giant form reaching 51.5 mm. in height is found 

 in the area of Chincoteague Island, Va., and is 

 considered a subspecies U. cinerea follyensis (fig. 

 389). As the common name indicates, the Uro- 

 salpinx attacks oysters and other moUusks by 

 drilhng a round hole in the shell. The hole, 

 usually made m the upper (right) valve of the 

 oyster, tapers toward the inner surface; the shape 

 of the hole identifies the attacker, and the presence 

 of drilled empty shells on oyster grounds is reliable 

 evidence of the mroads made by the snail on an 

 oyster population. 



For a long time bormg was considered an en- 

 tirely mechanical process. Observations made by 

 Carriker (1961a) showed that both chemical and 

 mechanical actions are involved. Secretion from 

 the accessory boring organ, called ABO for short, 

 softens the shell, probably by an enzyme acting 

 on the conchiolin, and the softened material of 

 the shell is removed by abrasive action of the 

 radula. Active drilling continues for a few min- 

 utes and is followed by a long period lasting up 

 to an hour of chemical action dm'ing which the 

 ABO gland remains in contact with the shell. 



The oyster is not the only victim of drills. 

 They show preference, in fact, to barnacles, and 

 usually stop drilling oysters if a rock covered witli 

 live barnacles is placed near by. A well-developed 

 chemical sense permits the drills to distinguish 

 between young and adult oysters. If both kinds 

 are offered to hungi'y snails kept in a large tank 

 with running sea water, the majority of active 

 drills will choose the young oysters. The drills are 

 positively rheotactic and in running water orient 

 themselves agamst the current. The orientation 

 is not, however, precise and the path of a moving 

 drill is a meandering line only generally directed 

 against the current. 



Light has an effect on the orientation of drills. 

 They move away from a strong source of light, 

 but move toward it at lowefr intensities (Carriker, 

 1955). In dim light, the phototactic response is 

 lost. In laboratory tests at Woods Hole, I 

 noticed no orientation of drills toward the window 

 side of the tank; the drills distributed themselves 

 at random. They have a tendency to climb 

 away from the bottom (negative geotaxis) and 

 congregate on rocks, pilings, and on the wall 



Centimeter 



1 I I 



2 



Mil I i meters 



Figure 389. — [/. cinerea follijensif: from Chincoteague Island region, Va. 1 — aportural view; 2 — abapertural view; 



3 — egg cases. 



FACTORS AFFECTING OYSTER POPULATIONS 

 733-851 O— 64 29 



431 



