Mucous Cocoon Protects From Predators

a bullethead parrotfish in Morotai, Indonesia, a slim, medium sized fish with slight blue tail, yellow and orange scales, lavender stripes throughout and other blended pastel colors

Biological Strategy

Daisy parrotfish

AskNature Team

Image: Rickard Zerpe / Wikimedia / CC BY - Creative Commons Attribution alone

Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

Search this Function

Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

Search this Living System

Glands of the parrotfish protect it from parasites and mask olfactory cues by secreting a mucous cocoon that surrounds the fish.

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.

Last Updated October 27, 2016

References

“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).

Journal article

Fish mucous cocoons: the 'mosquito nets' of the sea

Biology Letters | 18/11/2010 | A. S. Grutter, J. G. Rumney, T. Sinclair-Taylor, P. Waldie, C. E. Franklin

embedly preview

Reference

“The mucous cocoon…is an extensive disulphate bonded network of small glycoproteins of about 21 kDa apparent molecular weight.” (Videler et al. 1999:1124).

Journal article