Besides its immune system, the fish has some
physical defenses against external bacterial
or parasitic attacks. The fishes' first layer
of protection is the epidermis, the outermost
layer of skin, which contains the relatively
impervious scales and is further protected
by mucus. Scattered among the cells of the
epidermis are gland cells that constantly
secrete the mucus that makes a fish proverbially
slippery. This mucus coating is continually
being sloughed away, and along with it, many
bacteria or fungal spores or sessile ciliates
that may have recently settled. I don't know
how long it takes for the mucus covering
a fish to be entirely renewed, but I'd like
to. Not more than 24 hours, surely?
Fish mucus also has a less passive role.
It contains antibodies and antibacterial
lysozymes, which are enzymes that digest or dissolve
organic material. Lysozymes degrade carbohydrate
bonds in the polyaccharides that build bacterial
cell walls, causing the bacterium to burst,
and lysozymes may even degrade the cell walls
of protozoans. In the 19th century, long
before Alexander Fleming isolated the first
human lysozyme in his own mucus, the skin
slime of the European Tench (Tinca tinca ) was used in clinics to reduce inflammation.
Other biologically active
secretions in fish
mucus include proteases,
alkaline phosphatase,
esterase. Mucus gland secretions
associated
with skin spines have evolved
in some marine
fishes to make them actually
toxic.
So fish mucus is not merely a kind of inert
protective gelatinous coating that could
be supplemented or replaced, perhaps by aloe vera gel.
In sick fish "increases in mucus production
may occur in protozoal infections and in
instances in which an irritant is present
in the water," according to Richard
Wolke's chapter "Physiological mechanisms
of fish disease," in Aquariology: Fish Diseases and Water Chemistry, John Grazek, et al, 1992, p. 44. An irritant
that may increase mucus production is salt
(sodium chloride). But ammonia levels in
seawater suppress the production of protective
mucus in marine fish, I've recently read,
and this may hold true for freshwater fish
too. (Ammonia is a surfactant that acts chemically
to break up slime.)
Acquired immunity. Fishkeepers have long noticed that, once
a fish had been attacked by "Ich"
and was cured, that individual seemed to
have developed some degree of immunity. At
other times, when almost all the fish in
an aquarium have been infested with "Ich,"
fishkeepers have noted that some individuals
have remained free of the parasite. Well,
inquiries currently being conducted by Dr.
Ted Clark and a team at Cornell University School of Veterinary Medicine
are confirming that these anecdotal hobbyists'
hunches are in fact well-founded. Dr. Clark
explains, "Although the parasite is
quite lethal, animals that survive epizootics
develop a strong acquired resistance, and
we can routinely immunize fish in the laboratory
by exposing them to sublethal infections
followed by chemical treatment of the water
to eliminate free-swimming stages that would
eventually kill fish. Resistance is seen
within 3-4 weeks of initial exposure."
He goes on to tell that part of the resistance
involves an antibody produced in the mucus
as a response to proteins in the parasite's
outer outer membrane. The antibodies bind
on the surface of the parasite, forcing it
off the fish prematurely, though not killing
it. The team is even working on Ich vaccines!
