I wonder what’s true about the so often cited “symbiosis” between anemones and anemone fish. I have a small tank which allows me for only a moderate sized anemone (Entacmaea quadricolor), which I added to the already present pair of anemone fish (Amphiprion ocellaris). The fish were delighted and all the time bumping into the anemone. The anemone slowly but steadily declined and after a year no more then a few centimeters is left from it. Of course anything can have caused its decline, but my hypothesis is that the anemone is simply withering from all the bullying of the fish. I read on several occasions that the fish prefer big anemones, say 20 cm diameter. In that case I would have simply saddled a too small anemone. But this also triggered me to try and find out more about the often quoted symbiosis between the two.
Wiki does mention the removal of parasites and defending against tentacle-eating fishes, which of course can be true. There is also mentioning of the fish luring other fish near the tentacles but I find that far-fetched to be honestly. The other way around, the benefit of the tentacles providing shelter to the anemone-fish seem obvious, though.
Still, I got my case of an anemone not pleased at all by its alleged companions. Just recently its shrunken appearance even moved away to the other side of the stone. The fish found it back all right, but left it more at ease (for the fish already had laid eggs on the former spot, where they of course wanted to stick to).
I am installing a backup tank right now and if this one is stable I like to tranfer the anemone overthere. If it recuperates this adds to my suspicion that the anemone did suffer from the fish instead of profit. If the suffering is only to do with providing not the right species and size of anemone, which might be the case in my tank, I of course rest my case. But within these circumstances we better not talk of symbiosis, but rather parasitism instead.
Like to close with summarizing an article of F. de Graaf in “Het Aquarium”, jr 38, nr 11. 1958, elegantly refuting that the fish might help the anemones by even feeding them their left-overs:
Anemonfish got their name from the fact that when possible they live in and around big sea anemons from geni Discosoma, Radianthus and Stoichactis. Sea anemons themselves are equiped with two types of nettle cells, one type stinging and paralyzing any object that touches the trigger hair on top of the cel, and yet another type ejecting slimy threads sticking to the prey and thus enabling the anemon to “grap” it and conduct it to its mouth via contraction of its tentacles. The species vary in amount of stinging, and some like red nipple anemon are for instance harmless for us to touch. The fact that they are still evaded by bigger fish that do not have to fear from their stinging capacity might be a mimicry effect, caused by look-alike anemons that inflict more painful stinging. This very fact of being evaded is obviously the main reason why anemon fish do seek their company: to receive indirect protection from enemies. They have achieved a method by which they are not harmed by the anemons, and the way they achieve this has long divided researchers into two camps.
The oldest theory was that there is a special relationship between the two: the anemon fish behave in such a way that the anemon refrains from stinging, the fish appears to be recognised. and the anemone refrains from firing its nettles. This assumes the anemons are capable to recognise and remember these signals, which is doubtful looking at their primitive neural layout. But is also assumes there is a benefit for the anemon in the relationship, as in a symbiosis. This is also doubtful, for there are only few observations reported of anemon fish feeding their host. In fact most commonly seen is that the fish steal pieces from the anemons tentacles. The only observations that de Graaf made the other way around was about pieces that were too big for the fish to chew. These were stuffed to the border tentacles of the anemon and subsequently chewed from until nothing left for the anemon again. When the anemon would manage to bring a left-over of such a piece to its own mouth the fish would retrieve it and once more put it on the border and resume chopping pieces from it till it is all eaten. So much for symbiosis. The observations of anemon fish feeding their hosts (e.g. Hackinger, 1959?) may therefore well be misinterpret. The second theory states the fish have a slimy skin containing substances that suppresses the stinging reflex of the anemon’s nettle cells. In favor of this thesis are experiments where the fish their skin was cleaned from its slime: the same fish was immediately stung when re-entering the anemon. This is also how you can mark a sick fish: since they loose capability to produce enough slime they will abandon their host to avoid getting stung. The slime-theory can also be elegantly proven by wrapping this slime onto a sponge. This piece of sponge, normally a welcome prey for the anemon, is now left at easy when put onto its tentacles with this soothing slime. Another observation, where fish are not directly indulging into a first contact with an anemon but instead first time make contact around its feet and swiftly tap a fin to a tentacle, was again feeding the first theory of a build up relationship. It can however also be explained by assuming the level of anti-stinging slime needs to be build up first via contact with the sample of stinging material from the anemon. This build up can be quite swift, sometimes within hours.
Since the anemon does not seem to profit from its fish we should determ this as commensalism rather then symbiosis. Or maybe even parasitism, since in captivity anemons without the company of anemon fish do live longer.