What makes a fish a fish? Fins, scales, gills, living in the water?

Surprisingly enough there are fishes without true fins or scales whose respiration does not rely mainly on "gills", and those that spend a great deal of time out of the water. One trait that all living fishes do have in common however, is that they have body slimes, a mucoid covering on their very outsides. This trait is very important, particularly for its protective properties.

First off, where does this slime come from? The answer is that it originates from dispersed glandular cells, uni- or multicellular in the fish’s epidermis. The type and displacement of these glands is of importance in classification. They produce a glycoprotein called mucin which, when mixed water, produces mucus. As an example in the extreme, consider a hagfish (closest family to the Lampreys), Psychedelic Gobies (aka Mandarins, family Callionymidae), or some species amongst the true eels. Hagfishes are particularly slimy; one of their common namses is "slime eel" though they are not closely related phylogenetically to the eels.

One method of capturing Hags is to fill a steel drum with fish heads and/or other offal, puncture this drum and lower it over a boat side on the continental shelf where these fishes are found. Hagfish will squeeze in to the holes in the punctured drum and eat so much they can’t squeeze back out when the drum is hauled to the surface. Once captured, there is difficulty preserving the catch. First they must be cleaned of the copious amounts of slime they produce. If you can grab one well enough to stick it in a bucket of clean water, this medium quickly becomes slimy as well. These animals possess large (pea-sized) multicellular slime glands. Ultimately, what has been done to prepare Hagfishes for preservation is to place a batch of these fishs in a hapless researcher’s washing machine with jumbo amounts of enzymatic detergent on a continuous rinse cycle. This finally results in a slime-free hagfish ready for alcohol or formalin preservation.

The skin of fishes, like that of all vertebrates (amphibians, reptiles, birds, mammals) consists of two principal layers:

1) Superficial epidermis, and

2) Deeper dermis.

The epidermis in turn consists of two or more layers. The deepest is a series of close-paced, discrete cells called the germinal layer, or stratum germinativum. The outer cells areformed of its daughter cells. There is much variation in the outer cells, depending on the group of fishes being investigated. Body slimes are the products of these daughter cells and their degradation and, as such, are continuously replaced.

The dermis consists of thick connective tissue made up of two basic layers. It is thicker and more stable than the epidermis.

The fact that all fishes have these body coverings is some indication of their importance. With either too much or too little of this slimy coat, any fish will soon die. The slime serves three functions for all fishes. It aids in:

1) Osmoregulation/Gas Transport: Slime provides a selective interface to maintain internal/externail ionic balance. One of the reasons freshwater fishes are constantly urinating is their bodies are "saltier" than the water around them and they tend to absorb water. The fish gets rid of this excess water by elimination. The opposite rationale applies to saltwater fishes. In addition to salt balance, the slime plays important roles in dermal respiration. Fish breathe through their skins, as do humans. If the amount or quality of the slime changes, it effects the efficiency of gas transport through the skin.

2) External Protection: Body slime prevents attachment of ectoparasites by making the surface of the fish slippery, sloughing off with the parasite and suffocating pathogens. It also acts as a bandage by covering over a wound caused by trauma or infection. Usually fishes with poorly developed scales are more slimy, for example, Characins (some known as Tetras) and their relatives.

3) Reduces Turbulence: Especially in fast-moving fishes the drag resulting from small spaces between scales and projecting body parts accounts for considerable energy loss (up to 30% by some estimates) in locomotion. The slime acts to smooth out these gaps.

In addition to the abouve functions, many groups of fishes benefit in other ways from their body slimes. For some they help:

1) Coagulate Particles: Providing clean water in the immediate area around the fish, thus improving movement and dermal respiration. Some filter-feeding fishes pass the mucus forward into their mouths and eat it. For example, some of the Wrasses.

2) Produce Toxins: For example, some of the previously mentioned Hagfishes (family Myxinidae), closely related to the Lampreys (Petromyzontidae) that have ruined fisheries in the Great Lakes, immobilize a host on contact with their body slimes, entering their vent and eating them. Pardochirus marmoratus, a Sole (type of Flatfish) in the Red Sea featured in the November 1974 issue of National Geographic Magazine, has a slime that contains a substance so effective in warding off shark bate that the attacker’s jaws are said to be frozen in mid-bite.

3) Cocoon Formation: The African Lungfish avoids desiccation during Summer and dry periods by making a shell of its body slime and "hibernating".

