No Nemo: Anemones, Not Parents, Protect Clownfish
By John Roach
for National Geographic News
|June 5, 2003|
In the hit movie Finding Nemo an overprotective clownfish named Marlin searches for his son Nemo who is swiped from the Great Barrier Reef and plopped into a fish tank at a dentist's office in Sydney, Australia.
The true protectors of clownfish in the ocean, however, are not parents but rather prickly, stinging sea anemones that live on reefs.
"There is no bond between a fish and its parents," said Daphne Fautin, an invertebrate zoologist at the University of Kansas in Lawrence and an expert on the relationship between clownfish and sea anemones.
Clownfish, which are also called anemonefish, are relatively small. The largest of them grow not much longer than 5 inches (13 centimeters) and they are prone to being devoured by any larger fish-eating fish such as grouper, said Fautin.
For protection, clownfish seek refuge amongst the tentacles of sea anemones. The tentacles contain harpoon-like stinging capsules called nematocysts that the anemones employ to capture prey and ward off predators.
In a yet-to-be resolved biological mystery, clownfish have mucus on their skin that somehow protects them against the sting of their host anemone. As a result, the clownfish are able to stick near their host which is avoided by most other fish in the sea.
"The clownfish gets protection by hiding sting-free among the tentacles. If you remove the clownfish, large butterfly fishes will eat the anemone," said John Randall, an ichthyologist at the University of Hawaii at Manoa.
Butterfly fish are predators of the sea anemone. In certain areas of the tropics where clownfish, sea anemone, and butterfly fish exist, clownfish scare off butterflyfish from their host anemone. Research has shown that if the clownfish are removed from the anemone, butterfly fish will move in and devour the anemone. So, the protection of the anemone afforded by the clownfish is part of the mutual relationship.
In addition to scaring off predators, some scientists speculate that clownfish waste may serve as a nutrient for the anemones. In certain cases, however, sea anemones appear to derive no apparent benefit from the clownfish, said Fautin.
There are more than 1,000 species of sea anemones found throughout the world's oceans. Only ten of these species share their niche with clownfish, which thrive in the tropical waters of the Indian and Pacific oceans.
Each individual host anemone is home to one group of clownfish, which contain a dominant breeding pair and up to four smaller, subordinate fish. There are 28 known species of clownfish, so more than one species of clownfish may take to any given species of anemone.
When clownfish larvae hatch, they all begin life as males and swim up to the ocean surface where they feed on plankton for a few weeks as they are swept around by the currents. Then the time comes for the maturing clownfish to seek out an anemone home.
In the boldest venture of clownfish life, the young guys swim down to the ocean floor where anemones are found on the reef. Easy and unprotected prey, many of them are munched before they find safety.
Those that do find safety within an anemone community enter it as a tiny post-larva. The fish only gets bigger when larger fish above it die. The biggest fish in the community is always the dominant female.
"The fish in an anemone are like stair steps in size," said Fautin. "When a fish in the anemone disappears all the smaller fish grow rapidly to close the gap in size left. And then there is space at the bottom of the queue for another tiny post-larva to settle."
Since the post-larva shift around on ocean currents for weeks as they feed on plankton, by the time they return to the embrace of an anemone, "there is little chance a settling larva would end up where it was an egg," said Randall.
When the dominant female dies, the dominant male changes sex and becomes the dominant female. The females lay their eggs under or near the anemone and are tended to by the dominant male.
The clownfish are very protective of their community. They chase off clownfish that don't fit into the hierarchy and many scuba divers tell anecdotes of being nipped at if they venture too close.
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 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."
While the sea anemone may offer clownfish protection from predators in the wild, clownfish do not need a protector in home fish tanks. Home aquarium hobbyists are urged to refrain from getting their clownfish a host anemone.
"I oppose the keeping of wild-caught anemone in home aquaria because of the negative impact on the environment collecting them has," said Fautin.
Scientists and marine conservationists also ask that aquarium hobbyists not place wild-caught clownfish in their tanks, but note that most of the clownfish in fish tanks today were bred in captivity. Most anemones, however, are taken from the wild.
Randall notes that things are beginning to change for the propagation of sea anemones in captivity. "I know that some aquarists have trouble with certain anemones proliferating in their aquarium, so at least some anemones reproduce in captivity," he said.
Fautin cautions that these known proliferating anemones are not hosts to clownfish and continues to urge against the use of anemone in home aquariums.
"Only one percent of known anemone species host anemonefishes, so what is true of other anemones may not be true of them," she said.
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