Although aquarists and fishermen may dislike handling “slimy” fish, the slime coat serves a highly important function. Like skin, a fish’s slime coat protects against invasion by bacteria and fungi, defends against minor damage, and serves as a barrier against harmful environmental conditions such as excess acidity or harsh chemicals. In short, the slime coat is an adaptation by nature to life in a watery environment.
Slime originates from continually replaced glandular cells in the fish’s skin that produce a glycoprotein known as mucin, which when mixed with water produces mucus. Fish with poorly developed scales tend to be more slimy.
Slime serves three main functions: 1) it helps to regulate the internal/external ionic balance and the efficiently of gas transport through the skin (osmoregulation); 2) it provides external protection, helping to prevent the attachment of parasites by making a slippery surface, and by sloughing off or actually suffocating pathogens, and acts as a bandage, covering wounds; and 3) it reduces turbulenceby smoothing over the areas between the scales, thus reducing energy loss and aiding movement.
For some fish, other benefits also accrue from slime: toxicity, for defense or offense; formation of a cocoon for sleeping (parrotfish) or moisture retention (lungfish); food for fry, as provided by discus or some of the Asian catfishes, whose protein rich slime is an important early food; alarm chemicals that spread through the water from injury or panic (fright syndrome); and nest-building materials, as for the labyrinth fishes.
Stress to the fish occurs whenever the slime coat or slime production are affected, either in amount or in viscosity. Too much or too little slime production, and a fish will die. Some medications can act as proteinaceous precipitants, causing excess slime production. Copper, in particular, is an irritant to the skin and to the gill membranes, which then produce excess mucus. When this excess mucus is sloughed off, the irritants (bacteria, etc.) go with it. However, excess slime can also impede gaseous exchange by the skin and gills.
What does all of this mean to the aquarist? Use care when handling fish, to as much as possible prevent damage to the mucus coat. And always use caution regarding what enters or comes into contact with tank water.
Reference: Fenner, Bob and Cindy, D.V.M. “The Function of Body Slime in Fishes.” Freshwater and Marine Aquarium, June 1986, pp. 75-79
G.C.K.A. Newsletter May 1998