. The biology of marine animals. Marine animals; Physiology, Comparative. 270 THE BIOLOGY OF MARINE ANIMALS to find the anterior region (of reception) nearly neutral, stomach and mid- gut acid, and intestine more alkaline, but this is not invariably the case. In any event enzyme activity is confined to certain regions where the pH becomes significant for that particular enzyme, amylolytic action in the stomach and digestive diverticula of bivalve molluscs. Conditions in fishes are somewhat peculiar, since the stomach produces a highly acid secretion (containing free HC1) with a variable p
. The biology of marine animals. Marine animals; Physiology, Comparative. 270 THE BIOLOGY OF MARINE ANIMALS to find the anterior region (of reception) nearly neutral, stomach and mid- gut acid, and intestine more alkaline, but this is not invariably the case. In any event enzyme activity is confined to certain regions where the pH becomes significant for that particular enzyme, amylolytic action in the stomach and digestive diverticula of bivalve molluscs. Conditions in fishes are somewhat peculiar, since the stomach produces a highly acid secretion (containing free HC1) with a variable pH that may drop to 3 or less in different teleosts and selachians. Pepsinogen (the precursor of pepsin) and acid are apparently produced by the same gastric cell, and the pH optima for pepsin lie well on the acid side of neutrality (from 1-5. 10 40 60 2 3 4-56 Temperature (°C) pH Fig. Digestion of Starch by the Style Amylase of Ensis siliqua {Left) Effect of temperature on digestion. Digest, pH 6, duration of experiment five hours. {Right) effect of hydrogen ion concentration on digestion. Temperature 22°C; time of digestion 18 hours. Ordinates, milligrammes reducing sugars liberated. (From Graham (28).) to 2-9 depending on the source and substrate) (Fig. ). On passing into the duodenum the gut contents become alkaline (pH 7-9), in which range pancreatic trypsin shows maximal activity (pH 7-5-8-5). In stomachless fish, such as Fundulus, the foodstuffs pass directly from the oesophagus into the alkaline duodenum and neither pepsin nor acid are secreted (11). Some other effects of hydrogen ion concentration may be noted briefly. The acidity of the vertebrate stomach has a bactericidal action and is also responsible for decalcification of skeletal structures in the food. The chang- ing pH values in different gut regions may be of importance in altering the viscosity of mucus, especially in ciliary-feeding animals. The isoelectric point of mucus in lamellibranchs approxi
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