614 U. S. BUREAU OF FISHERIES 



sand, one sack of cement, and one-fourth to one-half sack of slacked 

 lime. 



When the seed oysters on the partitions had reached an age of 

 11/^ to 2 months, they were transplanted from the shallow inshore 

 areas to the deeper waters of Long Island Sound. It was found that 

 the partitions could be easily broken apart and the seed oysters 

 separated by simply forcing the collector together by pressure across 

 any two diagonally opposite corners. This required but a single 

 operation which detached a large number of the seed and broke up 

 the partition into GO small squares, 2 by 3 inches. The oysters 

 on many of these squares were kept under observation and were 

 found as they grew larger to gradually break apart and separate as 

 single individuals. After the partitions are broken up into squares, 

 the seed oysters can be easily shoveled back on deck in a pile and 

 then thrown overboard on suitable growing grounds. 



PROTOZOAN PARASITE OF OYSTER GILLS 



Many oysters from Milford Harbor, Conn., were found to have 

 decidedly undersized gills with frayed, ragged edges, which indicated 

 that they were being injured, or had recently recovered from dam- 

 age, by some unknown agency. A cytological study of these speci- 

 mens by Mr. Prytherch at the University of Pennsylvania showed 

 that the gill tissue had been invaded by ciliate protozoa, which 

 attacked the delicate, nonciliated cells lining the water tubes and 

 epibranchial cavities. In these cells the protozoa were found to be 

 reproducing rapidly and destroying this thin layer of tissue which 

 separates the blood vessels of the gills from the sea water that con- 

 tinually passes over it. The portion of the gill in which the para- 

 sites were found is supposed to function in the oxygeneration of the 

 blood so that injury in this region would not only affect the respira- 

 tion of the oyster, but would also indirectly bring about destruction 

 of the ciliated cells and gill filaments by disrupting their blood 

 supply. The parasite is a ciliate protozoan of the order Hetero- 

 trichida, and varies in length from 8 to 10 microns at the time 

 of first invasion to 15 to 25 microns when full grown. It was 

 found to be present in the gills of spat only 1 month old and in 

 adult oysters of various ages. In a few oysters over 50 per cent 

 of the gill tissue had been destroyed, so it is evident that this parasite 

 may easily interfere with the respiration, retard growth or cause 

 death of the oyster. 



CAUSES or OYSTER MORTALITY IN VIRGINIA 



During the winter of 1929-30 the oyster planters of Virginia, par- 

 ticularly those in Mob jack Bay and York River, suffered from some 

 unknown cause a heavy loss of their stock, amounting to over one- 

 half million dollars. 



As a result of this high mortality of oysters and a decline of over 

 3,000,000 bushels in the annual production from 1904 to 1927, the 

 State of Virginia requested the U. S. Bureau of Fisheries to under- 

 take an investigation of this valuable sea food resource. 



