ONE of the great nocturnal mysteries of coral reefs has finally been solved - the spectacular mucous cocoons that some fish wrap around themselves like duvets each night are the fish equivalent of mosquito nets.
Parrotfish and wrasses spend an hour each night producing the cocoons from glands inside their mouths. They were thought to protect the fish against predators such as moray eels, but lab experiments on the bullethead parrotfish suggest the cocoons ward off the bites of bloodsucking parasites called gnathiids that spread diseases.
Lexa Grutter and her colleagues at the University of Queensland in Brisbane, Australia, placed 38 bullethead parrotfish into individual tanks. At midnight, they delicately removed the cocoon from half the fish without waking them. Then they added 20 gnathiids to each tank (Biology Letters, DOI: 10.1098/rsbl.2010.0916).
They found that 94 per cent of the unprotected parrotfish were attacked by the parasites, compared with 10 per cent of the cocooned fish. Likewise, between 10 and 25 per cent of the gnathiids recovered from the tanks of unprotected fish were gorged with blood, compared with only 4 per cent from tanks of cocooned fish. "Fish that built cocoons before tucking in to bed at night were protected, much like humans putting on a mosquito net," says Grutter.
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