| Fish are ectothermic animals,which will be directly or indirectly affected by the water temperature.Each type of fish has a range of temperatures in which it is best suited to grow.Within this temperature range,their growth and survival rates are optimal.The growth of fish will increase rapidly with the increase of temperature until the optimal growth temperature is reached.When temperatures exceed the optimal growth temperature,their growth slows and then stops.If the temperature continues to rise,the fish will die.The rise of global temperature has a profound effect on the metabolism and survival of fish.Although there have been many studies on the acute lethal effects of heat on fish,little information is available on the effects of heat stress on the survival and Routine metabolism rate of fish caused by climate change.Temperature is an important environmental factor affecting the metabolism and growth of tilapia.Tilapia has strong adaptability to the environment,simple feeding,low oxygen tolerance,strong disease resistance and fast reproduction.It is a fish living in high water temperature.Fish have higher adaptability to high temperature than low temperature.Tilapia is one of the most important economic cultured fishes in China,and occupies a very important position in China’s aquaculture.Tilapia is mainly cultured in southern China,such as Hainan,Guangdong,Fujian,Guangxi,are tilapia cultivation provinces.The study found that the summer water temperature in these provinces exceeded 33℃for about 40 days,often resulting in loss of appetite and slow growth rate of tilapia.Therefore,to study the effect of high temperature on tilapia,to increase the survival rate of tilapia,to improve tilapia breeding efficiency in summer,to reduce water pollution caused by surplus feed,etc.,it has important practical significance in production.In this study,F1 progeny were generated by a one-to-one cross of Gift tilapia.400larvae from F1 family were stocked in 144 L tanks（28±1°C）and fed with a commercial diet for 42 days to acclimate them to the culture condition before the trial started.After42 days in captivity,the young tilapia were subjected to temperature treatments at 28°C（optimal）,32°C（slightly above optimal）and 36°C（thermal stress）（8parallel/30 fish each parallel）,105 days of domestication.At the end of temperature treatment,acute hypoxia test and respiratory metabolism test（three parallel experiments at each temperature）were conducted on tilapia in three temperature treatment groups.At the end of treatment,the mean survival rate,routine metabolism and tolerance to hypoxia were significantly reduced while the respiratory rate was increased in 36°C groups compared to 28°Cand 32°C（p<0.05）.Then the gills were fixed with 4%PFA in three temperature-treated groups under normal oxygen condition and the gills were fixed with 4%PFA in three temperature-treated groups under the condition of acute hypoxia.Gill histopathological changes including curling of lamellae and the hyperplasia of the end of filament were observed in the 36°C group,and these morphological deformity became more obvious after acute hypoxic treatment（0.2mg/L O2）.In addition,the transcriptome technology were applied to investigate the molecular regulation mechanisms of GIFT tilapia under long-term thermal stress.In the comparison of the three temperature acclimated groups,differentially expressed genes（DEGs）were significantly enriched in:DNA replication,metabolic pathways,mismatch repair,nucleotide excision repair,protein processing in endoplasmic reticulum,citrate cycle（TCA cycle）,etc.Noteworthy,nine genes involved in the TCA cycle pathway were significantly down regulated at both high temperature acclimated groups and acute hypoxic groups.It is suggested that there is a comparable correlation between the molecular regulation of high temperature and hypoxia in Gifu tilapia.The results show that long term high temperatures can have a profound effect on fish,especially if they are already living in high temperatures close to their upper limit,may be more susceptible to temperature increases. |
