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Low levels of sialic acid, a type of sugar, in the skin mucus of clownfish prevent stings from anemones. Credit: Nick Hobgood, CC BY-SA 3.0.
One of the most famous symbiotic relationships in the animal kingdom is the clownfish and its anemone. Symbiosis is the close association between two different species where at least one benefits from the arrangement. Scientists have long known the advantages clownfish and anemones offer one another, but one mystery has stumped them for a century: How do clownfish avoid getting stung by the nematocysts, or stinging cells, on an anemone’s tentacles, which it uses to catch its prey?
To understand this mystery, first recall how the species assist each other. The clownfish chases away predators that would feed on the anemone, keeps the anemone clean and free of parasites, circulates the water around the anemone, and provides the anemone with nutrients from its waste. Anemones, meanwhile, provide shelter and a safe home for the clownfish, which can hide from predators among the stinging tentacles. Clownfish also get scraps of prey from the anemone and feed on parasites that would otherwise harm the anemone.
Scientists knew that the mucus on a clownfish’s scales protects the fish from anemone stings, but they didn’t know how—until now. A team of international biologists studied the components of skin mucus from clownfish, anemones, and damselfish. They found that a sugar called sialic acid is present only in very low levels in anemone mucus, adult clownfish mucus, and the mucus of a young damselfish, which can safely live with anemones only as a juvenile. But the adult damselfish and larval clownfish—which can be stung by anemones—have higher levels of sialic acid. The scientists found that sialic acid triggers the nematocysts, so the low levels in the mucus mean the anemone doesn’t sting the clownfish—or itself. Mystery solved at last!
