Like other animals, fish possess an array of digestive enymes by which large macromolecular nutrients are broken down into smaller molecules that can be assimilated. Most fish possess seven main digestive enzymes – trypsin, maltase, amylase, two +aminopepsidases (carboxypepsidase a, carboxypepsidase b), lipase, and alkaline phosphatase. Almost all the major enzymes are present in all fish regardless of their food habits, however, the relative concentration and activity varies according to food preference. Pepsin is localized in the stomach where it functions at optimum pHs between 1 and 4, while the others are found in the intestine at more alkaline pHs. The optimum pH for each enzyme varies with different regions and between different species. In general, the optimum pH for trypsin lies between 6.8–7.8, for carbohydrases 5–7, and for lipases the most alkaline > 7.8. There are two sources of enzymes for the mid-gut – the pancreas and the secretory cells in the gut wall. Since the pancreatic tissue is often diffuse and closely adhering to the liver, portal veins, and gall bladder, it is often difficult to determine the exact origin of many digestive enzymes. Many fish appear to produce amylase and other carbohydrases, but others rely on the activities of gut microflora to supply these enzymes.
Chitinase has already been mentioned as occurring in the stomach, but in some fish it is found only in the intestine where it is secreted by the pancreas. Absorption efficiencies of carnivorous and herbivorous teleosts differ significantly. In carnivorous species the energy absorption efficiency is around 80% (up to 97% for adult sea lampreys feeding on blood), and for herbivores much less, around 40–50%. The only elasmobranchs examined have been lemon sharks (Negaprion brevirostris), which have been found to have absorp- tion efficiencies up to 83%. It is interesting that lemon sharks and carnivorous teleosts should have similar energy absorption efficiency values, for the shark digestive strategy is very different to that of most teleosts. Thus, compared to teleosts, these sharks have a low rate of food intake (1% of body weight per day in the sandbar shark, Galeaspis), extended retention time, greatly increased surface area – with the spiral valve at the hinder end of the gut – and grow slowly.