used in the test, was high, and the number of non- 

 spawning oysters gradually increased toward the 

 end of the experiment (table 35). 



When oysters failed to respond to spawning 

 stimulation, the testing was repeated at 4- to 

 5-day intervals for 5 consecutive weeks. Nega- 

 tive results were assumed to indicate the oysters 

 were nonfunctional sexually, and they were re- 

 turned to the holding tanks for another year. In 

 several instances the oysters that failed to spawn 

 became sexually active the following breeding 

 season. It is not known at present whether the 

 increased number of failures to spawn and in- 

 creased mortality (table 35) should be attributed 

 to aging or to unfavorable conditions in the win- 

 ters. On several occasions the holding tanks and 

 live cars in which the oysters were kept were 

 swept by stormy waters and everything inside was 

 covered with a deposit of mud. 



Table 35. — Changes in the percenlage of sexes in a selected 

 group of C. virginica tested consecutively for 4 years 



[Initial number of males and females 4 years old does not represent the normal 

 sex ratio which was about 1 to 1] 



During the 5-year period of observations the 

 number of spawning males decreased from 119 to 

 15 in 9-year-old inollusks. The number of females 

 decreased from 63 to 18. Consequently the sex 

 ratios of males to females of the experimental 

 oysters changed from 1.9:1 at the beginning to 

 0.8:1 at the end of the observations. The pre- 

 dominance of oysters of female sex in the surviving 

 oysters can not be attributed to more frequent sex 

 changes from male to female. In table 36 no 

 significant differences were recorded in the rates of 

 sex alternation in the two groups. The predomi- 

 nance of females at the end of the test could be 

 explained, therefore, by greater sm-vival rate of 

 oysters at the female phase. This interesting 

 point requires further corroboration. 



Out of the 68 survivors at the end of the breed- 

 ing season of the fourth year, 31 had alternated 

 their sex at least once during the period of testing 



ORGANS OF REPRODUCTION 

 733-851 O— 64 21 



(Galtsoff, 1961). The frequency of changes were 

 as follows: 



One change 18 instances. 



Two changes 10 instances. 



Three changes 2 instances. 



Four changes 1 instance. 



The oyster which changed sex every year was a 

 male at the beginning and returned to the male 

 phase at the last test. No distinct pattern is 

 apparent in the rhythm of changes except the 

 greater persistence of the female phase. The sex 

 ratio within the sex-reversed oysters changed from 

 23 males and 8 females at the beginning to 1 1 males 

 and 20 females at the end of the observations. 



Table 36. — Frequency of sex alternation in adult C. 

 virginica from 5 to 9 years old 



( The figures in the table indicate the number of males (column 2} and females 

 (column 5) of each age and the number and percentage of changes to females 

 (columns 3 and 4) and to males (columns 6 and 7) that occurred during each 

 year) 



The cause of sex instability and the factors 

 which may influence the shift of a gonad from one 

 sex to another are not known. Coe believed that 

 the physiological state of the organism in each 

 breeding season is the key to the determination of 

 the sexual phase of the oyster. No concrete proof 

 to substantiate this idea can be found in his ex- 

 periments. Egami (1952) attempted to trans- 

 plant pieces of gonad of C. gigas to another oyster 

 of the same or of the opposite sex. He found no 

 evidence that the sex of the host affected the sex 

 differentiation of the graft and concluded that the 

 sex of the grafted pieces has been determined at 

 the operating season (December) at their mor- 

 phologically undifferentiated state. In another 

 work (Egami, 1953), he corroborated the results 

 of Amemiya's observation on the decrease of 

 growth rate of C. gigas by the removal of gill tis- 

 sue and the increase in the percentage of males 

 among the mutilated oysters. He concluded that 

 maleness could not be attributed directly to mu- 

 tilation but was associated with the decreased 

 growth rate of oysters without gills. Egami found 

 tiiat among normal individuals of C. gigas those 

 growing more rapidly during the autumn tended 



317 



