The Visceral Temperatures of Mackerel Sharks (Lamnidae)

The viscera of mako Isurus oxyrinchus and porbeagle Lamna nasus sharks are significantly warmer than the water. The large suprahepatic rete mirabile in these fish is identified as a heat exchanger which retains metabolic heat to warm the visceral organs. An argument is presented that the efficiency of heat exchange in the rete must be greater than 97% to maintain the observed tissue temperatures and that this requirement explains the large size of the rete. A venous passage with a muscular wall which runs through the suprahepatic rete could allow blood to bypass the heat exchanger, and its presence suggests a simple mechanism for controlling temperature. However, measurements of temperature by acoustic telemetry from freeswimming sharks over a 4.5-day period do not give a convincing indication that stomach temperature is altered in a manner independent of the environment.