amphipod enters the egg case of the drill through the operculum, possibly by 

 burrowing, Within the egg case it constructs its mud tube. Whether it eats the 

 young therein has not been determined, but no eggs or live drills were found in 

 cases inhabited by the amphipod. In foyster Bay, Washington, of 62 egg cases 

 which were examined, 23 contained amphipods . 



Utilization of Local Conditions 

 Low salinity 



Since the activity of drills is suppressed by low salinities . . .the exact 

 salinity value varying with the temperature of the water, duration of the low sal- 

 inity, stage of the life cycle of the drill, and the previous salinity history of the 

 drill. . a fluctuating salinity barrier may be said to exist beyond which drills will 

 not be found. According to Glancy (1953) this barrier affords a highly effective 

 natural method of control which has been widely applied, particularly in the culture 

 of oyster seed which thrive in the brackish drill free waters upbay from the salinity 

 barrier Applications of this method have been effective in such estuaries as 

 Delaware Bay, New Jersey (Galtsoff et al , 1937; Stauber, 1943;Engle, 1953) and 

 in the James River s Virginia (Engle, 1953) . Drills tend to slowly repopulate these 

 fringe areas whenever salinities above approximately 12-17 o/oo (seems to vary 

 with the geographic location) and summer temperatures persist for a sufficient 

 time, the former as a result of periods of low rainfall T. C. Nelson (1922) and 

 J. R. Nelson (1931) explain that in Delaware Bay it has been possible to build an 

 industry yielding five million bushels of oysters annually only because of the ex- 

 istence of the natural normally drill free oyster seed beds upbay in water of low 

 salinities . When the young oysters are transplanted to saltier water after about 

 a year their shells have thickened sufficiently to afford more protection against the 

 drill. 



A discussion of the use of low salinities in the control of drills on oyster 

 grounds i s not complete without some mention of the effect of low salinities on 

 oysters, since a difference in salinity of only a few parts per thousand may be 

 sufficient to establish spawning beds of oysters protected from drills by the salin- 

 ity barrier . Loosanoff (1952), in an important contribution on the behavior of 

 oysters ( Crassostrea virgrnica) in water of low salinities, provides this information. 

 Using oysters dredged in waters of salinity 27 o/oo maintained in running sea water 

 of various concentrations and at various temperatures, he found that between 23 and 

 27°C only two oysters out of 50 died in a salinity of 7,5 o/oc in 30 days; at all 

 salinities the rate of survival increased as the temperature decreased, and young 

 oysters resisted unfavorable salinities as successfully as adult oysters. In a 

 salinity of 7.5 o/oo oysters fed normally; started growing, though" slowly; and 

 normal gonad development took place, though oysters with ripe gonads spawned in a 



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