Visceral endothermy results in increased digestive enzyme activities in the shortfin mako shark, Isurus oxyrinchus

Visceral endothermy, the ability to maintain the temperature of visceral organs elevated above the ambient water temperature, is thought to increase the rates of digestion and food processing within the digestive tract and allow thermal niche expansion of a species. I hypothesized that, at in vivo temperatures, the endothermic shortfin mako shark, Isurus oxyrinchus, would have higher specific activities of three digestive enzymes - gastric pepsin and pancreatic trypsin and lipase - than do two species that cannot maintain elevated visceral temperatures, the thresher shark, Alopias vulpinus, and the blue shark, Prionace glauca. The activity of all three digestive enzymes was significantly higher in the shortfin mako at 25°C than in the thresher and blue sharks at 15°C. The thresher shark had the lowest activities for each enzyme. I also hypothesized that the ratio between the enzyme activities at 25°C and 15°C (Q10) would be higher for the mako shark than for the thresher and blue sharks. However, the mako and thresher sharks had similar Q10 values for all three digestive enzymes. The higher in vivo digestive enzyme activities in the mako shark should result in higher rates of food processing. Higher food processing rates may have been a selective advantage for individuals within the populations of ancestral lamnid sharks that could elevate visceral temperatures and may have led to the evolution of visceral endothermy in extant lamnid sharks.