The anemonefish symbiosis: what is known and what is not

Asterisks (*...*) surround words or phrases that are to be italicized.The 26 species of anemonefishes (or clownfishes: *Amphiprion* and *Premnas*) are obligate symbionts of 10 species of sea anemones. The fish are unaffected by actinian nematocysts because of their mucus coating; whether the mucus originates with fish, anemone, or some combination has yet to be resolved. The source of their protection may bear on what happens when a post-larval fish first encounters an actinian. There is conflicting evidence about whether the fish displays acclimation behavior - then or ever. This may differ with species of both fish and anemone. Indeed, not all combinations of species interact identically, so data from one species pair may not be generalizable. An obvious manifestation of such differences is that only some combinations of species exist in nature. This specificity has at least four components: conditioned or innate preferences by the fish (the more motile and shorter-lived partner, and therefore responsible for the pattern), environmental requirements, competition by fish for hosts, and chance. One benefit of the symbiosis to the fish is obvious:its major source of protection is its anemone, which forms the core of its territory. The symbiosis has commonly been regarded as facultative for actinians. However, in at least some species, anemonefish provide protection against predatory fishes (especially chaetodontids) in the short term. Ultimately, their presence may have permitted evolution of the widely flared oral disc incapable of retraction that is characteristic of many host actinians. This unusual anatomy produces an enormous surface area to intercept light, upon which zooxanthellae depend. The intimacy of this three-way symbiosis invites investigation of the degree to which the actinians depend on their algae for fixed carbon and their fish for nitrogen and possibly other nutrients