For parrotfish living in Australia’s Lizard Island coral reefs blood-sucking sea bugs can be a bother at night. These parasites called gnathiid isopods also prey on parrotfish during the day, but at this time other fish called cleaner fish remove the parasites from the parrotfish and eat them. It’s a good deal. Parrotfish are rid of parasites and cleaner fish get an easy meal.
Parrotfish, or Chlorurus sordidus, as they are known in the scientific world, help keep coral reefs healthy. Image by Julien Bidet for Seamarc/Wikimedia Commons.
At night, however, when parrotfish sleep, cleaner fish go off duty, but the blood-sucking parasites do not. Alexandra Grutter, a coral reef expert in Australia, tells me that she suspected that parrotfish had other ways to avoid being infected by isopods at night.
“Parrotfish sleep inside a cocoon of mucus. This looked like a way to avoid isopod bites, just like a mosquito net would protect us from bugs while we sleep,” Grutter said. I was intrigued. How would they test this idea?
Jennifer Rumney, a marine biologist working in the Grutter lab, took on the challenge of testing whether a cocoon of mucus in parrotfish would work like a mosquito net does in people. For Rumney, the experimental design meant tiptoeing in a completely dark room around sleeping parrotfish, dealing with blood-sucking isopods, and sleeping very little on her part.
“I began the experiment by placing parrotfish in individual containers in the lab,” Rumney explained. “At night, I turned the lab lights off and waited for the fish to make their cocoons and fall asleep.”
Each night, parrotfish make the mucus in an organ that is near their gills. The mucus exits through the gills and, in about an hour, surrounds the body of the fish. The next morning, the fish leaves the mucus envelope and goes about its daily activities. Next evening, when parrotfish go to sleep again, they make another cocoon.
Shhhhhh…
A few hours after the fish had gone to sleep, Rumney quietly entered the room and checked on them. She did not want to wake up the fish. If they woke, they would exit the cocoon and Romney would not be able to do the experiment. So, she did not turn the lights on; she found her way around the dark room with a flash light that put out red light that did not wake up the fish.
Rumney checked that all the fish had made their cocoons. Then she gently pushed some of the fish out of their cocoons without waking them, and took the slippery cocoon out of the container. She left the rest of the fish inside their cocoons. Next, she added isopods to all the containers and left the room.
“Rumney went back to the room at regular times during the night to make sure the fish continued to sleep,” Grutter said. “The last visit was before the fish woke up. She then collected the parasites on the fish and also the parasites in the container. Rumney didn’t sleep very much.”
Rumney counted the number of isopods she had recovered from parrotfish, both with and without cocoons. She found that fish that spent the night without a cocoon had more parasites attached to their bodies than fish that had remained inside their cocoons. Also, the parasites Rumney recovered from the fish without a cocoon had more blood inside them than the parasites from the fish inside cocoons.
The scientists found that the mucus cocoon parrotfish make at night does indeed reduce the isopod’s attacks by working like a mosquito net does for people. It is possible that the cocoon may also play other roles, such as discouraging moray eel or other predators from eating the fish or maybe preventing fish scent from attracting predators. These other possible roles have yet to be tested in the lab. Here is a short video showing parrotfish inside a mucus cocoon.
Thank you for reading! Will you join me next time for more intriguing animal secrets?
Ana Maria
Ana Maria S. Rodriguez
Here is the reference for Alexandra Grutter’s original publication on this topic:
A.S. Grutter, et al., Fish mucous cocoons: the ‘mosquito nets’ of the sea, 2011, Biology Letters, Vol. 7, p. 292.
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