Their creations may not be pretty, but certain fish have developed what could be the world's most effective and ecologically sound parasite deterrent: "mosquito nets" constructed out of fish mucus.
The sight of fish sleeping soundly while surrounded by mucus has long fascinated scuba divers and scientists, but no one until now knew its function.
A new study, published in the latest Royal Society Biology Letters, demonstrates that the parrotfish and wrasse slimy nets, like human-made mosquito nets, prevent pesky parasites from biting the fish.
"Gnathiid isopods bite fish and suck blood, and they do so all day long and all night," lead author Alexandra Grutter told Discovery News. "Fish, unlike humans, do not have limbs to swat at them, so during the day they go to cleaner fish and shrimp who remove and eat them. But at night, when they are asleep, they cannot escape them."
That's when the fish build their mucous nets, according to Grutter, a coral reef ecologist at the University of Queensland, and her team.
The researchers made the determination after exposing coral reef parrotfish to the parasites. In some trials the fish had their net-like cocoons made out of mucus. Those with the protective mucus slept like a baby. Those without were like human mosquito magnets, suffering from multiple bites.
"The fish (in their nets) sleep all night until it starts to get light," Grutter said. "They are very sound sleepers, and go to sleep as soon as it gets dark."
"When sleeping in the nets, they remain completely motionless except for their breaths that involve small movements of their gill covers," she added. "They appear to have a small opening in the (mucous) cocoon at the mouth that lets them breathe."
In captivity, the fish may sometimes even pair up to share a cozy mucous net made for two.
Grutter explained that parrotfish and wrasse produce the mucous nets using large glands, about the size of a quarter, situated under the gill cover and behind the gills. Right after the fish go to sleep, "the mucus slowly comes out of the mouth and then moves back around the body until it completely envelops the fish," she said, adding that it "feels like soft Jello and is completely invisible."
It only becomes visible when a material like sand or silt falls on it, so that's how scuba divers are able to see the snoozing fish.
Howard Choat, a professor at James Cook University's School of Marine and Tropical Biology, told Discovery News that he agrees with the new findings. Choat said we now know that the nets prevent parasites from pesting the fish. "Gnathiid isopods constitute a cost to the host species by removal of blood," he added.
The net-constructing fish seem to not be lacking in mucus, as they create their nets each night. The fish can even make a second one if they are disturbed and must abandon the first net.
The scientists aren't exactly sure how the nets ward off the fish blood-sucking parasites. It could be that the nets act as a simple physical barrier, much as mosquito nets used by humans do. They also could function as a chemical barrier, like mosquito nets sprayed with an insecticide.
Alternatively, the nets might block the odor of fish so that 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!"