| Digestive enzyme activities were measured in two tuna species---albacore tuna (Thunnus alalunga), a species that warms both the muscle and viscera---and yellowfin tuna (Thunnus albacares), a species that warms its muscle but not its viscera---and in two closely related ectothermic scombrid species, eastern Pacific bonito (Sarda chiliensis) and chub mackerel (Scomber japonicus). I tested the hypothesis that endothermic tunas have a higher enzymatic activity at their elevated visceral temperature (25°C) compared to their ectothermic relatives at their in vivo temperatures (15--20°C). The specific activities of pepsin in the stomach and of lipase in the caecal mass and intestine were measured at 15°C and 25°C, and trypsin activities in the intestine and caecal mass were measured at 20°C and 25°C. Even though the albacore tuna maintains elevated digestive tract temperatures resulting in high digestive enzyme activities, none of the enzymatic activities were higher in albacore tuna at 25°C than in the other species studied. Histological analyses showed that pancreatic tissue is dispersed throughout the caecal mass. The albacore tuna had a significantly higher percentage of pancreatic tissue in the caecal mass than the mackerel, yet it had lower caecal mass digestive enzyme activities per gram of tissue. The relative masses of the three digestive organs studied were highest in the chub mackerel. Therefore, this study does not provide support for the hypothesis that increased digestive enzyme activity is a selective advantage of visceral endothermy. |
