Does exposure to moonlight affect day/night changes in melatonin and metabolic parameters in Amazonian fish?

Lunar cycle modulates the rhythmic activity patterns of many animals, including fish. The effect of the moonlight cycle on daily melatonin and metabolic parameters was evaluated in matrinxã (Brycon amazonicus) subjected to external natural lighting. Eighty juvenile were distributed in 4 tanks of 1m3 (20 fish/tank) and divided into two groups. One group was exposed to the full moon and the other group to the new moon for 30 days, which corresponds to the duration of the lunar period. At the end of the lunar phase, 6 fish from each group were anesthetized to collect blood, tissue and eye samples at midday and midnight. The comparison between the light and dark periods revealed a significant increase in plasma and ocular melatonin in the last period. However, there was no significant difference for plasma melatonin between moons. Ocular melatonin presented higher concentrations during the new moon. Glucose, total proteins, cortisol, liver glutathione and gill lipid peroxidation were higher in the full moon compared to in the new moon. Plasma triglyceride was higher during the night for the full moon, and the opposite was found for the new moon. Total cholesterol values were higher at night regardless the moon phase. Glutathione in the gills and lipid peroxidation in the liver showed no significant differences. These results highlight the importance of considering both the day and lunar cycles for melatonin and metabolic parameters in species of commercial interest and susceptible to stressful situations in rearing conditions.

Understanding behaviour to improve the welfare of an ornamental fish.

Some common practices in aquaculture, ornamental trade and fish facilities may disturb the behavioural repertoire of fish and its natural adaptive value, reducing welfare and impairing fish production. Hence, it is necessary to understand fish behaviour, as well as the factors affecting it, to improve the quality of fish's life under artificial environment. Here, we reviewed the behaviour of the angelfish Pterophyllum scalare, an Amazonian cichlid used worldwide both as an ornamental fish and as a fish model in scientific research. We characterized social, reproductive and feeding behaviour, as well as the amazing cognitive ability of the angelfish. In addition, we reviewed the effects of environmental enrichment and suggested some important variables that need to be considered for rearing P. scalare. In this review, we show for the first time a synthesis on behaviour and a best practice overview to improve the welfare of angelfish as a target species. Nonetheless, most topics reviewed fit a broader set of fish species, particularly ornamental ones. This synthesis can therefore open a path for further behavioural research applied to the welfare of angelfish and bring insights to other fish species.

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.

Photoperiod modulation of aggressive behavior is independent of androgens in a tropical cichlid fish.

Photoperiod is a major environmental cue that signals breeding conditions in animals living in temperate climates. Therefore, the activity of the reproductive (i.e. hypothalamic-pituitary-gonadal, HPG) axis and of the expression of reproductive behaviors, including territoriality, is responsive to changes in day length. However, at low latitudes the seasonal variation in day length decreases dramatically and photoperiod becomes less reliable as a breeding entraining cue in tropical species. In spite of this, some tropical mammals and birds have been found to still respond to small amplitude changes in photoperiod (e.g. 17min). Here we tested the effect of 2 photoperiod regimes, referred to as long-day (LD: 16L:08D) and short-day (SD: 08L:16D), on the activity of the HPG axis, on aggressive behavior and in the androgen response to social challenges in males of the tropical cichlid fish Tilapia rendalli. For each treatment, fish were transferred from a pre-treatment photoperiod of 12L:12D to their treatment photoperiod (either LD or SD) in which they were kept for 20days on stock tanks. Afterwards, males were isolated for 4days in glass aquaria in order to establish territories and initial androgen levels (testosterone, T; 11-ketotestosterone, KT) were assessed. On the 4th day, territorial intrusions were promoted such that 1/3 of the isolated males acted as residents and another 1/3 as intruders. Territorial intrusions lasted for 1h to test the effects of a social challenge under different photoperiod regimes. Photoperiod treatment (either SD or LD) failed to induce significant changes in the HPG activity, as measured by androgen levels and gonadosomatic index. However, SD increased the intensity of aggressive behaviors and shortened the time to settle a dominance hierarchy in an androgen-independent manner. The androgen responsiveness to the simulated territorial intrusion was only present in KT but not for T. The percent change in KT levels in response to the social challenge was different between treatments (SD>LD) and between male types (resident>intruder). The higher androgen response to a social challenge in residents under SD may be explained by the time course of the androgen response that due to the long time it takes to fight resolution under LD, might have been delayed. This result illustrates the importance of incorporating time response data in social endocrinology studies.

Effect of body size heterogeneity on the aggressive behavior of larvae of matrinxã, Brycon amazonicus (Characiformes, Bryconidae) / Efeito da heterogeneidade no tamanho corporal sobre o comportamento agressivo de larvas de matrinxã, Brycon amazonicus (Characiformes, Bryconidae)

Brycon amazonicus is a native Amazonian fish that is important for aquaculture in South America. Larval mortality is high in this species in intensive breeding systems due to aggressiveness among larvae. The present study investigated experimentally the effects of body size heterogeneity on the aggressive behavior and survival of B. amazonicus during the early stages of larval development. Two treatments (larvae groups with homogeneous and heterogeneous body size) were evaluated throughout early larval stages tested at six time points: 12, 24, 36, 48, 60 and 72 hours after hatching (HAH). Two experiments quantified, respectively, aggressive interactions and mortality rates among larvae at each time point. The frequency of aggressive interactions exhibited by the less aggressive larvae in each replicate was higher in the homogeneous size treatment. Aggressiveness was higher at 12 HAH, decreasing thereafter, and increasing again at 72 HAH. The mortality rate significantly increased with the larval stage, and was higher in the homogeneous than in the heterogeneous sized groups. Our results showed that aggressiveness in B. amazonicus larvae is affected by size variability and larval development stage. This knowledge about larval behavior is important to develop measures to improve larval health and survival in intensive production systems for this species.

Brycon amazonicus é um peixe nativo da Amazônia que apresenta importância para a aquicultura na América do Sul. Em sistemas de criação intensiva, a mortalidade é alta nessa espécie devido à agressividade entre as larvas. O objetivo deste estudo foi testar se o tamanho diferencial entre indivíduos afeta o comportamento agressivo e a sobrevivência de B. amazonicus durante os estágios iniciais do desenvolvimento larval. Dois tratamentos (grupos de larvas com tamanho homogêneo e heterogêneo) foram avaliados ao longo do desenvolvimento larval em seis pontos de observação: 12, 24, 36, 48, 60 e 72 horas pós-eclosão (HPE). Em dois experimentos foram quantificadas, respectivamente, as interações agressivas e a mortalidade em cada ponto de observação. A frequência das interações agressivas exibidas pelas larvas menos agressivas foi maior no tratamento de tamanho homogêneo. A agressividade também foi maior às 12 HPE, diminuiu nos períodos intermediários e aumentou novamente às 72 HPE. A taxa de mortalidade aumentou ao longo dos estágios larvais, sendo observada maior mortalidade total nos grupos de tamanho homogêneo. Os resultados deste estudo indicam que a agressividade em B. amazonicus é modulada pela variação do tamanho dos indivíduos e pelo estágio de desenvolvimento das larvas. O conhecimento sobre o comportamento de larvas é importante para desenvolver medidas que melhorem a saúde e a sobrevivência em sistemas de produção intensiva para esta espécie.