Social relationship as inducer of immunological and stress responses in matrinxã (Brycon amazonicus).

Matrinxã (Brycon amazonicus) is a freshwater neotropical fish species with a social interaction and aggressive behavior, especially in crowded environments. This species' social structure is established by agonistic interactions, which increase significantly at the fifth hour, when compared to the first and second hours of confinement, when each dominant fish competes with other individual for a conditioned territorialism. This social relationship also induces a complex physiological response in the organism, which generates an acute stimulation from the stressor agent. A stress situation modulates the physiology of the subordinate fish, which undergo significant increases in cortisol, glucose, hematocrit, and hemoglobin, when compared to the control fish. The immune system also indicates a modulation caused by cortisol, which results in an increase in neutrophils and a significant decrease in thrombocytes in subordinate fish, in comparison with control fish; however, the dominant fish show a significant increase in monocytes and a decrease in lymphocyte levels, when compared control fish. The agonistic interactions in B. amazonicus during crowding are not favorable to the physiology and immune system of the fish, mainly subordinate fish, and should be avoided in rearing systems.