Increase in environmental temperature affects exploratory behaviour, anxiety and social preference in Danio rerio

Abstract

The aim of this work is to investigate the effect of a temperature increase on the behaviour of adult zebrafish (Danio rerio) maintained for 21 days at 34 °C (treatment) and 26 °C (control). The temperatures chosen are within the vital range of zebrafish and correspond to temperatures that this species encounters in the natural environment. Previous results showed that the same treatment affects the brain proteome and the behaviour of adult zebrafish by producing alterations in the proteins involved in neurotransmitter release and synaptic function and impairing fish exploratory behaviour. In this study, we have investigated the performance of treated and control zebrafish during environmental exploration by using four behavioural tests (novel tank diving, light and dark preference, social preference and mirror biting) that are paradigms for assessing the state of anxiety, boldness, social preference and aggressive behaviour, respectively. The results showed that heat treatment reduces anxiety and increases the boldness of zebrafish, which spent more time in potentially dangerous areas of the tank such as the top and the uncovered bright area and at a distance from the social group, thus decreasing protection for the zebrafish. These data suggest that the increase in ambient temperature may compromise zebrafish survival rate in the natural environment.

Figure 1

Zebrafish mass at the beginning and end of the thermal treatment. Data are expressed as the mean ± S.E.M. and were analysed by two-way ANOVA. No significant differences were detected (p > 0.33).

Figure 2

Schematic representation of the experimental design. The adult zebrafish were maintained for 21 days at 26 °C (control) and 34 °C (treatment). After thermal treatment, the fish were subjected to a battery of behavioural tests: NTT (a), LDT (b), MBT (c) and SPT (d). Behaviour was video-recorded for 10 min in each test.

Figure 3

Novel environment exploration behaviour in the NTT. Immobile episodes (a), time immobile (b), maximum speed (c), average speed (d), total distance travelled (e), transition between areas (f), total rotations (g), clockwise (CW) and counter-clockwise (CCW) rotations (h), absolute turn angle (i), and latency to enter the top area (j). Data are expressed as the mean ± S.E.M. and were analysed by unpaired Student’s t-test. *P ≤ 0.05; **P ≤ 0.01, N = 12.

Figure 4

Top area exploration in the NTT. Time spent (a,d), distance travelled (b,e) and number of transitions (c,f). Total (a–c) and 1-min interval (d–f) analyses are reported. Data are expressed as the mean ± S.E.M. and were analysed by unpaired Student’s t-test (a–c) and by repeated measures ANOVA (d–f). *P ≤ 0.05; **P ≤ 0.01, N = 12. (d–f) Solid and dotted lines refer to 26 °C and 34 °C, respectively.

Figure 5

Scototaxis behaviour in the LDT. Time spent in the bright area (a) and number of passages between bright and dark areas (b). Data are expressed as the mean ± S.E.M. and were analysed by unpaired Student’s t-test. *P ≤ 0.05; N = 12.

Figure 6

Shoaling behaviour in the SPT: whole tank analysis. Immobile episodes (a), time immobile (b), maximum speed (c), average speed (d), total distance travelled (e), transition between areas (f), total rotations (g), clockwise (CW) and counter-clockwise (CCW) rotations (h), absolute turn angle (i), and meandering (j). Data are expressed as the mean ± S.E.M. and were analysed by unpaired Student’s t-test. *P ≤ 0.05 **P ≤ 0.01, N = 12.

Figure 7

Shoaling behaviour in the SPT: conspecific area analysis. Parameters analysed in the conspecific compartment: number of entries (a), time spent (b), distance travelled (c), and latency to first entry (d). Data are expressed as the mean ± S.E.M. and were analysed by unpaired Student’s t-test. *P ≤ 0.05, N = 12.

Figure 8

Aggressive behaviour in the MBT. Mirror approach area: entries (a), time (b), mirror bites (c) and mirror-biting latency (d). Data are expressed as the mean ± S.E.M. and were analysed by unpaired Student’s t-test; **P ≤ 0.01, N = 12.