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, e.g. 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. 6.9. 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. 6.10). 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 approximates the pH actually measured 

 in the stomach, and resultant changes in mucus-viscosity may influence 

 speed of digestion. 



