The relationship between the sea anemone and clownfish allows the other to flourish through symbiosis.

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Of the over 1,000 anemone species that live in the ocean, only 10 species coexists with the 26 species of tropical clownfish. Within these species, only select pairs of anemone and clownfish are compatible. Together, they are obligatory symbionts, which means that each species is highly dependent on the other for survival. Symbiosis between the two species is achieved in a variety of ways including a mutual protection from predators, an exchange of nutrients, and the clownfish’s tolerance of anemone nematocysts.

In order to live among the anemone, clownfish first and foremost protect themselves from nematocyst strikes. Nematocysts are harpoon-like stingers on the anemone’s tentacles used to capture prey and ward off predators. While most fish try to eat the nutrient-rich tentacles, the possibility of being stung while eating deters the clownfish from nibbling on it. In return, the anemone has evolved to not strike the clownfish.

On the off chance the clownfish is struck, it is protected by a thick mucus layer. The mucus layer is three to four times thicker than other fish, and can be a combination of both anemone and clownfish mucus.The clownfish is born with a mucus layer that is already thicker than average, but as it grows, it can mix its mucus with that of the anemone’s to create a stronger barrier.

In return for a safe and protective home, the clownfish benefits the anemone in several important ways. These include cleaning the anemone, providing nutrients in the form of waste, and scaring away predatory fish such as the butterfly fish.

This summary was contributed by Allie Miller.

Check out this related strategy:
Mucus coat protects from sea anemone: clownfish

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“The clownfish Amphiprion clarkii is able to live unharmed amongst the tentacles of the sea anemone Stichodactyla haddoni. The latter has a powerful stinging response and would be capable of capturing any non-symbiotic fish that entered the tentacles. The presence of clownfish affects the anemone’s behaviour but does not impair its stinging ability, suggesting that a general inhibitory effect mediated by the anemone’s nervous system is not involved. A. clarkii achieves protection from stinging by means of its external mucus layer. This layer appears to be three to four times thicker than that of related fishes that do not inhabit anemones and consists largely of glycoprotein containing neutral polysac­charide. The mucus of A. clarkii remains inert after exposure to extreme denaturing conditions, suggesting that it does not contain specific nematocyte inhibitors or excitatory substances that are masked chemically; its inert nature probably results from a lack of those stimulatory compounds that are present in the mucus of non-symbiotic fishes.” (Lubbock et al. 1980:35)

Journal article
Why are clownfishes not stung by sea anemones?Proc. R. Soc. Lond. B 207, 35-61 (1980)R. Lubbock, David Cecil Smith

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