A mucous ‘mosquito net’ may be the true function of the cocoons some species of fish doze in.

Many fish on coral reefs go to a lot of trouble before they go to bed to encase themselves in a mucous cocoon in which they doze away till dawn.

Exactly why they do this is something of a mystery. In the past there have been some suggestions – repeated as gospel to and by many divers on said reefs – that the cocoons protect fish from hungry eels and other such predators.

Alexandra Grutter, of the University of Queensland in Australia, has another explanation: the makeshift shelters are the underwater equivalent of a mosquito net.

Her team placed Chlorurus sordidus fish in bins with parasitic isopods overnight. They found only 10% of fish in cocoons were parasitized by these critters versus 94.4% of fish who had been teased out of their shelters by the researchers.

Specialized ‘cleaner fish’ that eat parasites are sought out to pick off the annoying ‘marine mosquitos’ during the day, notes Grutter in a statement. “At night, when cleaner fish sleep, mucous cocoons act like ‘mosquito nets’, allowing fish to sleep safely without being constantly bitten, a phenomenon new to science.”

As she notes in her paper in Biology Letters, that the cocoons decrease attacks from parasitic pests does not mean they do not also help prevent predation from larger animals. But the benefit they bring in blocking bites doubtless helps offset the energy that goes into building the protective sheaths, calculated by Grutter at around 2.5% of a fish’s daily energy budget.

Image: A. Grutter