Gnathiids are a family of isopod crustaceans whose larvae feed on the blood of fish. During the day, infected parrotfish seek out cleaner fish to consume the parasites; however, at night they are relatively vulnerable to attack. Parrotfish overcome this vulnerability by secreting a mucus cocoon before sleeping which envelopes their bodies with a protective biopolymer that functions similar to a mosquito net. The mucus is secreted from large glands in the gill cavity and is composed of small glycoproteins which are extensively cross-linked through pyrosulfate bonds. This exopolymer net allows small molecules to permeate but prevents the parasitic gnathiids from entering. The process is thought to involve a combination of blocking odorants which the isopods use to target the fish and physically preventing them from approaching the fish. The nightly mucus secretion only consumes ~2.5% of the daily energy budget of the parrotfish which makes it a very efficient strategy.
Alternative strategies used by other organisms to deter parasites include chemical ones like the secretion or production of toxins and behavioral ones like scraping along surfaces, avoiding infected individuals/habitats, and seeking cleaner fish; these alternatives are relatively energetically costly compared to the mucus cocoon.
“Ectoparasitic gnathiid isopods (Gnathiidae), which feed on the blood of fish…One of the most notable nocturnal behaviours of coral reef fishes, mainly some wrasses and parrotfishes, is the large mucous cocoons that they envelop themselves in at night…Gnathiid isopods attack many coral reef fishes, especially at night. During the day, parrotfish repeatedly seek cleaner fish, which only control gnathiid infestations during the day but it is not clear how parrotfish control gnathiids at night…Significantly, more fish from which the cocoon had been removed (94.4%), compared with fish with cocoons not removed (10%), were attacked by gnathiids.” (Grutter et al. 2010:292).“By remaining in a mucous cocoon at night, parrotfish C. [Chlorurus] sordidus may avoid being attacked by gnathiids, which regularly attack fish at night and possibly other parasitic isopods, of which there is a wide range on coral reefs…Cocoons may prevent infestation by masking olfactory cues used by gnathiids to find fish or act as a physical or chemical barrier…The moderate investment in cocoon production, estimated at 2.5 per cent of their daily energy budget…may explain why fish can produce cocoons nightly and also could produce a second cocoon on the same night if needed…Anti-parasite behaviours fish can engage in, including seeking cleaner organisms, avoiding infectious habitats and infected individuals, chafing along a substrate, reducing activity and shoaling are relatively energetically costly. Using mucous cocoons, however, circumvents this limitation by deterring parasites in a moderately energetically efficient way…In contrast to the astonishingly diverse behavioural adaptations and the use of toxic compounds in other animals, parrotfish use a physiological adaptation to deter parasites. This involves large highly specialized glands in the gill cavity and/or under the operculum, which secrete a structure that not only protects the whole fish but also allows the fish to sleep, a combination of features not known to occur in any other animal…Mucous cocoons, in contrast, are more reminiscent of the barriers, such as mosquito nets, constructed by humans to control biting arthropods.” (Grutteret al. 2010:293).
Fish mucous cocoons: the 'mosquito nets' of the seaBiology LettersNovember 18, 2010
“The mucous cocoon…is an extensive disulphate bonded network of small glycoproteins of about 21 kDa apparent molecular weight.” (Videler et al. 1999:1124).