Fish create ‘mosquito nets’ to fend off parasites

By Amanda James 19 November 2010
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Tropical parrotfish create mucous cocoons to protect themselves against blood-sucking aquatic parasites.

A BRIGHTLY COLOURED species of fish popular with scuba divers have been found to perform a neat trick – they tuck themselves in at night in mucous ‘nets’ which protect them from blood-sucking parasites.

Parrotfish in mucous cocoons have long-fascinated divers and can be seen throughout the tropical parts of the Indian and western Pacific Oceans. These fish have special glands behind their gills about the size of 10-cent pieces, which produce mucus that they spread over their heads and bodies.

“Fish that spend their time building the cocoons before
tucking into bed at night are protected, much like humans putting up a
mosquito net,” says Dr Alexandra Grutter a coral reef ecologist at the
University of Queensland in Brisbane.

Vampire gnathiids

Experts had assumed that these cocoons protected the fish from nocturnal predators such as moray eels, but a new study, published this week in the journal Biology Letters, turns that idea on its head.

Alexandra and her student Jennifer Rumney report evidence that the fish use the cocoons to guard themselves against aggressive aquatic parasites, protecting against blood-borne diseases in the process.

The laboratory experiment that Alexandra and Jennifer devised to test the cocoon’s function resembled a midnight heist. First, they gave around 40 bullethead parrotfish (Chlorurus sordidus), in separate tanks, time to create their mucous cocoons.

Next, the researchers stealthily removed the cocoons of half of the fish by “gently pushing” them out of their cocoons without waking them, says Alexandra, and then scooped out the mucous. Many fish took the time to build themselves a second cocoon after the first one was removed, even though that takes about 45 minutes.

The second step in the experiment saw the researchers add blood-sucking ‘gnathiid’ aquatic parasites (Gnathia aureusmaculosa) to the tanks. They found that fish still protected by mucous cocoons suffered fewer attacks than those that had been deprived of them.

2.5 per cent of energy intake

“We were surprised by the great lengths that fish in this group go to in order to avoid being attacked by the parasites,” Alexandra told Australian Geographic. “[They] have evolved these huge glands, and every night they make this huge structure to protect themselves.”

The researchers estimate that constructing a cocoon takes around 2.5% of the total energy provided by daily food intake.

“All fish have an epidermis [skin layer] that produces mucus to protect them from bacteria, but the parrotfish has a special adaptation,” comments Dr Robert Adlard, a marine zoologist at the Queensland Museum who was not involved in the study. He says that gnathid parasites present the same kinds of threats to parrotfish as malaria-carrying mosquitoes do to humans.

Alexandra says that the reason the fish only require protection at night is because during the day cleaner fish help rid them of parasites. Each cleaner fish can eat around 1,200 parasites a day. “They’re just constantly being attacked,” she says.

The next step for the researchers is to study how the protective shield works – whether it makes the odour of the fish undetectable to parasites, or if it is a chemical or physical barrier that deters them.

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