Uc) Of the clams in all seven size groups, 10 /^ were burrowed at the end of 

 30 minutes. This oercentage was 2.5 lower than the percentage burrowed at 

 13 cm, per second. 



Conclusions 



As previously mentioned, the effects of ciirrents on various sizes 

 of juve lile clams vrere similar i:i some ways to those on coarse sand and 

 gravel. However, the important difference between clams and sediments, out- 

 side of shape and specific gravity, is that clams are alive and have some 

 influence over their ovm disoersal. This has been emphasized in the foregoing 

 results by the fact that the smaller clams (2-13 mK=) which were moved most 

 extensively by the currents also burrowed more rapidly and in larger numbers 

 per allotted time than did the larger clams (lU-22 mm,). The explanation for 

 this seeming paradox lies in the greater activity and superior burrowing 

 ability of the smaller clams, which is partly due to the relatively larger 

 foot. While the clams are being moved along by the current, there are brief 

 intermittent periods when thej-- become stationary, enabling the small, active 

 clams to extend the foot quickly and burrow. On the other hand, the larger, 

 less active clams, which require longer periods at rest in order to extend 

 the foot, are repeatedly set in motion before they can begin to burrow. 

 Also contributing to the successful burrowing of the small clams are minute 

 depressions in the flat which afford some protection from the full impact of 

 the current. 



According to the data presented, there appears to be a natural line of 

 demarcation of juvenile clams at about 13 mm, beloxv' which they are very 

 active and above which they become progressively less active. This is sig- 

 nificant in view of the fact that almost all the clams of Smith's (19^2) 

 movement trays were under 13 nun. in length, orobably indicating that clams 

 over 13 mm. are beginning to acquire the sedentary habits of mature clams. 



In attemoting to interpret field conditions in the light of the results 

 herein, many natural factors should be considered, such as water depth, 

 v/ave action, variation in current velocities through tidal cycles, and typr 

 of sediment. For instance, wave action may prevent the burrowing of exposed 

 clams 'bj constantly agitating them, even though the current velocitj' maj' be 

 too slow to move the clams. Oftentimes the ability of a clam to burrow may 

 depend upon the stage of the tide, which governs the velocity of the tidal 

 currents. It would be relatively easy for the clam to burroiv' near slack 

 ebb or slack flood tide, but difficult at mid-tide. However, clams which 

 are able to t-jithstand the high-velocity currents of mid-tide by byssus 

 attachment or by landing in depressions would be able to burrow at the 

 approach of slack tide. 



V/hen a flat is composed of shifting sand, juvenile clams may be 

 alternately eroded from their shallow burrows and buried by the moving 



12 



