l6o BULLETIN OF THE BUREAU OF FISHERIES. 



NUTRITION BY SEAWEEDS AND PROTOZOA. 



The nutrition of oysters by fragments of seaweed was also studied. This idea seems 

 to have been little considered because of the generally prevailing view that algse and 

 other microscopic life constitute the food of oysters." In these experiments oysters 

 were fed in glass jars of aerated water with sea lettuce (Ulva lactuca) chopped into fine 

 fragments and added fresh at intervals of one or two days. It was found that the ma- 

 terial had to be in fresh condition, because if it darkened and decomposed, or if the water 

 in the containers was not changed frequently enough, the oysters would die. That this 

 material was ingested was shown by repeated microscopic observations of the stomach 

 contents of oysters so fed. All the sea water used was carefully filtered through several 

 thicknesses of fine filter paper so that, as shown by subsequent microscopic examina- 

 tion, it was freed from algae. It is true that some algse clung to the sea lettuce, and 

 Protozoa and bacteria from the same source were also present in the preparation. The 

 multiplication of these organisms pro\-ided, therefore, a part of the nutrition for the 

 oysters. Several experiments were started, but only one was successfully continued 

 long enough to be of value. The result is shown in Table 6. It indicates that the sea- 

 weed fragments contributed to the nourishment of the oyster. 



Whether accumulations of seaweed on oyster beds may cause the death of oysters is 

 a question of practical interest to oystermen. Gorham '' observed the death of oysters on 

 beds where seaweeds in which Ulva predominated had lodged in masses sufficient to 

 cause putrefaction. In his opinion the seaweed was the cause of death. 



The following experiments on this point were undertaken: Wooden boxes about 

 30 inches square and 3 inches deep were filled with sand and gravel, and 25 oysters 

 were laid on top of the gravel in each box. A basket of galvanized wire netting was 

 made so as to fit over the top of each box with a space of 3 or 4 inches above the oysters. 

 Two such boxes, one with the wire basket loosely filled with seaweeds and the other 

 without them, were then anchored together in each of several locations on the shores of 

 Narragansett Bay in waters suitable for oyster culture. The seaweeds used were mostly 

 Ulva with some eelgrass. In one case Ulva alone was used. Four such experiments 

 were carried out. In two of them the oysters were taken up for observation after 29 

 days, in one after 25 days, and in one after 14 days. In every case the oysters under 

 seaweeds were found to be dead and badly decomposed, with the meats running out 

 of the shell. Of the 100 oysters so treated only 7 were found alive and 6 of these had 

 discolored meats showing signs of incipient putrefaction. All the control o>-sters showed 

 no pathological conditions or signs of decay, and from the new shell formation during 

 the progress of the experiment, showed abundant e^idence of flourishing growth. It 

 would seem, then, that although seaweed may ser^'e as food for oysters, accumulations 

 of it in places where the tide does not keep it sufficiently in motion must be guarded 

 against in oyster culture. 



The nutrition of oysters by Protozoa was also investigated. Rich cultures of various 

 Protozoa were added daily to a dish of filtered, aerated sea water containing oysters. 

 As algcC were excluded from this experiment, only Protozoa and bacteria, but with 

 Protozoa predominating, ser\'ed as food for the oysters. They appeared after 17 days, 



o Brooks, William K.: The 0>*steT. 335 p., 1905. Baltimore. 



b Gorham: Annual Report, Rhode Island Shcll&sh Commission. 1914. 



