What makes a fish a fish? Fins, scales, gills, living in the
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.
General Structure of the Skin:
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
The dermis consists of thick connective tissue made up of two basic
layers. It is thicker and more stable than the epidermis.
What the Slime Does for Fishes:
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
Additional Functions of Body Slime:
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.
Where I Got This Stuff From; & You Can Too:
Bond, C.E. 1979. Biology of Fishes. W.B.Saunders Co., Philadelphia.
Bratt, B.L.H., Grosse, D.J. 1982. A reproductive pheromone in the
Mexican poeciliid fish Poecilia chica. Copeia, no.1, pp
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.
Norman, J.R. revised by P.H. Greenwood. 1963. A History of Fishes.
Pandey, A.K. Chemical signals in fishes: Theory and application.
Acta Hydrochim. Hydrobiol.;vol. 12, no. 5. pp. 463-478; 1984.