Fish have varying degrees of phenotypic plasticity (PP), in which traits like behavior, morphology, and growth, are influenced by complex genetic and environmental interactions. PP can provide coping mechanisms to variable environmental conditions brought on by climate change, without altering genetic makeup, and could induce long-term benefits to individuals’ well-being and survival. Our previous study using larval Zebrafish (Danio rerio) as a model showed potential adaptive responses after 24 hours exposure to high temperature including improved growth, feeding efficiency, and stress and inflammation responses, when the fish were re-exposed to the same temperature later in life. However, the 24hr high temperature treatment also significantly decreased survival shortly after exposure. To improve survival and still induce plasticity, this study aimed to investigate the impacts of a shorter duration of early temperature exposure (12 vs 24hr) on Zebrafish 1) survival and growth, 2) sex ratios, and 3) stress and inflammation, when exposed to similar conditions later in life.
This study was conducted in three phases: environmental programming (EP; 5-7 days post-hatch; dph), common grow-out (8-36dph), and environmental challenge (EC; 37-64dph). Fish were distributed into 12 (9L) tanks at 2dph (149 larvae/tank). The EP phase consisted of three groups: 1) fish at the control optimal temperature of 26°C (LC); 2) fish exposed to 32°C for 12 hours (P12); and 3) fish exposed to 32°C for 24 hours (P24). After EP, all treatments were raised at 26°C during a common grow-out phase. At 36dph, the initial treatment groups (LC, P12, and P24) were equally split and distributed at 54 ± 3 fish/tank for the EC period, a chronic high temperature exposure simulating long-term environmental change. The treatments were either: 1) kept at the optimal temperature of 26°C (LC-O, P12-O, P24-O), or 2) kept at the high temperature of 32°C (LC-H, P12-H, P24-H). All groups were in triplicates. All fish were fed using a restricted feeding approach during EC. Sex ratios were evaluated when fish reached sexual maturation at 93dph.
The was no significant effect of programming temperature (early acute exposure) or challenge temperature (later chronic exposure) on survival or growth during EC. However, there was a significant effect of chronic exposure on FCR (p=0.023). Fish challenged at 32°C (LC-H, P12-H, P24-H) presented lower FCR values. The temperature during EC also significantly affected il1b expression, an inflammatory gene, and hspb1, a heat shock gene, with higher expression across 32°C exposed treatments (LC-H, P12-H, P24-H). Temperature during EC also affected sex ratios, with non-challenged treatments experiencing male-skewed ratios (LC-O, P12-O, P24-O; Fig. 1). These results present contradictory evidence on the effectiveness of early programming in relation to growth. The sex ratio results also conflict with previous literature on temperature sex determination in Zebrafish. Future research should continue to investigate the potential for EP as a viable method to improve responses to changing environments due to climate change. Aspects related to EP effectiveness could be further studied including increasing duration or temperature variability during programming, beginning programming earlier, or utilizing trans-generational plasticity through programming broodstock at higher temperatures.