anemone under some circumstances to be deprived of their natural association in an ideal captive environment.

Let me start by explaining how the clownfish / anemone interaction works. The ability of clownfishes to contact anemones results from an odd mixture of innate defenses (which vary by species), behavioral adaptation and developmental stage. The eggs of all species of clownfishes were not stung by any species of host anemone, but the larvae of all species were captured and killed by all the host anemones, regardless of species. Newly metamorphosed juvenile clownfish appear to have innate defenses against only the anemones with which they are typically associated in the field (e.g., Premnas, the maroon clown, is protected against only the sting of the bubble-tip anemone Entacmaea, while other species of anemone typically capture and consume the larvae following contact with the tentacles). Only Amphiprion percula (the "true percula" clown) was immune to the stings of all potential host anemones as a juvenile, with most other species being stung by at least one species of anemone (most frequently the carpet anemone Stichodactyla gigantea). Only a few species were able to contact Stichodactyla without being stung and eaten, while all the species of clownfish tested were capable of contacting the tentacles of the bubble-tip anemone E. quadricolor without being stung as soon as they metamorphosed into juveniles (but were captured and eaten as larvae as I mentioned above).

However, clownfish can also acclimate to most any of the host anemone species if given sufficient time and exposure (assuming that they are not captured and eaten at the first contact). This acclimation, known as behavioral adaptation, occurs largely through the accumulation of anemone mucus through repeated contact and complexing compounds in the water in close proximity to the anemone but also appears to involve some biochemical changes by the clownfish to adapt their mucus to match that of the host anemone. This behavioral adaptation requires a substantial effort of the clown to maintain the coating of anemone mucus on their body, and much of the "bathing" behavior of clowns appears to be a mechanism of mucus collection, because innately protected juveniles do not appear to spend as much time performing this behavior. Furthermore, in tanks the "rules" of host preferences and associations often seem to break down. It is not uncommon for a pair of clowns to spend all their time in a long-tentacled plate coral, hairy mushroom or some other such "inappropriate host" even when there is an anemone of the "correct" species nearby. Juvenile clowns tend to spend more time performing the bathing behavior when found in hosts to which innate protection is lacking. It is unclear whether innate protection lasts into adulthood, however, because virtually all adult clownfish perform bathing behaviors regardless of the host species, and even for species in which the juveniles have innate protection and at first introduction swim directly into the tentacles of the anemone, adults usually still go through a period of acclimation in which they repeatedly contact the chosen host and slowly migrate into spending more and more time among the tentacles of the animal. If your ultimate goal is watching the clowns bathe among the tentacles of their host, you may actually want to encourage them to accept some host other than their natural one – many aquarists have reported that bathing behavior is more commonly displayed by clowns in the “wrong” host species.

Determining whether a given association is a positive or negative one involves looking at the benefits (if any) each partner derives from the relationship, and discounting those benefits by the associated costs each partner pays to maintain the association. The reason that the clown may actually be better off without a host in the aquarium is that the bathing and acclimation behavior are quite energetically expensive and potentially stressful for both the clown and the anemone (the clowns are stung repeatedly during initial acclimation and may actually rip tentacles off the anemone when escaping initial contacts), and the continual expenditure of energy to maintain the symbiosis by both partners is a cost. In experiments in which animals are kept with and without hosts in similar conditions it has been found that removal of the cost of maintaining the association frequently leads to increased health, growth or reproductive output of the animals (depending on the specific conditions and experiment). In this case both the participants, the clownfish and the anemone, expend significant energy in maintaining the relationship, and when the costs and benefits are added together for life in an aquarium, they are almost always weighted towards the negative with this particular symbiosis.

These animals need one another in the wild, because both partners gain protection predators through their association with one another. In many regions the removal of either the clowns or the anemone results in almost immediate predation on the other. Both the anemone and the clownfish gain far more from the association than it costs to be without it in the wild -- both gain protection from predators by their association and that benefit far outweighs the energetic cost of maintaining the association for either participant. Considering that the aquarium is a relatively safe haven for clownfish (because few people keep clowns with predatory fish that are likely to eat the clowns without a protective anemone), there is little biological reason to provide them with that protection in the aquarium.

Add to our overestimation of the importance of this association the dismal success most hobbyists have with anemones in aquaria (this was the focus of my original anemone article to which you refer in your question), and I feel that it becomes difficult to argue that there is any good biological reason to maintain an anemone host with a clownfish. I certainly appreciate the aesthetical one of us as aquarists feeling good to watch the clowns in the anemone, however -- I must admit that it was one of the reasons that I first got into keeping marine tanks, too. However, I think that it is important for people to realize that it is for our benefit and not that of the clownfish or anemone that this association be maintained in captivity. There is no basis to claim that it is in any way "cruel" to maintain one species without it's symbiont in this case, and in fact research would suggest that, if anything, they are no different, or possibly even better off without one another when maintained in predator-free aquaria. Once you come to the realization that it is purely for our personal impression of the animal "happiness" that a host be provided for either clowns or anemones in captivity, I think you are in a much more objective and realistic place to make a decision about whether or not your aquarium design and skill are sufficient to attempt to keep a clownfish and host anemone. I keep a pair of clowns with an anemone in my tank, and after 5 years of being together I am still enthralled by them every time I watch them, but I have no misconceptions that the association is of primary benefit to me rather than them. Consider that a similar pattern of association has been found for a herbivorous crab (Mithrax) associate of corals in the wild, but no one argues that it is cruel to the crabs to maintain them in an aquarium without their coral symbionts...


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