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Mucus Cocoons Protect Fish from Parasites

Just as some peoplemust sleep with mosquito nets to ward off the bloodsuckers, coral reef fishapparently spin cocoons of mucus before slumbering to keep away bitingparasites, scientists find.

Coral reef fishessuch asand wrasses sleep soundly in mucus cocoonsthat have long fascinated recreational scuba divers and are often a mainattraction on night dives. Surprisingly, until now, no experimental studies hadexamined their function. []

Past observationalstudies have suggested these gooey blankets, which are secreted from thefishes' mouths, somewhat protect fish from nocturnal predators such as morayeels. However, researchers also found many cocooned fish were still eaten.

Fishy mosquito nets

To find out whythese cocoons are created, scientists concentrated on parasitic crustaceansknown as gnathiid isopods (tiny shrimp-like creatures), which feed on the bloodof fish. During the day, parrotfish seek outto remove these "marinemosquitoes," explained researcher Alexandra Grutter, a marine biologist atthe University of Queensland in Australia.

For the study,researchers focused on the bulletheadparrotfish (Chlorurus sordidus). Each parrotfish has to be caughtseparately by a scuba diver using a large 6-foot-by-45-foot (1.8 meter by 13.7meter) net; swimming underwater with a rolled-up net takes stamina, Gruttersaid. Also, "parrotfish are extremely slippery so getting them from a handnet to a catch bag is very tricky," Grutter added. "Parrotfish alsohave a mouth shaped like a beak, hence their name, so you want to avoidsticking your finger in their mouth when putting them in a bag."

Back at the lab, theresearchers compared fish coated with mucus with ones the researchers gentlypushed out of their cocoons without disturbing their snooze. The slumberingfish were then exposed to the parasites for 4.5 hours.

The scientists foundthat nearly 95 percent of the fish without the cocoons were attacked, comparedwith just 10 percent of those coated in mucus.

"At night, whencleaner fish sleep, mucus cocoons act like 'mosquito nets,' allowing fish tosleep safely without being constantly bitten, a phenomenon new toscience," Grutter said.

Eating mucus

The researchersestimated crafting these cocoons takes up 2.5 percent of the fish's dailyenergy.

"The amount ofeffort that goes into building these cocoons, which requires fish to havedeveloped very large glands about the size of [a] quarter to produce thecocoons, is extraordinary," Grutter said. "Parasites must exert anenormous pressure on these fish in order for the fish to have evolved such aspecific way of avoiding the parasites."

The parasites mightnot simply only wound the fish. "These parasites have been implicated intransmitting a blood parasite, which resembles malaria," Grutterexplained.

It remains uncertainexactly how the cocoons protect the fish. "Are they a physical and/orchemical barrier, or do the cocoons prevent the parasites from detecting fishby blocking ?" Grutter asked. "What other organisms aredeterred by the cocoons? Research in the wild would be interesting, but this isinherently dangerous due to the dangers of working underwater in the dark andother nocturnal predators of humans, such as sharks."

It is also an openquestion as to whether the fish can recoup part of their investment by eatingthe cocoons.

"I haveobserved on occasion a fish at dawn with what appeared to be mucus stuffed inits mouth," Grutter told LiveScience. "I have observed another fishthat produces cocoons, the cleaner fish Labroides dimidiatus, peckingaway at its old cocoons in the morning."

The scientistsdetailed their findings Nov. 17 in the journal Biology Letters.

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