272 



THE BIOLOGY OF MARINE ANIMALS 



generally lie below 30°C and temperature optima above such a figure have 

 no physiological significance. Food passes through the gut slowly, and the 

 time thus consumed may correspond to the period for maximal enzymatic 

 action at the normal environmental temperature. In Tethyum food takes 

 35 hours to pass through the alimentary canal at 15°C. Purified pepsin, 

 which has now been obtained from several species offish, still shows appreci- 

 able activity at the low temperatures which the fish encounter in nature 



840 



100 



- 50hourSj 



560 



o 



so 





280 



140 



26 hours 



10 hours. 



3hours 



10 20 30 40 50 



Temperature C°C) 



Fig. 6.11. Effect of Temperature on Digestion by the Protease 

 of Haloeynthia {=Tethywn). (From Berrill (15).) 



(Fig. 6.7). Since food remains for a long time in the stomach (some 18 

 hours in Pleuronectes), there is opportunity for prolonged pepsin hydrolysis 

 (6,7, 15, 55). 



Temperature certainly influences other processes besides enzyme hydro- 

 lysis, and thus affects total digestion time. A rise in temperature increases 

 gut motility and probably influences rates of secretion and absorption (53). 



Factors controlling Secretion 



The secretion of enzymes is concerned with the digestion of food and it 

 is not surprising to find that the mode of secretion is related to feeding 

 habits. In lamellibranchs, as we have noticed, feeding is continuous while 

 the valves are open, and depends on ciliary mechanisms. The best-known 

 extracellular enzyme is an amylase, which is continuously released by 



