For all living organisms, stress represents a major concern when dealing with the maintenance of an appropriate physiological status. In a wide sense, the discomfort associated to stress, either physical or environmental, can trigger in individuals a series of known physiological short and long-term responses in order to face and adapt to impose changes. Particularly, aquarium fish are subject to a broad variety of different stressors because their homeostatic mechanisms strongly depend on prevailing conditions in their immediate surroundings1. There is a well-known set of aquarium fish-specific stressors, the most relevant being: i) improper water parameters (ammonia, nitrate, pH, temperature, salinity, oxygen…), ii) inadequate tank size, iii) lack of hiding places, iv) overstocking of tank, v) harassment from other fish, vi) infections with pathogens that cross-react with different host species of different geographical origins, vii) contaminants (chemicals, medication), and vii) malnutrition of captive fish because of suboptimal feed formulation, as the precise nutritional requirements for aquarium fish species is not well-determined.
Physiological responses to stress include changes in heart rate, blood pressure, blood sugar, and release of adrenergic (such as adrenaline) and glucocorticoid hormones such as cortisol, which is considered a hallmark of the stress response. Although activities of these hormones are beneficial to tackle short and sudden stressors, negative health effects will be elicited when hormonal stress responses persist and become chronic, including depletion of energy reserves, muscle breakdown, interference with osmoregulatory processes, and immune suppression2. Thus, the lowered immune response will eventually allow parasites, bacteria, and fungi to infect a stressed fish and cause disease and death.
In this scenario, dietary supplements that contribute to enhance fish resistance against infection diseases and/or to act as immunostimulants in order to improve immune status of aquarium cultured fish are currently of major concern in these ecosystems.