From the instant life appeared on our planet, species started to multiply, grow and adapt within an interactive system of increasingly complex rules. In the conquest and preservation of its own living space, each species has developed unique foraging and reproductive abilities. Eventually, the pressure exerted by organisms on their environment had an overwhelming impact on the shared resources of several habitats. The situation forced many species to interact more closely and to establish intricate relationships that would help them survive. Sea anemones are no exception to this.
Partnership with zooxanthellae: Similar to corals, sea anemones can be subdivided in two main categories: those that live in association with algae and those that do not. Asymbiotic species will depend exclusively on external sources of food that they must capture themselves from the environment. In contrast, many shallow-water tropical species are reminiscent of hermatypic reef corals in that they have developed a symbiotic association with algae. These species will consume what falls into their tentacles, but they will additionally rely on zooxanthellae to sustain or complement their needs. Zooxanthellae are photosynthetic light-brown algae whose waste products are used by the host as nutrients. Because zooxanthellae require light to carry out photosynthesis, zooxanthellate sea anemones will have requirements similar to mini-reefs.
Partnership with clownfishes: For almost everyone, the first example of symbiosis that comes to mind is that of the weird little fish that lives among the sweeping tentacles of their sea anemone host. Is this relationship commensal (beneficial only to the fish) or mutualistic (beneficial to both)? Some biologists have argued for the former. Since they are poor swimmers, clownfishes (also called anemonefishes) make easy prey if they cannot dive into the safety of the tentacles. That is why clownfishes are obligate symbionts and rarely occur outside sea anemones in the wild.
On the other hand, the host sea anemones do occur alone in the sea, which is why some say they gain less from the partnership. However, it was found that sea anemones (Entacmaea quadricolor) rapidly fall prey to butterflyfishes when clownfishes are removed from them. Thus, that particular species apparently needs protection from its fish associates. Another side to this coin is whether sea anemones consistently do better when they host clownfishes, even though they can survive on their own. Some researchers postulate that the presence of symbiotic fishes has enabled many species of sea anemones to spread and grow larger, hence the term “carpet anemones,” because they do not need to retract to protect themselves anymore.
There are 28 known species of clownfishes, mostly of the genera Amphiprion and Premnas and 10 species of host anemones. How does the clownfish-anemone symbiosis establish itself? The tentacles of sea anemones are dressed with millions of stinging cells called nematocysts, which discharge poisonous harpoons at the slightest touch. How can a clownfish live there? The fish has to acquire a protective mucus coating by repeatedly passing through the stinging tentacles. The acclimation period may last from a few minutes to a few hours. Whether the mucus layer comes from the fish, the anemone or is a mix of both is still subject to speculations. One thing is sure: the fish cannot venture away from its host for too long or else it will have to reacclimate itself all over again. But this is an advantage for the aquarium hobbyist who can purchase clownfishes and anemones separately and let them get to know each other.