In 1954, the German behavioral scientist (ethologist) Irenäus Eibl-Eibesfeldt dived on a coral reef in the Galápagos Islands. What he discovered there was unbelievable—even to an experienced zoologist. As he was resting by a rock, a large grouper swam up from the dark depths and stopped above the rock. It opened its mouth wide, as if it were yawning, and simultaneously, it spread its gill cover. Then, it remained motionless. Two slender wrasses then swam up, their sides adorned with a striking pattern of stripes. To the researcher’s amazement, these midgets then swam directly into the mouth of the predator. They nibbled at the roof of its mouth and poked between its teeth for food remnants. They then slipped into its gill openings, where they cleaned the walls and the outer skin from tiny crabs before returning to the gullet to clean some more. The grouper suddenly closed its large mouth again—and the observer thought the cleaning team was gone forever. But the grouper reopened its mouth immediately, and the small helpers left their dangerous workplace unscathed. With a shake of its head, the predator fish signaled the end of the cooperation.
Cleaning as if on a conveyor belt
No sooner had the grouper disappeared into the reef, a second one appeared in front of the wrasses, who were bobbing up and down. The cleaning team went back to work again. What Eibl-Eibesfeldt saw on the Galápagos was the alliance of two very different partners. The predator fishes were freed from their annoying parasites, and the wrasses found their nourishment during their cleaning work. Since then, this symbiosis has been observed in many places in the oceans, whereby very specific locations act as ‘beauty salons’ in the reefs. The clientele, who range from sharks to butterfly fish, cover almost the complete diversity of reef inhabitants. “We have observed that the fishes were actually jostling one another to have their turn in these cleaning stations. And the more incompatible they are in other places, the more peacefully they behave here,” said the researcher, summarizing his experiences.
There are also tricksters
We now know of more than fifty types of small fishes that clean larger fishes in the coral reefs. The blue-streak cleaner wrasse, a wrasse from the Indo-Pacific Ocean, appears to be especially efficient. It can deal with up to 300 fish in six hours.
Communication is necessary to ensure that the predator fish does not regard the small helpers as prey. For their part, the wrasses pacify the predator with their dance-like movements and their striped pattern. By remaining motionless with its mouth wide open and its gill covers spread, the larger fish signals its peaceful intentions to the cleaner fish. This mutual understanding beyond the “language barrier” of the individual species is certainly the result of a long learning process in the course of evolution.
However, as so often happens in nature, there are also cheats who disturb this mutual trust. Disguised with the blue-black “cleaning uniform” of the wrasse and swimming up-and-down just like them, a fish called the ‘false cleanerfish’ approaches big fishes. When the client fish obediently opens its mouth and gills for treatment, the venomous dwarf strikes like lightning, tearing skin and fin parts from its surprised victims.
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