Researchers had speculated that the reason certain parrot fish and wrasses envelop themselves each night with a big blob of mucus might be to protect against settling silt or to deter hungry predators such as moray eels. But definitive experiments were lacking. Now scientists from the University of Queensland in Australia have done the dirty work. The team placed parrot fish in plastic tubs and after midnight, when all the fish had made their mucus cocoons, the researchers gently scraped off the cocoons from half the fishes. Then the team introduced tiny parasitic isopods — blood-sucking crustaceans that are taxonomically closer to lice than to mosquitoes — into the tubs.

Tallying each fish’s blood-engorged parasites showed that the mucus acts as a slimy sea version of bug netting: 94 percent of fish without cocoons had bites, versus 10 percent of fish with intact cocoons. The cocoon-challenged fish also had far more bites on average than their counterparts, the researchers report in a paper to appear in Biology Letters.

Making the mucus cocoon, which begins at the fish’s mouth and envelops the entire body within an hour, is an efficient protection strategy, costing a mere 2.5 percent of the fishes’ daily energy budget, the researchers estimate. This is relatively cheap, compared with scraping yourself on rocks or sand, avoiding areas with parasites or seeking parasite-eating cleaner fish (which the fish do during waking hours).

Keeping bugs at bay is a new role for fish mucus — the thinner slime layer employed by some species appears to protect against UV rays and pollutants, or can aiding in maintaining the proper balance of electrolytes and fluids.

Images: 1) Each night the parrot fish Chlorurus sordidus envelops itself in a cocoon of mucus, a slimy nightgown that protects it from biting parasites. Alexandra Grutter 2) The mucus cocoons made by some reef-dwelling fish protect the sleeping animals from being bitten by the louse-like parasite Gnathia aureusmaculosa (a marine isopod). Nico Smit

See Also:

• The Usefulness of Dolphin Snot
• T. Rex of Leeches Found in Amazon Swimmers’ Noses
• Video: How Leaping Fish Species Left the Water — For Good
• Ocean Acidification Gives Young Fish a Death Wish
• Fish See Their Enemies’ Faces in Ultraviolet
• Parasites Could Be Good for You

Authors: Rachel Ehrenberg

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