What do you think of this fact?
Some people sleep with a mosquito net. Parrotfish make their own, and they make it out of slime.
A sleeping bag from the gills
When night falls on a coral reef, many parrotfish and wrasses tuck themselves in. They release mucus that forms a see-through cocoon around their whole body. According to National Geographic, they build it in under an hour. The slime comes from special glands in the gills, which Live Science describes as roughly the size of a quarter.
This bedtime routine isn’t cheap. Making the cocoon costs about 2.5 percent of the fish’s daily energy. So why bother?
The parasite test
Marine biologist Alexandra Grutter from the University of Queensland in Australia tested one idea in 2010. Her suspect: gnathiid isopods, tiny crustaceans that suck fish blood at night and may also pass on a malaria-like blood parasite.
Grutter worked with bullethead parrotfish from the Great Barrier Reef. From half of the sleeping fish, she carefully removed the cocoon, so gently that none of them woke up. Then she released the parasites into the tanks for 4.5 hours.
The result, published in the journal Biology Letters: about 95 percent of the fish without a cocoon were attacked, but only about 10 percent of the fish in their slime bag. National Geographic compared the cocoon to a mosquito net. What exactly keeps the parasites out is still open. It could be a physical barrier, a chemical one, or the slime might hide the fish’s smell.
Bonus fact: The idea that the cocoon protects against predators is older. In 1959, researchers observed that three mucus-weaving species got eaten by moray eels less often than a species that slept uncovered. But Live Science notes that many cocooned fish still got eaten.
The morning after has its own twist. According to Live Science, Grutter watched a cleaner wrasse peck at its own old cocoons and saw a fish at dawn that seemed to have mucus stuffed in its mouth. Maybe last night’s pajamas are this morning’s breakfast.