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A peculiar behaviour in some fish, which has long puzzled divers and scientists, has been explained by new Australian research.

When roundhead parrotfish living in the Great Barrier Reef bed down at night, they spend up to an hour each evening exuding a mucous cocoon around their body.

Now, new research from the University of Queensland, published in the journal Biology Letters, has found these jelly-like cocoons protect the fish from parasite attack.

Previously, researchers believed that the mucous cocoon, which is a solid mass much like a balloon filled with gelatine, protected the fish from predators. The mucous, was thought, to mask the smell of the fish and help it avoid detection.

But Dr Lexa Grutter and colleagues, always wondered how such a transparent, fragile and easily removed structure would protect a fish from voracious predators like moray eels and sharks.
Evading blood sucking parasites

Parrotfish are plagued by gnathiids, a 2 millimetre long parasitic isopod distantly related to the slaters and pill bugs, which inhabit most suburban gardens.

Gnathiids act like aquatic mosquitoes, sucking blood for up to an hour and sometimes transmitting a blood-borne fish disease, similar to malaria.

Grutter knew the green, blue and yellow parrotfish relied heavily on cleaner fish, called wrasse, to pick off their parasites during the day. But she wondered what the fish did at night, and whether the mucous cocoon might have a role to play.

She and her colleagues tested the theory by placing parrotfish in separate tanks. When the fish had spun their cocoon for the night, the cocoons were gently removed from half the fish. Parasitic gnathiids were introduced into all the tanks.

At the end of the night, the researchers found that fish with intact cocoons had far less gnathiid parasites attached to them than unprotected fish.

"Fish that spend their time (up to an hour) building the cocoons before tucking into bed at night were protected, much like humans putting on a mosquito net," says Grutter.

"We don't know if it's a physical barrier, a chemical barrier or whether it actually just blocks the odour of the fish so the parasites can't find them. No one knows."

She says the mucous is produced by special spongy glands, the size of a ten cent piece, tucked in inside the operculum, the bony flap which covers the gills.

Grutter says fish leave a small opening near their mouth so they can still breathe.

"It's kind of like if you're in a sleeping bag and you have the thing tight around you, but you leave a little hole to breathe from by your mouth because it's really cold," she says.
Biological arms race

Parasitologist Dr Robert Adlard who is head of marine biology at the Queensland Museum described the mosquito analogy as 'spot on'.

"Certainly we know every fish has mucous covering their scales and it has antimicrobial properties, which acts as a barrier against infection. Clearly these fish have taken this one step further in the evolution of those cocoons.

"It's another fantastic demonstration of the arms race between the host and parasite, whether it's developing immunity, or coming up with these sorts of physiological mechanism to protect you. "

Fish biologist Dr Culum Brown from Macquarie University in Sydney says, during the day fish can take evasive action either by dislodging the parasites by rubbing on the sand or rocks, or visiting a cleaner wrasse for a daily check-up.

"During the night this can't happen without waking up and wrecking a good night's sleep; if one could prevent parasites attaching by producing a relatively cheap security blanket, then it's clearly a good option."

Grutter says the research had important implications for the aquarium trade.

"In the aquarium trade, cleaner fish are now one of the top 10 most popular aquarium fish traded in the UK and the USA. So over-exploiting wrasse has knock-on effects for parrot fish," she says.

This research was originally part of an Honours project by Queensland University student Jenna Rumney.

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