The familiar phrase “Good night, sleep tight, don't let the bedbugs bite†is just as applicable to parrotfishes as it is to humans.
It has long been known that some coral reef fishes, mainly parrotfishes and some wrasses, wrap themselves up cocoons made of their own mucus when they retire for the night. Various scientists have proposed a number of hypotheses regarding the cocoon’s function: to deter predators, to protect against settling silt or bacteria, or as a contact-based early warning system.
In a series of experiments, Alexandra Grutter and colleagues demonstrate that the mucous cocoons of these fishes protect against nocturnal attacks from ectoparasites (typically blood-sucking larval forms of isopods in the family Gnathiidae). The research is to be published in a forthcoming issue of the journal Biology Letters.
The authors captured Bullethead parrotfishes (Chlorurus sordidus) from the Great Barrier Reef, and divided them into two groups of fishes placed individually in enclosures. In the first group, the fish were allowed to produce their mucous cocoons and sleep in them unmolested, while in the second group, the fish were gently pushed out of their mucous cocoons at midnight (when all of the cocoons had been produced) and left exposed for the remainder of the night.
Twenty unfed third-stage gnathiid isopods (Gnathia aureusmaculosa) were then introduced into each enclosure, and were removed after four and a half hours. The authors then scored gnathiid feeding success, which was expressed as the number of fed gnathiids relative to the total number recovered (fed gnathiids were easily identified by their engorged, red bellies).
The authors found that while only 10% of the fishes in the cocoons were attacked by gnathiids, a whopping 94% of the fishes without the cocoons were attacked. The proportion of gnathiids that had fed on fish was also higher on fish without than fish with cocoons.
The authors also measured the energy content of the mucous cocoon using a bomb calorimeter, and found that the fishes used an estimated 2.5% of their daily energy budget to produce the cocoon.
Although other methods of parasite removal are available to the fish (eg. visiting cleaner fish, avoiding infected areas and infected individuals, rubbing their bodies against the substrate, and reducing shoaling and activity levels), not all of them are effective at night and most are energetically more costly than cocoon production. Cocoon production is thus a moderately energy-efficient method of deterring parasite attacks.
The use of a physical barrier (the mucous cocoon) by the parrotfishes to deter biting arthropods is reminiscent of the use of mosquito nets by humans, add the authors. This adaptation shows the tremendous selective pressure that parasites can impose on fish.
For more information, see the paper: Grutter, AS, JG Rumney, T Sinclair-Taylor, P Waldie and CE Franklin (2011) Fish mucous cocoons: the ‘mosquito nets’ of the sea. Biology Letters doi: 10.1098/rsbl.2010.0916