Conn. (Loosanoff, 1956). Apparently it has no 

 difficulty in entering oyster spat through the 

 slightly opened valves. On the Pacific coast, the 

 flatworm Pseudosti/lochus ostreophagus Hyman 

 (Hyman, 1955) was reported to cause mortalities 

 of from 6 to 42 percent among the imported 

 Japanese seed oysters on various grounds. The 

 worm bores keyhole perforations in the shells of 

 young oysters (Woelke, 1957). 



Crabs 



Ryder (1884) was the first to include the blue 

 crab, Callinectes sapidiis Rathbun, and the com- 

 mon rock crab. Cancer irroratus Say, in the list 

 of oyster enemies. He quoted complaints of oys- 

 termen working in Great South Bay, Long Island, 

 N.Y., who stated that the crabs eat small oysters 

 up to the size of a 25-cent com and invade the 

 the oyster planting grounds. 



For many years crabs were not mentioned in 

 oyster literature as potential enemies, but in the 

 1930's and 1940's there were reports from the 

 U.S. Bureau of Fisheries Biological Laboratories 

 at Milford, Conn., and at Pensacola (Gulf Breeze), 

 Fla., that under certain conditions the blue crab, 

 the rock crab, and the green crab, Carcinides 

 moenas (Linneaus) destroyed oysters kept in 

 outdoor tanks or placed in baskets with the crabs. 

 Lunz (1947) reported that at Wadmalaw Island, 

 S.C., the blue crab was probably the most serious 

 pest in 1946 and destroyed more than 80 percent 

 of the young oysters set on collectors. The 

 crab's diet includes a great variety of food, includ- 

 ing oysters. There is no evidence that they are 

 attracted specifically by oysters, but it is apparent 

 that they may destroy many small oysters in 

 clusters by cracking their shells. 

 Mud prawns and fish 



Brief mention should be made of the family 

 Calianassidae (genera Upogebia and Callianassa), 

 popularly known as "mud prawns" or "burrowing 

 shrmips", which excavate deep burrows under 

 oyster bed dikes. This activity drains water 

 from the grounds, exposes the beds of 0. lurida, 

 and smothers the young oysters with material 

 thrown up in burrowing (Stevens, 1928). 



In the southern waters of the Atlantic coast, 

 oyster beds are often invaded by schools of 

 black drum, Por/oiuas cro7ms (Linnaeus), which 

 feed on mollusks and occasionally cause extensive 

 destruction of oysters, leaving behind piles of 



crushed shells. The fish uses its powerful pharyn- 

 geal teeth to crush the shells (fig. 398). 



The diamond stingray of the Pacific coast, 

 Dasyafis dipterurus (Jordan and Gilbert), also 

 devours oysters, crushing them with powerful 

 teeth. To ward off attacks by this fish, oyster 

 grounds in California are surrounded by high 

 fences, a practice used for the same purpose by 

 French oystermen. 



Birds 



Various species of ducks are enemies of small 

 0. lurida of the Puget Sound area. The extent of 

 damage to oyster grounds near Olympia, Wash., 

 was estimated in the fall of 1928 by the United 

 States Biological Sm-vey. McAfee, who con- 

 ducted the field studies, reported (quoted from 

 GaltsofT, 1929) that at that time 87 percent of 

 the bluebills {Nyroca marilla and A'', affinis) fed 

 principally on oysters, which comprised 80.5 

 percent of the bulk of the food found in their 

 stomachs. In 38 percent of white-winged scoters 



A. 



B. 



Centimeters 



Figure 398. — Pharyngeal teeth of small size black drum 

 P. cromis, used for crushing oyster shell. A — upper 

 teeth; B — lower teeth. 



FACTORS AFFECTING OYSTER POPULATIONS 



439 



