detritus and that "the animate food (i.e., living 

 microorganisms) never exceeded 10 percent of the 

 total" (by volume). He also advanced a hypoth- 

 esis which, however, lacks experimental con- 

 firmation, that growth of Oxford oysters was due 

 mainly to the inanimate food (detritus) and that 

 fattening was caused by diatoms (Nitzxchiella 

 longwH'ima f. parva). He found no evidence of 

 selection of food by the oysters and commented 

 that the actively feeding oyster appears to ingest 

 anj^thing that it can capture. 



The extreme view that phytoplankton is of no 

 direct significance as food of 0. edulis in Danish 

 water was expressed by Blegvad (1914), who 

 classified tliis moUusk as a "pure detritus eater." 

 Phytoplankton, according to liis view, contributes 

 tf) the food only as part of the detritus after tiie 

 death of the algae. 



Petersen and Jensen (1911) attributed great 

 importance to eel grass, Zostera, as a possible 

 source of food for bottom organisms. On the 

 basis of their observations Sparck (1926) ex- 

 perimented with 0. edulis, which he kept in a 

 tank with sea water to which he added a liberal 

 supply of old brown Znstera. Examination of tlie 

 stomach contents of these oysters showed many 

 species of flagellates and some Zostera detritus, 

 but the cjuaiitity of the latter was by no means 

 greater than in the oysters from the natural 

 bottoms in the fjord. Decaying Zostera probably 

 fertilized tlie water and stimulated the growth of 

 the plankton. Danish investigators emphasized 

 the fact that pentosan released from the decaying 

 Zostera is a principal source of organic food for 

 bottom invertebrates. The substance is ap- 

 parently useless to oysters because they are unable 

 to digest it, as has been shown by Yonge's experi- 

 ments (1926a). The cjuestion of tlie extent of 

 utilization by the oyster of the organic detritus 

 which is always present in its natural environment 

 has not yet been settled. 



Naked flagellates and infusoria are frecjuently 

 found in the contents of the alimentaiy tract. 

 Under tlie influence of gastric fluids these forms 

 are rapidly destroyed and, therefore, cannot be 

 enumerated with any degree of certainty. The 

 same problem applies to the bacteria which reacli 

 the alimentaiy canal. That tliey may play a 

 considerable role in the feeding of lamellibranclis 

 is indicated by the experiments of ZoBell and 

 Landon (19:57), and ZoBell and Feltham (193s). 

 with the California mussel, which was fed known 



amounts of red coccus and a spore-forming 

 bacillus. Within 3 hours the mussel removed 

 about 200 million bacteria per 1 ml. of water. 

 The microorganisms were actually ingested and 

 after 6 hours disappeared from the digestive 

 tract. In 9 months the mussels which were fed 

 red coccus gained an average of 12.4 percent, the 

 bacillus fed animals gained 9.7 percent, and the 

 fasting mussels, kept as controls, lost about 6.8 

 percent. These experiments suggest an explana- 

 tion of tlie observations by Kincaid (1938) that 

 oysters kept for several months in water with 

 nothing to feed on except bacteria appeared to be 

 normal and even increased their glycogen content. 

 Kincaid's experiment should, of course, be re- 

 peated and the cpiestion of the role of bacteria 

 should be adequately studied before a conclusion 

 can be made of their significance in the feeding of 

 oysters and other bivalves. 



By feeding the oyster known concentrations of 

 coliform bacteria, Galtsoff and Arcisz (1954) 

 found that 15 minutes after the start of addition 

 of the culture the two oysters retained from 21 to 

 49 percent of Esrherischia coli available in sea 

 water. The accumulation of bacteria soon I'eached 

 the point at which no more microorganisms were 

 retained and the effluent leaving the oysters 

 contained more E. coli than the surrounding 

 water. Retention and elimination of microorgan- 

 isms are probabty associated with the secretion 

 and discharge of mucus by the gill epithelium. 

 These results confirm the previous observations 

 by Galtsoff (1928) that over 50 percent of the 

 bacteria pass through the gills and that only a 

 fraction of then- total number is retained. 



The organisms found in the stomach of the 

 oyster reflect the composition of plankton and 

 nannoplankton present in the surrounding water. 

 Selection is made primarily by the size and shape 

 of food particles, although the ability of the 

 oyster to discriminate between two suspensions 

 of microorganisms of different colors but of the 

 same size was suggested by I^oosanoff's experi- 

 ments (1949). A more detailed study should be 

 made, however, before the discriminating ability 

 of the oyster is confirmed. 



There are several weaknesses common to all the 

 studies on the feeding of oysters. The conclusions 

 are based on examinations of the contents of the 

 stomach and composition of feces without giving 

 proper consideration to the nutritive value of 

 different forms and their digestibility. The simple 



ORGANS OF DIGESTION AND FOOD OF THE OYSTER 



233 



