To avoid being constantly bitten by nocturnal predators and get a good night's sleep, fish develop their own 'mosquito nets'.
Alexandra Grutter from University of Queensland Centre for Marine Studies said that while biology textbooks presumed mucous cocoons protected fish from nocturnal predators such as moray eels, no experimental studies had examined their function.
Coral reef fish spin cocoons of mucus before slumbering to keep away biting marine parasites.
"In our study, we exposed coral reef parrotfish with and without cocoons to ectoparasitic gnathiids overnight," Grutter said according to the journal Biology Letters.
Gnathiidae are temporary parasites that, like mosquitoes, feed intermittently on the blood and tissue fluids of fishes, a statement of Centre for Marine Studies said.
"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," 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.
Grutter and her team are believed to be the first to investigate this remarkable occurrence.
"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."