"Non-replacement of dead food" in- 

 dicates that after a food animal was 

 drilled and eaten and the drill had 

 moved away, the animal was taken out 

 and cleaned of whatever meat was left. 

 The shell was then replaced in its 

 original location on the design. 



After 70 days of Experiment I, tanks 

 1, 3, and 4 selected by random numbers 

 were provided with test animals to begin 

 Experiment II. The original test ani- 

 mals were kept in tank Z throughout 

 Experiment II to determine if any 

 changes occurred in their feeding be- 

 havior over a longer test period. 



The tests in tanks 1 and 4 of Ex- 

 periment II in which dead food was 

 not replaced were conducted to deter- 

 mine if drills would completely finish 

 a preferred species before turning to 

 another. This phase of the experiment 

 was designated "non-replacement of 

 food" test. 



An Ocinebra usually stayed on one 

 individual food animal for several days. 

 As soon as the drill moved off or left 

 a food organism, this animal was 

 taken out of the aquarium and examined 

 for drill holes. If there was a drill 

 hole, the animal was considered dead. 

 When it was found that a hole had been 

 drilled through a valve of a mussel or 

 clcim, the soft parts of the visceral 

 mass were gone, leaving intact the 

 harder parts such as the siphon in the 

 clams, mantle edges, foot, and parts 

 of the adductor muscles. Usually, only 

 fragments of the adductor muscles 

 were left in oysters. These findings 

 were in general agreement with those 

 who worked with other species of 

 drills [Carriker, 1954; McConnel, 1954; 

 and Chapman, 1956], 



Carriker (1955) points out that the 

 radula of UfOSalpiflx is ineffective in 

 rasping tissues such as the adductor 

 muscles of adult oysters until partial 

 autolysis has taken place. It is pos- 

 sible that the temperature of the water 

 (11.9° C.) in the two experiments may 

 have delayed nornial autolytic action 

 in the hard parts of the visceral mass 

 of drilled bivalves. Ocinebra may have 

 been discouraged by this delay and 



consequently left the hard parts of the 

 visceral mass partially or wholly in- 

 tact in the bivalves. 



Effects of Current 



A series of experiments by Federighi 

 (1929) showed that Urosalpinx cinerea 



(Say) orients precisely and will move 

 against a current of water (rheotrop- 

 ism). Therefore, prior to Experiments 

 I and II of this study, tests were con- 

 ducted on the possible effects of water 

 currents on the drills' initial move- 

 ments in each tank. Prior to Experi- 

 nnent I, a total of 72 unmarked drills 

 within the specified size range were 

 placed without food on the 18 positions 

 marked in each of the four tanks 

 (figure 4). The drills were observed 

 every half hour for 4 hours. No defi- 

 nite pattern was observed, and move- 

 ment of the drills was assumed to have 

 been random. The same applies for the 

 54 unmarked drills tested in tanks 1, 

 3, and 4 before starting Experiment II. 



RESULTS 



A summary of the major observa- 

 tions is presented in tables Z through 

 8. In these tables, column A repre- 

 sents the species of food animal at- 

 tacked (C = clam, M = mussel, O = 

 Olympia oyster, and P = Pacific oyster); 

 column B represents the location; and 

 column C represents the nximber of 

 days the drill remained on the victim. 



Attacks by Drills 



" Replacement of food" test . --There 

 was a difference between the relative 

 numbers of different types of pelecy- 

 pods attacked by drills in the four 

 different tanks of Experiment I. This 

 was confirmed by a chi- square value 

 (X^) of 33.04, which was significant 

 at the 5 percent level, with 9 degrees 

 of freedom (table 9). When the results 

 of tank Z in Experiment I were ex- 

 cluded in the test, a X^ value of 5.18 

 was obtained (table 10). This indicates 

 that the distribution of attacks between 

 the different species of pelecypods of 

 tanks 1,3, and 4 of Experiment I were 

 not significantly different. 



