section_path: Anatomy and physiology/Endothermy
section_name: Endothermy
paragraph_text: Endothermy
  
  Most fish are exclusively cold-blooded or ectothermic. However, the Scombroidei are warm-blooded (endothermic), including the billfishes and tunas. The opah, a lampriform, uses whole-body endothermy, generating heat with its swimming muscles to warm its body while countercurrent exchange minimizes heat loss. Among the cartilaginous fishes, sharks of the families Lamnidae (such as the great white shark) and Alopiidae (thresher sharks) are endothermic. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the bluefin tuna and the porbeagle shark, which maintain body temperatures more than 20 °C (68 °F) above the ambient water.