As all of you reading this blog probably knows, anemonefish are associated with host anemones and need such an association to survive. Biologists would call that obligate symbiosis. The obligate part is that the anemonefish need the anemone in order to survive, and the symbiosis is that they live together with the anemone. Maybe not so well known, far from a random occurrence in any species of anemone, most species of anemonefish are limited to one or a couple different anemone species.
The life of a young anemonefish is not easy. They spend the first part of their life as plankton, be it shorter time than most marine fishes. When they finish the planktonic stage, they have to find an anemone that is compatible with them. Not only that, the adult anemonefish that are resident in every anemone, will limit access to the anemone, only letting a new juvenile in if there are “free” spots in the anemone. Excluded fish will almost certainly be dead in very short time, as they have very little survival chance if they need to find another anemone.
Anemonefish can in captivity survive in an anemone that is not of the species they usually associate with. However, they cannot raise juveniles in such anemones. Thus, evolution should select hard against such mistakes, and to see anemonefish in the “wrong” anemone in nature should be extremely rare. However, during my last visit here to Lembeh, I have seen quite a few very small anemonefish in anemone species that will be of no use for the anemonefish with regard to reproduction. The evolutionary pressure for them to find a space before they die being outside an anemone will of course have to be traded against the pressure for being sure that the anemone is of the right species. Thus, it is probably more common than one first would think that anemonefish make such mistakes. Sadly enough, such a wrong choice will definitely cut them out of the gene pool, aspiring to the Darwin award of anemonefish!