The roles of nucleotides and metabolites in diets of bony, sturgeon, cold, warm and ornamental
fishes have been sparingly studied for over 25 years. Beside possible involvement in diet
palatability, fish feeding behavior and biosynthesis of non-essential amino acids, exogenous
nucleotides have shown promise most recently as dietary supplements to enhance immunity and
disease resistance of fish produced in aquaculture. Research on dietary nucleotides in fishes has
shown they may improve growth in early stages of development, enhance larval quality via
broodstock fortification, alter intestinal structure, increase stress tolerance as well as modulate
innate and adaptive immune responses. Fishes fed nucleotide-supplemented diets generally have
shown enhanced resistance against viral, bacterial and parasitic infections. Despite occasional
inconsistency in physiological responses, dietary supplementation of nucleotides has shown rather
consistent beneficial influences on various fish species. Although nucleotide nutrition research in
fishes is in its infancy and many fundamental questions remain unanswered, observations thus far
support the contention that nucleotides are conditionally or semi-essential nutrients for fishes, and
further exploration of dietary supplementation of nucleotides for application in fish culture is
warranted. Hypothesized reason(s) associated with these beneficial effects include dietary provision
of physiologically required levels of nucleotides due to limited synthetic capacity of certain tissues
(e.g. lymphoid), inadequate energetic expenditure for de novo synthesis, immunoendocrine
interactions and modulation of gene expression patterns. However, currently there are numerous
gaps in existing knowledge about exogenous nucleotide application to fish including various aspects
of digestion, absorption, metabolism, and influences on various physiological responses especially
expression of immunogenes and modulation of immunoglobulin production. Additional information
is also needed in regard to age/size-related responses and appropriate doses and timing of
administration. Thus further research in these areas should be pursued.