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 fluid, serous and muc ...

covering the body of all

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

. An important part of

fish anatomy Fish anatomy is the study of the form or Morphology (biology), 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 ...

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

making up the slime coat is secreted by

goblet cells Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 2 in the lower gastrointestinal tract, and mucin 5AC in the respiratory tract. The goblet cells mainly use the merocrine method of secretion, secreting ...

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 Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

. 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 Prokaryote, prokaryotic and eukaryota, eukaryotic cells that may ...

and other antimicrobial components such as

lysozyme Lysozyme (, 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 hydrolase ...

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 ...

. 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. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study 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 the educational institution (and, in the case of in-person learning, the building) designed to provide learning environments for the teaching of students, usually under the direction of teachers. Most countries have systems of fo ...

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 The fus ...

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, have myocytes and are able to move, can reproduce sexually, and grow from a ...

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 An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

. In the absence of a

stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...

, 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 Parrotfish (named for their mouths, which resemble a parrot's beak) are a clade of fish placed in the tribe Scarini of the wrasse family (Labridae). Traditionally treated as their own family (Scaridae), genetic studies have found them to be dee ...

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 (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...

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 Predation, prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, na ...

: 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 Fish farming or pisciculture involves commercial animal husbandry, breeding of fish, most often for food, in fish tanks or artificial pen (enclosure), enclosures such as fish ponds. It is a particular type of aquaculture, which is the controlled ...

and

fishkeeping Fishkeeping is a popular hobby, practiced by aquarists, concerned with keeping fish in a home aquarium or garden pond. It is a practice that encompasses the art of maintaining one's own aquatic ecosystem, featuring a lot of variety with various w ...

, 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 chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

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 or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resis ...

Locomotion

As a defensive adaptation

Human importance

See also

References

Further reading