This research aims to describe the effects of a variable number of Danio rerio fish subjects, ranging from one to eight, in the light/dark box preference test. Four hundred eighty adult male short-finned phenotype zebrafish were tested in the light/dark box. There were four groups in this experiment and a different number of subjects was used in each group: the control group had only one subject, whereas the experimental groups had either two, four, or eight subjects simultaneously inside the apparatus in every session. The average occurrence (AO) of subjects in the white side of the aquarium and the first choice average (FC) were recorded. The AO revealed no difference between the control group and test groups with two and four subjects. The results for the test group with eight subjects showed significant difference when compared to the control group and from the test group with two subjects. The FC also showed no difference between the control group and test groups with two and four subjects. There was significant variation between the control and the test group with eight subjects. The results reflect a conflict between the animal's preference for dark places and the innate drive to explore new environments. Zebrafish are highly social animals, exhibiting preference for swimming in groups and other patterns of social cohesion. The reduced white avoidance behavior in the test group of eight subjects may possibly reflect the role of shoaling, which is a defensive behavior, in reducing anxiety and stress. On the other hand, the absence of difference between the control group and test groups with two and four subjects suggest that it is feasible to run the light/dark test with up to four subjects, becoming an alternative to streamline and simplify data collection and test analysis.

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gland cells contain photoreceptors and the suprachiasmatic nucleus seems to be involved in biological rhythm just like in other vertebrates. This work aims to describe the role of luminosity and day periods in Danio rerio's Light/Dark preference. For this purpose 160 naive adult undetermined sex Zebrafish were divided into 5 groups of 32 subjects and kept in isolated aquaria with light control. Afterwards, each fish was tested in the Light/Dark aquaria, with water column of 5 centimeters. There was an habituation period of 5 minutes (300 seconds) followed by removal of the sliding doors, allowing the subjects to explore the apparatus for 15 minutes (900 seconds). Tests were performed in different day periods, as follows: Morning (06:00 -12:00 h); Afternoon (12:00 -18:00 h); Night (18:00 -24:00 h); Late Night (24:00 -06:00 h). Our results show that the time of permanence is sensitive to day period. The number of crossings and latency were not influenced by the period of the day. Further studies regarding biological basis of Light/Dark Preference should be carried out in order to understand the role of circadian function in Danio rerio's behavior.

Mercury chloride (HgCl2) is a toxic mercury salt and a major pollutant, that can be found in soil, water and air, with influences on behavior, physiology and adaptation to the environment. In this study two experiments were designed to examine interactions and effects of HgCl2 on some behavioral patterns of Siamese fighting fish (Betta splendens). In the first experiment we tested the effect of a progressive dose (five 0.04 mg) on aggressive display with exposure to a mirror, whereas in the second experiment we tested the effect of an acute dose (0.2 mg) on the aggressive display with exposure to a mirror. The experiments were performed on 5 consecutive sessions at intervals of 18 hours between sessions. Differences of performance were shown by subjects in the acute and progressive treatments when compared with a control treatment in the majority of behaviors evaluated, namely Floating, Slow Swimming, Wavy Swimming, Emerging, Bend, Square Move and Motor Display Components. Acute treatment was different from control only on Show Body, while the progressive group differed on Resting, Horizontal Display and Appropriate Display Components. Differences between Correlate Display Components and Total were also shown. Both the acute and progressive contamination with HgCl2 decrease the motor activity in the aggressive display, mirror-image test of Betta splendens, mainly on the progressive dose. This implies an impairment on feeding behavior, predator avoidance, reproductive behavior, mate choice and territoriality. These results suggest that in this fish species, the progressive dose has a greater effect on behavior in general and that both the acute and progressive contamination with mercury chloride affect many other aspects of behavior.