Many Parrotfishes (family Scaridae) produce a mucus "tent" at night to protect themselves against predation. As an experiment, some of the Parrotfishes of the genus Scarus which construct such sleeping bags and an equal number of similar species of the similar-appearing genus Sparisoma which do not produce cocoons were placed in a tank with a few large Moray Eels (family Muraenidae). It seems Parrotfishes are a favorite food item of many Morays. These fishes were left together overnight. The Scarus built cocoons and were not eaten but the Sparisoma were consumed. During the night the Morays were observed approaching the camouflaged Scarus: although they couldn’t see what was contained within the enclosed envelopes, it was evident they understood what was contained within. The eels "tasted" the mucus and left the Scarus alone.

4) Feeding: Several fishes, including some of the Mystus (Asian catfishes) and the Discus (Symphysodon) secrete body slimes to feed their young. Baby Discus feed on an overabundance of slime which develops on the sides of the parent fish at breeding time. The substance is highly proteinaceous in nature and is produced by specialized skin cells. This situation is not the same as lactation in mammals; the slime is different chemically and there is not permanent organized structure for secretion. This is an important source of food for the young who need it during the first week of life. There are no suitable naturally occurring substitutes.

5) Alarm Substances: A lot of aquarium fishes such as Tetras, Barbs, freshwater "Sharks", Rasboras, Loaches, Catfishes and others have a number of blind cells; that is, they have no opening to the outside of the body, that are involved in producing, storing alarm substances. When the skin is broken, these cells release a fright contagion that notifies others that something is going wrong. These substances are not necessarily species specific. They are responsible for producing the fright syndrome German aquarists refer to as "shrekstoff". A situation that hobbyists everywhere should be aware of and guard against, through careful netting, handling practices, and adequate filtration and maintenance.

6) Nest Building Materials: In some species such as the Gouramis (family Anabantidae) and Bettas, slime is utilized in the construction of "Bubble Nests" that males spit eggs and keep young safe in till they’re able to fare on their own.

7) Cement: In western India the mucus of Snakeheads (family Channidae) is used in the construction industry to increase the strength of mortar.

Practical Aquarium Significance:

What does all this mean to an aquarist? As can be seen from the previous discussion, body slimes are eminently important to fishes. Stress to the fish can and does occur by affecting body mucus amount or viscosity, and vice versa.

Aquarists should be especially careful when netting their fishes. Commercially, we never touch fine scaled fishes with our hands. If a fish drops to the floor, pick it up with a wet net or towel and try to preserve the integrity of the animals slime layer.

Metallic ion medications among other types act as proteinaceous precipitants, making the fish produce more slime as irritation increases. Concerning the fishes, the copper ions (as well as malachite green) sold as marine and freshwater ich remedies act as an irritant to the skin and gill membranes of fishes, which in response produce copious amounts of mucus to protect these tissues.

If disease organisms are present on the gills and skin, the mucus produced engulfs the organisms. When the mucus is sloughed off, the disease organisms are lost as well. Under high dosages or prolonged treatment with such medications, a loss of fish livestock results from direct uptake of medicants and mucoid production so great as to impede gaseous exchange by the gills and skin.

Much of these free metal ions can be found even in fresh tapwater. Some water treatment products are designed to make aquarium fishes more slimy to protect against such irritation.

Once again, the best policy for maintaining aquatic life is not to change too much too quickly and provide the optimum suitable environment. Change part of your water frequently, vary the diet and watch what you put into your tanks. A factor as seemingly "simple" as fish sliminess can be a determining factor in the well being of our aquatic charges.

Bond, C.E. 1979. Biology of Fishes. W.B.Saunders Co., Philadelphia. P. 28,29.

Bratt, B.L.H., Grosse, D.J. 1982. A reproductive pheromone in the Mexican poeciliid fish Poecilia chica. Copeia, no.1, pp 219-223.

Herald, E.S. 1961. Fishes of the World. Doubleday & Co. New York. P. 204, 205.

Jonsson, L. 1979. Chemical Stimuli: Role in the Behavior of Fishes. Environmental Physiology of Fish.; Plenum Press: New York, N.Y. pp. 353-363.

Norman, J.R. revised by P.H. Greenwood. 1963. A History of Fishes. P. 157.

Pandey, A.K. Chemical signals in fishes: Theory and application. Acta Hydrochim. Hydrobiol.;vol. 12, no. 5. pp. 463-478; 1984.