The mucus coat of clownfish protects the fish from sea anemone's sting via innate or acquired immunity.

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Clownfish and sea anemones have a complex and mutually beneficial relationship. Clownfish live in and are protected by some species of sea anemone; without this protection, they cannot survive in the wild. Anemone tentacles sting and kill other species of fish, but the clownfish is protected from the anemone’s sting.

It is believed that the clownfish is protected due to a mucus coat on the outside of its skin. Studies have suggested that the clownfish’s protection can be innate, acquired, or both, depending upon the species. It is suggested that some species of clownfish are innately protected from an anemone’s sting before ever coming into contact with the anemone. This is because the mucus coat they produce is sufficient to protect them from the anemone’s sting. Others, however, must acclimate to the host anemone before they can move freely among its tentacles without being stung. They do this by rubbing themselves on the anemone’s tentacles over and over again. Initially, the clownfish are stung by the tentacles, but over time, they appear to be unharmed.

Interestingly, even clownfish that are innately protected exhibit this “acclimation behavior.” After initial contact, they acquire antigens from the anemone they have encountered. It has been proposed that these antigens serve as a type of “chemical camouflage” for the fish. With the acquisition of these antigens, the anemone is no longer able to distinguish between itself and the clownfish. The result is that the anenome no longer reacts to the fish by stinging.

This summary was contributed by Alexis Dean.

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Intricate relationship allows the other to flourish: sea anemone and clownfish

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“The mystery behind the clownfish and sea anemone relationship is how the clownfish avoids being stung and killed by its host anemone. Of the numerous theories that have been presented over the years to explain this relationship, the focus is now on a layer of mucus that coats the clownfish. ‘The fish are not immune to being stung,’ said [Daphne] Fautin. ‘But their mucus coat protects them. The debate is the source of the mucus.’ One theory holds that the fish produce the mucus themselves and that it contains chemicals that prevent the anemone nematocysts from stinging as they do other fish in the sea. The other theory is that the clownfish rub themselves against the anemone tentacles in elaborate dances, smearing anemone mucus over themselves. This coating tricks the anemone into confusing the fish for itself. ‘There is evidence for both,’ said Fautin. ‘And since there is a wide variety of anemone hosts, and 28 species of fish, I am convinced these views present two ends of a spectrum, and a combination is probably true for many.'” (Roach 2003)

No Nemo: Anemones, Not Parents, Protect Clownfish

Journal article
The anemonefish symbiosis: What is known and what is not

“This study indicates that naive anemonefishes [also known as clownfish] are innately protected from certain anemone species… Even though most of the naive anemonefishes examined in the present study were protected from anemones during the initial encounter, the fish still displayed the acclimation behaviors described by previous workers (e.g., Davenport and Norris, 1958; Mariscal, 1971; Schlichter,1976). These appear to be stereotyped behaviors that the fishes invariably exhibit upon initial contact with an anemone, whether or not the fishes are already protected.” (Elliot 1997: 287)

Journal article
Acclimation or Innate Protection of Anemonefishes from Sea Anemones?CopeiaApril 26, 2006
Joel K. Elliott, Richard N. Mariscal

“Anemonefishes are known to have a protective mucous coat that allows them to contact the tentacles of their host anemone without being stung. There are two conflicting hypotheses as to the source and biochemical properties of this mucous coating. One hypothesis proposes that anemonefishes acquire anemone substances from their hosts during the behavioral process of acclimation, that protect the fish from being stung. Anemonefishes are considered to use anemone mucus as “chemical camouflage” or “macromolecular mimicry” to avoid recognition as “not-self” by the anemone, and possible subsequent stinging. Another hypothesis is that anemonefishes produce their own protective mucus coat, which lacks substances that elicit cnida (nematocyst and spirocyst) discharge by their hosts…ELISA (enzyme-linked immunosorbent assay) tests showed that anemone mucus antigens were present in the mucous coating of associated anemonefish, but not naive fish. This showed that an innately protected A. clarkii does not produce a mucus coat that is biochemically similar to that of anemones, but that the same fish does acquire anemone substances in its mucus coat when it associates with anemones in aquaria.” (Elliot 1994)

Journal article
Do anemonefishes use molecular mimicry to avoid being stung by host anemones?Journal of Experimental Marine Biology and EcologyAugust 7, 2003
J.K. Elliott, R.N. Mariscal, K.H. Roux

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