Why Don't Clownfish Get Stung by Anemones? Mucus Facts

How anemone stinging cells work

Sea anemones are animals, relatives of corals and jellyfish, and like them they hunt with stinging cells. Each tentacle is packed with thousands of tiny capsules called nematocysts. When something touches the tentacle, a nematocyst can shoot out a hollow thread that pierces the prey and injects venom, stunning small fish and shrimp so the anemone can pull them into its mouth. The process is explained in How do sea anemones eat?

Stinging cells do not fire at everything they touch. They usually need two signals at once: physical contact plus a chemical cue, such as certain compounds found in the mucus of fish. This is also why an anemone does not sting itself when its tentacles touch its own body: its own mucus does not carry the right trigger.

The mucus shield

A clownfish is covered in a coat of mucus that behaves like the anemone’s own. Rather than presenting the chemical signals that set off stinging cells, it seems to lack them, so the tentacles treat the fish as if it were part of the anemone.

Researchers have looked at several features of this mucus:

• Missing triggers. Fish mucus normally contains sugar-based compounds, including sialic acids, that help trigger stinging cells. Some studies have found much lower levels of these compounds in clownfish mucus than in other reef fish.
• Chemical camouflage. Clownfish mucus may mimic the molecules of the anemone’s own mucus, so the anemone “recognizes” the fish as itself.
• Thickness. Clownfish mucus has been described as thicker than that of related damselfish, but the evidence that thickness matters is mixed.

The best-studied host is the magnificent sea anemone, a large anemone that hosts about a dozen clownfish species, including the ocellaris clownfish.

The acclimation dance

When a clownfish meets a new anemone, it rarely dives straight in. Instead it swims around the anemone, brushing its belly and fins against the tips of the tentacles, nipping and fanning them, then darting away. In early contacts, the fish may flinch as if lightly stung. Over a period that ranges from minutes to days, the touches get longer, until the fish can bury itself among the tentacles without any reaction.

This behavior was described in detail in the 1950s, when researchers Demorest Davenport and Kenneth Norris studied clownfish and anemones in aquariums in California. They concluded that the protection lay in the clownfish’s mucus. In some experiments, clownfish kept away from their anemone for a while appeared to lose part of their protection and had to acclimate again.

Does the protection come from the fish or the anemone?

This is the part scientists still argue about. There are two main explanations:

1. The fish borrows the anemone’s mucus. During acclimation, the clownfish rubs against the tentacles and picks up a layer of anemone mucus. Coated in it, the fish smells and tastes like the anemone and is not attacked.
2. The fish makes its own protective mucus. In a 1980 study, Robert Lubbock concluded that clownfish produce a special mucus themselves that simply lacks the signals for stinging cells to fire, rather than borrowing the anemone’s.

Today most researchers think both processes may play a part and that the balance probably varies between clownfish species and their hosts. Some clownfish seem protected almost at once, while others need a long acclimation. Recent research has also found that clownfish and their anemones exchange skin bacteria, which may help the fish blend in chemically.

What each partner gets

The clownfish and anemone relationship is a classic case of mutualism, a partnership in which both sides benefit.

The deal is uneven. Clownfish cannot survive long on a reef without a host, but many anemones live without clownfish. Even so, anemones with clownfish tend to do better, and in one study they grew and regenerated faster.

Which anemones host clownfish?

Only 10 of the more than 1,000 known sea anemone species host clownfish, and the 28 or so clownfish species use them in different ways. Clark’s anemonefish is a generalist that uses all ten. Others are specialists: the tomato clownfish and the maroon clownfish live only with the bubble-tip anemone. The ocellaris clownfish uses three hosts: the magnificent sea anemone, the giant carpet anemone and Mertens’ carpet anemone.

Host anemones need sunlit water, because algae living in their tissues provide much of their food, so clownfish are limited to shallow tropical reefs. Where they live is covered in Where do clownfish live?