There are night time pests in the reef!! The gnathiidae are a family of isopod crustaceans whose juveniles feed on the blood of fish.
Some species of parrotfish and wrasses have developed a unique method of getting a good night's sleep while staying safe from blood sucking parasites: They make a cocoon to sleep in, out of their own mucus produced by an organ in their heads, like a mosquito net.
During the day, cleaner fish remove these pests but at night the cleaners as well and the parrotfish are left vulnerable to the bloodsuckers. So they create their nets each night. The fish can even make a second one if they are disturbed and must abandon the first cocoon.
Past observational studies have suggested these gooey blankets, which are secreted from the fishes' mouths, somewhat protect fish from nocturnal predators such as moray eels. However, researchers also found many cocooned fish were still eaten.
Researchers from Queensland University had a different theory: the mucous cocoons protect against gnathiids, in order to test the novel hypothesis that, several fish were exposed with and without cocoons to gnathiids overnight and measured the energetic content of cocoons. The scientists found that nearly 95 percent of the fish without the cocoons were attacked, compared with just 10 percent of those coated in mucus.
The researchers also estimated crafting these cocoons takes up 2.5 percent of the fish's daily energy. "The amount of effort that goes into building these cocoons, which requires fish to have developed very large glands about the size of [a] quarter to produce the cocoons, is extraordinary, Dr. Alexandra Grutter from Queensland University said. "Parasites must exert an enormous pressure on these fish in order for the fish to have evolved such a specific way of avoiding the parasites."
The isopods might not simply only wound the fish. "These parasites have been implicated in transmitting a blood parasite, which resembles malaria," Grutter explained.
However, the scientists still don’t know exactly how the nets ward off the fish blood-sucking parasites. The nets act as a simple physical barrier as a mosquito net or they act as a chemical barrier by blocking the fish odor so the parasites cannot smell and find their blood meal targets.
What is certain, Grutter said, is that the nets constructed out of mucus "are a natural, biodegradable and energetically efficient way of warding off pests. What an ecologically friendly solution!"
Join us during the night dives and maybe you can see one of those amazing fish sleeping inside the cocoon.