Parrotfish sleep in a mosquito net made of mucus
It’s time for bed, and what better way to ensure a comfortable sleep than snuggling into a cocoon of your own mucus?
Humans could probably think of some alternatives, but for many coral reef fish, mucus sleeping bags are all part of a good night’s rest. Many species of parrotfish and wrasse belch out their own cocoons every night, covering themselves in under an hour. And Alexandra Grutter from the University of Queensland thinks she knows why – the mucus deters vampires.
There are many possible explanations for the cocoons: they’re an early warning system; they act as a dust sheet; they protect the fish from DNA damage caused by sunlight; the list goes on. The most popular theory is that the cocoons protect the fish from predators. It dates back to 1959 when Howard Winn and John Bardach found that three species of mucus-weavers are less often eaten by moray eels than a fourth species that sleeps uncovered. But the duo never considered other differences between the four species and they still found that the eel occasionally ate the allegedly protected individuals. To date, the idea has never been properly tested.
Grutter thinks that the mucus protects the sleeping fish, not from the big jaws of hungry eels, but from the little mouthparts of hungry parasites. At night, reef fish are attacked by tiny blood-sucking crustaceans called gnathiid isopods. During the day, cleaner fish help to keep these pests at bay but they don’t work at night. So when darkness falls, the fish wrap themselves in a ‘mosquito net’ of their own making.
Grutter collected daisy parrotfishes from the Great Barrier Reef and at night, all the fish dutifully produced cocoons. But half of them were in for a restless night. Grutter pushed them out of their sacs and fished out the mucus, and she did it so gently that none of the fish woke up.
She then added gnathiids to each tank and found that the tiny vampires attacked just 10% of the cocooned fish, but virtually all of the unprotected ones. By analysing the chemical content of the scooped mucus, Grutter calculated that making one takes up just 2.5% of the parrotfish’s total energy budget – a small ask for a good night’s sleep.
The study raises more questions than it answers. How do fish that sleep without mucus protect themselves? Do they bury themselves in sand, use toxic skin, or sleep in open areas away from gnathiid-infested reefs? And why do the cocoons ward off the gnathiids? Most anti-parasite measures tend to involve chemicals, but Grutter thinks that the mucus is probably more of a physical barrier, like a human mosquito net.
Mucus-producing reef fish are the only animals that secrete a covering that wraps their entire body and allows them to sleep. The ability involves special glands in their gills and Grutter says that the evolution of these glands is testament to the pressures that parasites place on their hosts.
I’m as big a fan of parasites as the next person but this claim seems a bit far-fetched based on the results of the study. The idea that the mucus is a mosquito net doesn’t disprove any of the other possible functions (as Grutter herself acknowledges). It could provide many benefits to the sleeping fish and the study as it stands says nothing about the pressures exerted by parasites in particular.
Reference: Biology Letters http://dx.doi.org/10.1098/rsbl.2010.0916
Image: by Richard Ling
More on fish:
- Malawi cichlids – how aggressive males create diversity
- Pocket science – swordfish and flatfish are close kin
- Bearded goby munches jellyfish, ignores toxic gases, is generally very hard
- Pregnant male pipefish abort babies from unattractive females
- Evolving guppies shape their environments
- Moray eels attack with second pair of ‘Alien-style’ jaws
If the citation link isn’t working, read why here
//
Related Topics
Go Further
Animals
- This ‘saber-toothed’ salmon wasn’t quite what we thoughtThis ‘saber-toothed’ salmon wasn’t quite what we thought
- Why this rhino-zebra friendship makes perfect senseWhy this rhino-zebra friendship makes perfect sense
- When did bioluminescence evolve? It’s older than we thought.When did bioluminescence evolve? It’s older than we thought.
- Soy, skim … spider. Are any of these technically milk?Soy, skim … spider. Are any of these technically milk?
- This pristine piece of the Amazon shows nature’s resilienceThis pristine piece of the Amazon shows nature’s resilience
Environment
- This pristine piece of the Amazon shows nature’s resilienceThis pristine piece of the Amazon shows nature’s resilience
- Listen to 30 years of climate change transformed into haunting musicListen to 30 years of climate change transformed into haunting music
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
History & Culture
- Meet the original members of the tortured poets departmentMeet the original members of the tortured poets department
- Séances at the White House? Why these first ladies turned to the occultSéances at the White House? Why these first ladies turned to the occult
- Gambling is everywhere now. When is that a problem?Gambling is everywhere now. When is that a problem?
- Beauty is pain—at least it was in 17th-century SpainBeauty is pain—at least it was in 17th-century Spain
- The real spies who inspired ‘The Ministry of Ungentlemanly Warfare’The real spies who inspired ‘The Ministry of Ungentlemanly Warfare’
Science
- Here's how astronomers found one of the rarest phenomenons in spaceHere's how astronomers found one of the rarest phenomenons in space
- Not an extrovert or introvert? There’s a word for that.Not an extrovert or introvert? There’s a word for that.
- NASA has a plan to clean up space junk—but is going green enough?NASA has a plan to clean up space junk—but is going green enough?
- Soy, skim … spider. Are any of these technically milk?Soy, skim … spider. Are any of these technically milk?
Travel
- What it's like to hike the Camino del Mayab in MexicoWhat it's like to hike the Camino del Mayab in Mexico
- Is this small English town Yorkshire's culinary capital?Is this small English town Yorkshire's culinary capital?
- This chef is taking Indian cuisine in a bold new directionThis chef is taking Indian cuisine in a bold new direction
- Follow in the footsteps of Robin Hood in Sherwood ForestFollow in the footsteps of Robin Hood in Sherwood Forest