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

Management of the First Feeding of Dorada Brycon sinuensis with Two Species of Cladocerans

The management of the first feeding is a critical stage in the viability of the larvae and fingerling rearing. So far, the first feeding of the bryconids record the best results when fed with forage larvae; thus, the aim was to evaluate two species of cladocerans as live prey in the first feeding of dorada Brycon sinuensis and to evaluate their effects on the control of cannibalism. Larvae (1.2 ± 0.15 mg and 5.9 ± 0.4 mm initial weight and total length) were fed Moina minuta (Mm), Macrothrix elegans (Me) or a mixture (50:50) of cladocerans (Mix) at a rate of 20 prey mL−1, once for 24 h. Another dorada larvae group were fed newly hatched larvae of Piaractus brachypomus (4.5 ± 0.9 mm) as forage larvae (FL) in a ratio of 2:1 (prey:predator). The larvae were stocked to 50 L−1 in aquaria with 5 L of useful volume (12 per treatment). The growth, survival, stress resistance, cannibalism mortality, and the number of prey in the gut contents were analyzed. Dorada larvae fed FL showed higher growth, but those fed Mm showed the highest survival rate (76.1 ± 6.6%) and the lowest cannibalism mortality (16.8 ± 3.7%) (p < 0.05). The use of the cladocerans allowed high survival and stress resistance (95.3 ± 2.4%), and M. minuta proved to be a suitable prey for cannibalism control in the management of the first feeding of dorada larvae.

Melatonin reduces aggressiveness and improves oxidative status of matrinxã (Brycon amazonicus) subjected to social challenge

Melatonin is a hormone that can modulate aggressive interactions in fish, stimulate antioxidant enzymes, and neutralize the biological damage caused by high free radical production. Fish, especially those kept in artificial environments, are subject to damage caused by free radicals resulting from stressful conditions, such as a social challenge. The matrinxã (Brycon amazonicus) arouses great commercial interest and has been widely used in farming systems. However, these animals have a high frequency of aggressive behavior and cannibalism, which is considered a limiting factor for the production system. Thus, we evaluated the influence of melatonin on aggressiveness and oxidative stress parameters in matrinxã juveniles, testing whether melatonin increases glutathione concentrations and reduces lipid peroxidation (thiobarbituric acid-reactive substances) during social challenge. For this, 24 fish were subjected to three concentrations of melatonin in water: control (0 μmol/L), low MEL (1 μmol/L), and high MEL (10 μmol/L), with eight replicates each. The fish were isolated for 96 h, subjected to the mirror test (social challenge), and then euthanized and dissected to remove the liver. Melatonin reduced total aggressive interactions in animals treated with melatonin (P = 0.036), increased glutathione concentrations (P = 0.002), and decreased levels of lipid peroxidation (P = 0.001). We concluded that melatonin reduces aggressiveness and acts as a potent antioxidant in matrinxã juveniles, contributing to the reduction in stress levels and thus improving the maintenance conditions of animals in production systems.

β-Glucan-induced cortisol levels improve the early immune response in matrinxã (Brycon amazonicus)

This study investigated the role of endogenous cortisol on the innate immune response in matrinxã (Brycon amazonicus) fed with β-glucan, prior to and after stressor exposure and bacterial challenge. For this, we evaluated the serum cortisol and plasma glucose levels, the serum lysozyme levels, the hemolytic activity of the complement system, and the respiratory activity of leukocytes, as well as the number of circulating erythrocytes and leukocytes of fish fed during 15 days with diets containing β-glucan 0.1% (β-G) or β-glucan 0.1% + metyrapone 30 mg kg^-1 fish (β-G + MTP). Dietary MTP was used to block cortisol production. After feeding, fish were air-exposed during 3 min, to endogenously increase the cortisol levels. Following that, they were challenged with intraperitoneal injection of Aeromonas hydrophila. Results were compared with a positive control group fed with a β-glucan-free diet. A negative control group, also fed with β-glucan-free diet but inoculated with PBS, was established to evaluate the effect of the handling during injection. Fish were sampled prior to the stressor exposure, 30 min after exposure, and 24 h post infection (hpi). Herein we observed that dietary β-G modulated the cortisol profile prior to and after the stressor, increasing the number and activity of leukocytes. Moreover, cortisol showed to be an efficient modulator of both humoral and cellular innate immune system by increasing lysozyme and complement activity, as well as neutrophil and monocyte populations. Our results suggest that β-glucan-induced cortisol increase is one important mechanism to improve the innate immune response in matrinxã.

Novelty of the arena impairs the cortisol-related increase in the aggression of matrinxã (Brycon amazonicus)

The dichotomic effect of a cortisol level rise in vertebrate behavior has been widely observed. Generally, a chronic increase of the hormone level inhibits aggression, while an acute rise increases aggression. However, in this study, we show that this increase in aggression through an acute rise of cortisol also depends on the context in which the agonistic interaction occurs in the tropical fish matrinxã, Brycon amazonicus. We combined two factors: the type of housing (resident or non-resident in the trial arena) and the level of cortisol at the beginning of the fight (normal level - control, or high level - hydrocortisone-treated fish). The cortisol treatment increased the aggressiveness in the resident fish, but this effect was not observed in the non-resident fish, which fought in an unknown arena. The novelty of the arena may have elicited an "alerted state" in the non-resident fish; in this situation the fight was not the priority, and the cortisol effect in aggression could be impaired by a conflict between motivational systems (fear and aggression). In our knowledge, in fish, the increase of aggression promoted by an acute rise in cortisol levels was always tested and observed in a resident context, and the inhibition of cortisol effect in the agonist behavior is demonstrated for the first time. As the cortisol effect in aggression is observed in several taxa, the inhibition of aggressiveness increased by the novelty of the arena should be investigated in other groups to clarify the dynamics of this effect of cortisol in animal behavior.

Physiological and health consequences of social status in zebrafish (Danio rerio)

Social status affects access to food, mates and shelter and has consequences for the physiology of individuals and their health status. In the zebrafish (Danio rerio), an emerging model for studies into animal behavior, the possible consequences of social hierarchy to an individual's physiology and health are unknown. To address this, in this species we assessed the effects of social interaction (for periods of 1-5days) on growth, stress, immune function and reproductive condition. Wide-ranging differences in physiology occurred between the social ranks, some of which were sex-related and time-dependent. In both sexes, dominant fish were larger than subordinates and dominant males had a higher growth rate during the trials. Subordinates had higher plasma cortisol and in males higher telencephalic corticotrophin-releasing hormone, neuropeptide y and glucocorticoid receptor gene expression. Splenic cytokine expression suggested differences in immune status between ranks in both sexes and hematocrit was elevated in subordinate males. In both sexes, dominants and subordinates differed in the expression of genes for various gonadal sex steroid receptors and steroidogenic enzymes and in dominant females the ovary was larger relative to body mass compared with in subordinates. Dominant males had higher plasma 11-ketotestosterone than subordinates and there was an increase in the number of spermatids in their testes over the duration of the study that was not seen in subordinate males. The wide-ranging physiological differences seen between dominant and subordinate zebrafish as a consequence of their social status suggest negative health impacts for subordinates after prolonged durations in those hierarchies.