Endothermy is the ability to generate and conserve metabolic heat to maintain body temperature above that of the surrounding environment.Endothermy enhances the physiological and ecological advantages of mammals,birds,and certain fish species.The opah,Lampris megalopsis(Lampridiformes),is the only known fish to exhibit whole-body endothermy.Currently,however,the underlying molecular mechanism for this remains unclear.Hence,the opah offers an excellent opportunity to study the evolutionary mechanism of whole-body endothermy in aquatic animals.In this study,we assembled a L.megalopsis genome(1.09 Gb in size)and performed comparative genomic analysis with ectothermic fish to reveal the genetic basis of endothermy.Based on analysis of positive selection,rapid evolution,and gene family expansion,we discovered several genes that likely contributed to thermogenesis and heat preservation.As the first reported L.megalopsis genome,our results not only clarify the possible molecular and genetic mechanisms involved in endothermic adaptation but also increase our understanding of endothermic fish biology.