Friends and Anemones

Why do sea anemones host and protect certain marine creatures but deliver poisonous stings to others?

• Kathryn Winiarski
• May 01, 1997

Ask a scientist about the relationship between sea anemones and anemonefish, and there's no way around it: The answer will involve mucus. Yes, mucus--the same type of viscous substance we humans depend on, whether we're aware of it or not, for the way it moistens and protects our membranes. In the sea, the slippery stuff is common; many marine organisms coat themselves with it. But between anemones and anemonefish, mucus seems to send a message, deterring the mostly stationary hosts from attacking their more mobile guests.

The anemone's beguiling beauty hides weaponry that fires stingers at almost any organism--from worms to nudibranchs--that touch its tentacles. The ammunition can paralyze would-be predators. Sometimes the anemone makes a meal of its victims. But a chosen few creatures such as anemonefish, also often called clownfish, nestle within the tentacles without being harmed by their toxin-filled capsules, called nematocysts. The key to that ability lies in the fish's protective coating, the source of which has turned out to be a complex and sometimes baffling scientific puzzle.

Anemonefish are not the only creatures to reside unharmed within the killers' reach. Dozens of other fish species are found near the protective tentacles--though sometimes only in early stages of development. And spider crabs and shrimp live hidden from predators beneath the blanketlike folds of the anemone's base. Many crabs even carry around small sea anemones as defensive weapons. How do such creatures survive their relationship with the deadly anemones? To find the answer, researchers have focused on anemonefish, which are among anemones' most brilliant and colorful guests.

Scientists are pursuing a few theories about the anemonefish's mucus shield--ranging from the idea that the fish must acclimate to their hosts, to the notion that the fish naturally develop their protection without contacting anemones. Each theory has supporters, and each may be true, depending upon the species of fish and anemone in question. "You cannot say something general about anemonefish," says biologist Joel Elliott of Queen's University in Ontario, Canada. "That's been a problem with studies conducted in the past: making broad generalizations from a small data set."

Anemonefish depend on anemones for survival. The fish dive within the hosts' tentacles when threatened and seldom venture a great distance from that refuge. The fish's mucus is thought to be such an effective defense because it is especially thick and lacks substances that trigger nematocysts. In other words, the fish don't elicit stings from their hosts.

The anemone is an animal that looks more like a plant, with tentacles extending from the creature's velvet-soft base in many branchlike projections that wave rhythmically in ocean currents. Living among the tentacles are algae that use photosynthesis to create sugar and feed themselves, and the anemone relies on the leftovers as its major source of energy. The anemones eat more than greens' leftovers, however. The animals also have a taste for fish and other marine organisms. Scientists have found remains of small fish, sea urchins, shrimp and crabs within anemones.

Only 10 of the approximately 800 anemone species in the world's oceans are known to host anemonefish, but those 10 species apparently receive many benefits from their guests, such as the removal of parasites. Researchers report that the fish may also feed the anemone, either via its waste products or by literally fetching food.

Most important, the fish are extremely protective. "If anemonefish are not present, anemones can be killed or badly damaged by predators," comments biologist Elliott, who has studied the fish extensively in Australia's Great Barrier Reef and in Papua New Guinea. Butterfly fish--which are unaffected by the anemone's stings--have been spotted ravenously eating anemones after anemonefish were removed, and turtles have been seen eating tentacle flesh. Most of the 28 species of anemonefish are found in tropical regions of the Indian and Pacific Oceans. In U.S. waters, the fish have been found in Hawaii and the Pacific Northwest.

Revelations about the anemonefish's protective mucus may hold important medicinal promise. Scientists are already testing mucus from other marine organisms for human applications. Corals and sponges, for example, may offer substances for anti-tumor medication, now being tested in clinical trials. Frogs may produce an antibiotic; the sea anemone Rhodactis howesii contains an anticoagulant; and the Moses Sole fish has yielded a shark repellent that disrupts the big predator's gill function.

One possible clue for the source of the anemonefish's protective coating comes from a process scientists call acclimation: The fish gently nestles among the anemone's tentacles, apparently enduring nonlethal stings in the process. Some scientists theorize that during the ritual, the fish bathes and coats itself in the anemone's mucus. The idea is that the fish then sneaks into the anemone undetected. Research by zoologist Dietrich Schlichter of the University of Cologne in Germany indicates that in the end, the fish and the anemone are chemically identical on the exterior.

Another theory comes from biologists William Brooks of Florida Atlantic University and Richard Mariscal of Florida State University, who propose that the anemonefish's own mucus protects it, altering as the need arises.

To test the idea, in 1984 the two researchers glued cut-up rubber bands to a petri dish to make a fake anemone. Then the team put one group of anemonefish in a tank with the fake anemone and another group of the same species of fish in a tank with the real anemone. Which fish changed its mucus secretions more quickly? The fish that had lived with the rubber imposter. For the researchers, that was proof the anemonefish, not an anemone, was the source of the protective mucus. Left unanswered for now is the question of whether the mucus would shield the fish from a real anemone.

The third theory is that the answer to the fish's protective mucus lies in the creature's genes: Maybe the fish's coating naturally wards off anemone stings. In 1995, Elliott and Mariscal exposed 30 hatchery-raised anemonefish, one by one, to anemones in a tank. In certain combinations of anemone and fish species, none of the anemones stung the fish. But in other combinations, the fish were stung. "It's a protection that's produced genetically," Elliott concludes. "We've shown the anemone doesn't have to be involved at all." Well, in some cases, anyway. In all, about half the fish were stung. He adds, "In some species combinations the fish are innately protected, and in other combinations they are not and may have to go through acclimation behavior to become protected."

Adds Mariscal, "What is not known is why some species seem to be innately protected and others do not. It seems like the more we learn, the more confused we get."

New York science writer Kathryn Winiarski, an avid diver, reports that for humans, an anemone's sting feels something like touching the sticky side of duct tape .