Clownfish - anemone mutualisms:
In nature, the 28 species of clownfish associate with only10 species of anemones out of the 1000 or so known species of true anemones (true anemones being members of the Anthozoan Order Actinaria. See Overview of Cnidarian Diversity). In the wild, some species of clownfish associate with just one anemone species, others associate with several, and one clownfish species naturally associates with any of the 10 host anemone species. In captivity, clownfish, depending on the species, will sometimes form associations with host anemones they would not normally be found with in nature, or even with anemones that don't naturally serve as hosts for any clownfish species (including Condylactis anemones from the Caribbean).
Though clownfish are always found with host anemones in the wild, they do perfectly well in captivity without them. In clownfish, the instinctual urge to associate with an anemone is strong, however, and sometimes captive clownfish lacking an anemone will adopt other sessile invertebrates as surrogate hosts. Our pair of percula clowns, for example, currently lives in our reef tank where it lacks an anemone host. This pair spends nearly all of its time among the long polyps of a large Xenia elongata soft coral colony on the right side of the reef tank, and every few weeks they lay a clutch of eggs on the rocks near the bases of the Xenia stalks.
Sex change in clownfish
Host anemones in captivity
Most of the ten different clownfish-hosting anemone species are very difficult to maintain long-term in captivity (much more difficult than most species of stony corals). Though some aquarists have had long-term success with a variety of host anemone species, the majority of clownfish hosting anemones in captivity die within a year or so. Indeed, even though the purple Macrodactyla doreensis anemone pictured here is one of the hardier species, we failed with this anemone (for undetermined reasons) after having it for just over a year. This is a shame, since barring accidents, other disasters, or predation, most anemones are essentially immortal; though not all anemones in the wild live to old ages, the ages of individuals of some species in the wild have been estimated at hundreds of years. Clearly, most clownfish-hosting anemones are better left in the wild. Though some species (such as Macrodactyla doreensis) are reported to be quite abundant in the regions where they occur in the wild (and according to some sources not currently in danger of depletion by collectors for the aquarium trade), we should be concerned about overcollection and depletion of wild host anemones in general, for wild clownfish are completely dependent upon their host anemones for protection from predators.
One host anemone species (the bubble-tip anemone, Entacmea quadricolor) has a better track record in captivity, often even reproducing through division in home reef aquaria. This anemone apparently is also abundant over a wide geographic range in the wild and is accepted by many clownfish species. For all of these reasons, this species is considered by many experts to be the best host anemone species to keep in captivity.
Though the idea of setting up a clownfish-anemone mutualism is very appealing to many marine aquarists, beginners should refrain from purchasing host anemones until they are skilled with the maintenance of reef tanks with live corals, and even then, only the hardier species of host anemones should be chosen. Unlike the anemones, however, clownfish are very hardy, typically living and even spawning for many years in captivity (and remember: clownfish do NOT require host anemones in captivity, though various hardy soft corals or long-tentacled stony corals can sometimes serve as surrogates).
Clownfish were among the first reef fish species bred successfully in captivity, and most clownfish species are now commercially available as captive-bred fish. For most clownfish species, there is now no good reason to buy fish captured from the wild. Captive-bred clownfish are generally hardier, more disease free, and in general better adjusted to life in aquaria than are their wild-collected cousins, and when fed properly they are just as colorful. Also, the buying of captive-bred rather than wild-collected clownfish prevents wild host anemones from having their clownfish removed from them (remember that the host anemones depend on their clownfish symbionts for protection from anemone predators). Furthermore, buying captive-bred clownfish supports the breeders who are tackling the often formidable challenges of developing captive-propagation techniques for a variety of other ornamental marine fish species as well.