In a study relating depth of perforation to length of shell of oysters, 

 Pope (1910-11) observed that holes 0.5 mm. deep were confined to oyster shells 

 not exceeding 3 .8 cm. in length, and perforations i .0 arum . and over were limited 

 to valves not exceeding 5 . 1 cm . Holes 3 .0 and 4.0 mm . deep were found in the 

 vicinity of the adductor muscle in oysters ranging in length from 3 5 to 4.4 cm. 



Apparently most rapidly growing oysters under 113 cm. in length are not 

 immune to the attacks of oyster drills (Pope, 1910-11), although larger so called 

 "dumpy" or thick shelled slowly growing oysters go unharmed. Pope noted that 

 whereas in the laboratory drills perforated oysters as large as 11.3 cm. in 

 length, he found no drilled oysters in the field larger than 7 cm . in length . He 

 states that the greatest mortality in the field occurs in oysters with an average 

 length of 3 ,2 cm. Since the gaping valves of smaller dead oysters are more 

 readily lost it appears that Pope failed to notice the great rate of destruction that 

 occurs among smaller oysters. NewcomJbe (1941-42) relates that in the laboratory 

 in no case did small drills kill medium or large oysters . In one instance drills 

 21 mm. in height did not attack oysters averaging 8. 5 cm, in length. Federighi 

 (1931c) and Stauber (1943) say that occasionally oysters over 8 cm. in length are 

 drilled, probably by larger drills. In laboratory investigations Stauber watched 

 newly hatched drills, 1.0 to 1.5 mm. in height, successfully attack oyster spat 

 up to 4 mm . in length. He adds that in the field drills under 15 mm . in height can 

 successfully drill oysters at least 2.5 cm. long. Cole (1942) in laboratory observa- 

 tions found that drills 3.5 mm. in height readily penetrate English oysters ranging 

 in diameter from 3 to 7 mm . 



Urosalpinx a ttack their prey with considerable pertinacity. (Pope 1910-11) 

 states that they frequently return to perforations which they begin. Orton 0930) 

 describes a case in which a drill resumed its position at a drilling site after four 

 interruptions during which it was removed a few centimeters from the perforation, 

 According to Federighi (1931c), Galtsoff et al. (1937), and Andrews and McHugh 

 (pers. com.), more than one drill may attack an oyster at the same time. Even 

 if one drill succeeds in piercing the oyster shell before another, the unsuccessful 

 drill continues rasping. A maximum of four distinct perforations on one oyster 

 shell is reported. In the field Pope found very few oysters in which the perfora- 

 tion was incomplete; he attributes the cessation of drilling to interruption by 

 oyster cultural operations . Andrews and McHugh (pers . com .) observed a number 

 of instances where a drill missed a living oyster and drilled instead some distance 

 into empty shell beneath. 



In the laboratory Urosalpinx continues to drill uninterruptedly throughout 

 the daily (diel) cycle, apparently unaffected by the alternation of day and night 

 (Pope, 1910-11). 



53 



