A large selection experiment is currently under- 

 way for spat and juvenile growth rate of C vir- 

 ginica. It is intended to keep this breeding experi- 

 ment going at Miiford for an indefinite number of 

 selection generations to estimate the duration and 

 extent of response for a particular inbreeding coef- 

 ficient and selection differential to upward and 

 downward selection for growth rate. It is hoped to 

 keep the experiment going on a scale large enough 

 to provide seed oysters of known genetic back- 

 ground for other projects 



A carefully chosen collection of about 6.000 wild 

 C. virginica was set up for mass spawning. Some 

 nonlocal C. virginica were included in this group 

 with the hope of increasing the base of genetic varia- 

 bility (Fig. 7). Of the approximate 6,000, 835 oys- 



ters spawned. Of these spawners. 859? were from 

 Long Island Sound and the rest from areas extending 

 from Prince Edward Island, Canada, to Virginia. 



Several million eggs from 835 spawners were cul- 

 tured, and the resulting larvae reared to metamor- 

 phosis. About 8,000juvenile oysters were obtained. 



At the age of 1 yr a portion of the surviving 

 juveniles was selected on the basis of their size. The 

 population was divided into a large-selected group, a 

 small-selected group, and a nonselected population. 

 The selection differential for the large juveniles was 

 18%; forthe small juveniles, 8%. The first generation 

 has been obtained from mass spawnings of the two 

 selected populations, and from the random- 

 breeding, unselected Fi control population. This 

 first generation from selected parents is currently 



Figure?. — Mass spawning of a group of .American ov>ters.(:ra.».s().\7;<'(/ \iri;inica. from different geographic areas. 



81 



