268 



THE BIOLOGY OF MARINE ANIMALS 



(Strombidae) are provided with a cellulase. These animals crop fine algae 

 and their cellulolytic enzyme will not only aid digestion by breaking down 

 cell walls, but will also provide an additional source of glucose. Recent 

 work shows that the crystalline style of various lamellibranchs (Mytilus, 

 Ostrea, etc.) possesses cellulolytic activity. The particulate food of these 

 animals contains dinoflagellates and a cellulase would appear to be 

 necessary for digesting the walls of algal cells. 



The seaweeds found on the shore and in shallow waters contain an 

 abundance of polysaccharides other than cellulose, and these form a 

 considerable proportion of the diet of many littoral animals. A hemicel- 

 lulase (lichenase) is widespread among invertebrates, and is known to 



8 



16 



40 



48 



24 32 



Time (hours) 



Fig. 6.8. Rates of Digestion of Various Substrates by Intestinal 

 Extracts from the Sea Urchin Strongylocentrotus purpuratus 



Substrates: 1 % casein at pH 6-8 and 30 C C; 1 % boiled starch, 0-1 % agar, and 002% 

 iridophycin at pH 6-8 and 30°C. (From Lasker and Giese (41).) 



occur in sponges, annelids, echinoderms, molluscs, arthropods and as- 

 cidians. Lichenase hydrolyses hemicelluloses (xylans, arabans, mannans, 

 galactans, etc.). There is some doubt whether the hemicellulases and 

 cellulases of some animals may not be identical. The ormer or abalone 

 Haliotis (a prosobranch gastropod) is said to possess enzymes capable of 

 digesting agar and alginic acid found in the brown algae on which it feeds. 

 In the sea-urchin Strongylocentrotus, on the contrary, there are no special 

 enzymes for attacking agar, nevertheless intact algal tissue disintegrates in 

 the intestine (Fig. 6.8). This effect is due to an intestinal bacterial flora, 

 and presumably, in some manner as yet unknown, it assists the digestive 

 processes of the urchin. 



A digestive chitinase has not been reported for marine animals, although 



