In
addition to other functions, mucus acts as a lubricant to decrease friction in swimming,
helps provide a barrier against and prevents the
attachment of pathogens, and contains biomolecules
important for immune defense. The fish
exterior generally contains either 1) scales (originating
from the dermal layer and comprised of
a type of bone) that are covered by the epidermis
(eg, koi, goldfish, cichlids, tetras, and loaches-that
have very fine scales) ; 2) no scales (eg,
channel catfish) ; or 3) bony plates (eg, Corydoras
sp. catfish, Plecostomus catfish [Hypostomus and
related genera], seahorses [Hippocampus spp.]).
The lateral line system, comprised of a series of
very small pits, is known as the organ of distant
touch. Among other functions, it facilitates
schooling behavior. Injury or lesions to the skin
can result in significant osmoregulatory compromise
and increased chance of infections.