Eldon's galaxias (Galaxias eldoni) - closeup of skin

The slime coat (also fish slime, mucus layer or slime layer) is the coating of

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

covering the body of all

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

. An important part of

fish anatomy Fish anatomy is the study of the form or morphology of fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. In practice, fish anatomy and fish physiology co ...

, it serves many functions, depending on species, ranging from

locomotion Locomotion means the act or ability of something to transport or move itself from place to place. Locomotion may refer to: Motion * Motion (physics) * Robot locomotion, of man-made devices By environment * Aquatic locomotion * Flight * Locomoti ...

, care and feeding of offspring, to resistance against diseases and parasites. The

mucin Mucins () are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most ...

making up the slime coat is secreted by

goblet cells Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secre ...

in the fish's

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

. The slime contains a variety of

antimicrobial peptides Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for a ...

and other antimicrobial components such as

lysozyme Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...

and

C-reactive protein C-reactive protein (CRP) is an annular (ring-shaped) pentameric protein found in blood plasma, whose circulating concentrations rise in response to inflammation. It is an acute-phase protein of hepatic origin that increases following interleukin-6 ...

. It contains

mycosporine-like amino acid Mycosporine-like amino acids (MAAs) are small secondary metabolites produced by organisms that live in environments with high volumes of sunlight, usually marine environments. The exact number of compounds within this class of natural products is y ...

s to protect from ultraviolet radiation.

Locomotion

The slime coat of some fish aids in more efficient swimming by reducing drag, attributed to the Toms effect. Slime can reduce the

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...

experienced by the fish by up to 65%. Generally, the faster the fish, the greater reduction in drag provided by the slime, but there are a few exceptions. In

schooling A school is an educational institution designed to provide learning spaces and learning environments for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is sometimes compuls ...

fish, slime shed by leading fish is thought to provide a hydrodynamic benefit to following fish.

As a defensive adaptation

The slime coat of

reef fish Coral reef fish are fish which live amongst or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad inhabitants, the fish stand out as colourful and interesting to watch. Hundreds ...

contains

s (MAAs) which protect the fish from sun damage by absorbing radiation. The greatest number of MAAs is found on the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

side of the fish, which is exposed to more radiation.

Animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

cannot synthesize MAAs, requiring fish to sequester them from their diet. Under water, fish are exposed to a greater number of microorganisms than animals whose skin is exposed mainly to

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

. In the absence of a

stratum corneum The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compact ...

, the slime coat serves to protect the fish from attack from harmful microorganisms. This is chiefly done by sloughing off microbes which become trapped in the slime coat, but the slime coat contains

and other defensive properties such as

and

Parrotfish Parrotfishes are a group of about 90 fish species regarded as a Family (biology), family (Scaridae), or a subfamily (Scarinae) of the wrasses. With about 95 species, this group's largest species richness is in the Indo-Pacific. They are found ...

create extra mucus during sleep which covers their bodies in a cocoon-like structure. It protects them from predators and parasites by masking their scent and providing a physical barrier against them. Hagfish Slime Predator Deterrence

The slime of the

hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, a ...

is unique due to its volume and dilution. In these fish it serves as an

anti-predator adaptation Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avo ...

: when grabbed by a predator fish, the hagfish ejects copious amounts of slime into the predator's mouth, causing the predator to gag and flare its gills, releasing the hagfish and moving away.

Human importance

pisciculture upright=1.3, Salmon farming in the sea (mariculture) at Loch Ainort, Isle of Skye">mariculture.html" ;"title="Salmon farming in the sea (mariculture">Salmon farming in the sea (mariculture) at Loch Ainort, Isle of Skye, Scotland Fish farming or ...

and

fishkeeping Fishkeeping is a popular hobby, practiced by aquarists, concerned with keeping fish in a home aquarium or garden pond. There is also a piscicultural fishkeeping industry, serving as a branch of agriculture. Origins of fishkeeping Fish have ...

, the slime coat is important to the health of fish, particularly during transport which can cause damage to it. High

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

levels in the water can also cause damage to the slime coat. The antimicrobial properties of fish slime have been studied as an alternative to antibiotic drugs to address

antibiotic resistance Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance. ...

Locomotion

As a defensive adaptation

Human importance

See also

References

Further reading