378 U. S. BUREAU OF FISHERIES 



which seed and adult oysters from the inshore reefs will be distributed 

 in the sprino; to serve as spawners. 



Destruction of oysters hy 'parasitic flat worm.. — A parasitic natural 

 enemy of the oyster known as the " wafer " or so-called " leech " has 

 become a serious menace to the continued natural production and 

 private cultivation of this shellfish in Apalachicola Bay, Fla. In- 

 vestigations conducted by Dr. Herbert F. Prytherch, in cooperation 

 with the Florida Department of Conservation, show that this pest 

 in the last 3 years has completely destroyed the seed and adidt oysters 

 on several of the best natural beds and is spreading rapidly to other 

 valuable areas. The parasite is a turbellarian flatworm, Stylochus 

 ini77iicits, measuring from one-half to three-fourths inch in diameter, 

 which enters the shell of the oyster and feeds gradually upon the 

 meat until it has killed its host. Since the fall of 1932 the parasite 

 has destroyed the oysters on five natural beds comprising an area of 

 800 acres on Avhich a crop estimated at 350,000 bushels, having an 

 approximate value of $175,000, has been lost. Recently this natural 

 enemy has spread to other beds at a distance of 12 miles from the 

 original outbreak and to a nearby oyster reef, which is the largest 

 in this region and contains nearly half of the entire crop. The 

 Bureau's investigations show that the rapid growth, reproduction, 

 and spread of the leech in Apalachicola Bay is associated with an 

 unusually dry period, a considerable decrease in river discharge, and 

 an increase in the salinity of the coastal water of this region. It is 

 desirable that extensive dredging operations be undertaken to destroy 

 the leech on its principal breeding areas and that scientific studies 

 be made of this parasite, which has been recently found on oyster 

 beds from North Carolina to Texas. 



PROTOZOAN PARASITE OF VIRGINIA AND LOUISIANA OYSTERS 



Oyster planters in Mob jack Bay, Va., and in Terrebonne Parish, 

 La., suffered a loss of their stock amounting to over a million dollars 

 during the winters of 1929-30 and 1932-33, respectively. Cytological 

 studies of oysters from both regions conducted by Dr. H. F, 

 Prytherch show that they were heavily infested with a parasitic 

 protozoan, which tentatively has been identified as one of the 

 Haplosporidia. This parasite was particidarly abundant in the tissue 

 of the adductor muscle, mantle, and gills, and was frequently found 

 in the muscle of the heart. Representative samples of oysters from 

 the mortality areas showed in every case a heavy concentration of 

 this protozoan, the number ranging from approximately 50 thousand 

 to 1 and 2 million per oyster. 



In the inunediate vicinity of the fisheries Inboratory at Beaufort, 

 N. C, it has been observed for several years that a large number of 

 oysters die at an age of II/2 to 2 years, following a period of abnormal 

 shell growth, similar to that observed in the oyster mortalities in 

 Virginia and Louisiana. Recent cytological studies have shown that 

 these oysters are also heavily infected Avith the same protozoan para- 

 site. In all three localities the mortality was highest in oysters grown 

 under croAvded conditions in Avhich large numbers of this minute 

 parasite Avere found. From a practical standpoint it is apj^arent that 

 oysters should be planted sparsely to prevent reinfection and spread 



