test of feeding the oysters inert materials such as 

 carmine powder, carborundum, clay, pulverized 

 williamite, and colloidal carbon would show that 

 these undigestible nuiterials, if fed gradually and 

 not in excessive quantities, are swallowed and pass 

 through the digestive tract. The fluorescent 

 mineral williamite, which I used extensively in my 

 studies, is particularly suitable for this purpose 

 because it permits easy detection of the most 

 minute granules of the mineral inside the intestinal 

 tract or in the feces when illununated by ultra- 

 violet light. The fact that some of the micro- 

 organisms found in the stomach are not destroyed 

 and can be recaptured alive in the feces has been 

 known for a long time. The dinoflagellate 

 Prorocentrum micans was seen by Blegvad (1914, 

 p. 47) to pass unharmed. Living Chlorella and 

 Nitzschia closterium given to C. virginica in large 

 quantities can be recovered alive from the feces 

 and recultured (Loosanofl" and Engle, 1947). In 

 studies of the effect of feeding oysters in the labora- 

 tory I frequently used a light suspension of 

 Fleishmann's yeast, and observed that such a 

 large number of yeast cells passed undigested that 

 the feces acquired a milky color. Thus, the 

 presence of an organism or its remnants in the 

 alimentary tract in itself is not a proof that it is 

 being used by the oyster as food and that it has 

 nutritive value. Neither the enumeration of the 

 organisms found in the stomach nor the deter- 

 mination of their volume gives satisfactory quanti- 

 tative data. It is at present impossible to judge 

 whether, for instance, one cell of Coscinodiscus 

 equals or differs in nutritive value from a single 

 cell of Pleurosigma, Skelefonema, Nitzschia, or 

 other forms. Information is lacking about the 

 caloric value and chemical composition of various 

 forms and, therefore, it is impossible to determine 

 the number that should satisfy the energy re- 

 quirements of the oyster. 



Through trial and error oyster growers know 

 that certain grounds in their possession are partic- 

 ularly suitable either for the growth or for 

 fattening and conditioning of oysters for market. 

 Sometimes a great difference in the productive 

 capacity of grounds may be found in the two 

 areas located a short distance apart. In an 

 ecological survey of the bottom it is relatively 

 easy to detect conditions which are unsuitable for 

 growth. It is, however, impossible at present to 

 evaluate the potential productivity on the bottom 



234 



because of the inadequacy of our knowledge of the 

 nutrition of the oyster. 



ARTIFICIAL FEEDING 



So far only a few experiments on artificial 

 feeding reported in the literature were successful 

 in producing an increase in the weight of the 

 oysters. As a rule oysters kept in the laboratory 

 show lack of nutrition and die sooner or later. 

 Better results may be obtained by keeping them 

 in large outdoor tanks adequately supplied with 

 sea water which has not been stored for any 

 length of time. Experiments by Martin (1927a, 

 1927b) in feeding oysters with pure cultures of 

 plankton forms resulted in very poor growth. 

 Sparck (1926), experimenting with Zosiera as a 

 potential food for the European oyster, emphasized 

 the fact that oysters "may thrive, increase in 

 size and even spawn in very small limited water 

 volumes without any renewal of water worth 

 mentioning." Such conditions occur in the Nor- 

 wegian oyster basins and in the French "parks" 

 which, however, must contain "some source 

 producing nourishment in sufficient quality and 

 quantity." This material presumably may derive 

 from the organic detritus. He also reports 

 that in his experiments the "development of 

 bacteria did not seem in any way to hurt the 

 oyster, rather the opposite." 



A unique experiment, unfortunately not well 

 known to biologists, was made by Gavard (1927, 

 quoted from Korringa, 1949) in Algiers. He fed 

 the oysters an artificial detritus prepared from 

 animal and plant material and obtained an increase 

 of 15 kg. per 1,000 oysters per season. Korringa 

 states that these results demonstrate the ability 

 of the oysters to gi-ow without using living organ- 

 isms as food. Without access to Gavard 's 

 original paper it is impossible to judge if the 

 detritus was directly consumed by the oysters 

 as food or whether it stimulated the growth of 

 bacteria and nannoplankton. 



Artificial enrichment of sea water by adding 

 commercial fertilizers at one time seemed to be 

 a simple answer to the problem of providing in- 

 creased food supply to the oyster. To test the 

 idea a series of experiments was conducted in the 

 Bureau of Commercial Fisheries Biological Labora- 

 tory at Milford, Conn., which resulted in the 

 interesting discovery that an excessive concentra- 

 tion of microorganisms {Chlorella sp., Nitzschia 

 closterium, Prorocentrum triangvlatnm, Euglena 

 viridis) adversely affects the feeding of oysters 



FISH AND WILDLIFE SERVICE 



