266 THE BIOLOGY OF MARINE ANIMALS 



been detected in a wide variety of animals. Lipases are secreted by the 

 hepatopancreas of cephalopods, the hepatic diverticula of decapod Crusta- 

 cea and the liver of ascidians. In fishes, fat digestion takes place in the 

 intestine through the agency of lipases secreted probably by both pancreas 

 and intestinal mucosa. 



The bile of vertebrates contains bile salts, such as those of glycocholic 

 acid and taurocholic acid, which facilitate hydrolysis and absorption of 

 fats. Bile salts reduce surface tension at fat and water interfaces, and so 

 permit emulsification of the fats. A larger surface area is thus presented 

 upon which the digestive lipases can act. Very little is known about the 

 physiological conditions of fat hydrolysis and absorption in lower animals, 

 but it has been shown that the digestive secretion of decapod crustaceans 

 is capable of emulsifying fats, and in this respect resembles liver bile of 

 vertebrates (7, 72). 



Proteases. Earlier studies on protein digestion were often concerned 

 with mixtures of proteolytic enzymes, and it is only comparatively recently 

 that different kinds of proteases have been recognized in lower animals. 

 Proteases are classified according to effective substrate, optimal pH, and 

 effects of activating and inhibiting agents. 



Peptidases split proteins and complex polypeptides. In this group are 

 included endopeptidases which attack central peptide bonds, e.g. trypsin, 

 pepsin and kathepsins; and exopeptidases which attack terminal peptide 

 bonds. 



Endopeptidases. Trypsin acts in an alkaline medium, from about pH 

 7-9. It is secreted by the vertebrate pancreas as trypsinogen which is 

 activated by an intestinal enzyme, enterokinase. Proteolytic secretions 

 having trypsin-like properties have been identified in Maia (crustacean) 

 and Murex (gastropod), and possibly other forms. Extracts which show 

 proteolytic activity over an optimal pH range of 4-5-6*5 have been noted 

 in a great many invertebrates and have been vaguely characterized as 

 intracellular and extracellular kathepsins. Instances are gastropods, 

 lamellibranchs and echinoderms. Crystallized pepsin has been prepared 

 from fishes as well as mammals, and its properties studied (Fig. 6.7). 

 It is secreted as pepsinogen and becomes converted to pepsin in the acid 

 medium of the stomach. Pepsin is not known among invertebrates (55, 

 56). 



Exopeptidases, including carboxypeptidases and aminopeptidases, 

 attack terminal peptide bonds, and dipeptidases act on dipeptides. These 

 enzymes have been identified in various invertebrates, those which have 

 been studied including Limulus, Maia, Murex, Octopus, etc. 



Sequential digestion by different proteases takes place to a limited extent 

 in vertebrates where distinct peptidases are secreted in the stomach (pepsin) 

 and intestine (trypsin), but in many invertebrates the enzymes are poured 

 into a single chamber and attack the foodstuffs concurrently. Endopep- 

 tidases split the proteins into peptide fragments ; carboxypeptidases and 

 aminopeptidases continue the hydrolysis, attacking the polypeptides from 



