Like humans, fish also sleep putting on 'mosquito nets', say scientists.
An international team, led by Alexandra Grutter of the University of Queensland, has found that fish have developed mucous cocoons, akin to a mosquito net, to avoid being bitten by parasites in their sleep, the Biology Letters reported.
According to the scientists, while most fish guide books and biology textbooks presumed mucous cocoons protected fish from nocturnal predators like moray eels, no experimental studies had examined their function.
"In our study, we exposed coral reef parrotfish with and without cocoons to ectoparasitic gnathiids overnight. Fish without mucous cocoons were attacked more by gnathiids, than the fish with cocoons.
"Fish that spent their time building the cocoons before tucking in to bed at night were protected, much like humans putting on a mosquito net," Dr Grutter said.
Fish sleeping soundly in mucous cocoons is a sight that has long fascinated recreational scuba divers and is often a main attraction on night dives.
Dr Grutter and her team are believed to be the first to investigate this remarkable occurrence.
"Fish seek cleaner fish to remove these 'marine mosquitoes' during the day. 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," she said.
Their findings showed that cocoons protect fish from the parasites that bite like mosquitoes.