which summarizes Coe's observations (1934) made 

 on oysters from four different places along the 

 coast of Massachusetts, Connecticut, and New 

 York. 



During the second breeding season the number 

 of males may still exceed that of the females, but 

 generally the sex ratio approaches ec}uality. Great 

 differences in the degree of protandry among the 

 1-year-old oysters is associated with the differ- 

 ences in the growth rate. Coe's observations 

 suggest that there is a correlation between the 

 development of ovocytes in the bisexual gonad and 

 the rate of body growth. At the first breeding 

 season the average size of the females is much 

 larger than that of males (Needier, 1932a, 1932b). 

 Coe (1934) found that the mean height of 389 

 yearling males from the New Haven area was 

 31.28 mm. (Std. dev. ±6.33) and that of the 13 

 females of the same age 38.54 mm. (Std. dev. 

 i8.12). The difference does not seem to be 

 statistically significant, and several interpreta- 

 tions were advanced by Coe. He suggested that 

 the females require more favorable conditions in 

 order to mature, that tliey are metabolically more 

 active, and that at the critical period of sex 

 differentiation the metabolic factor determines 

 the predominance of the male or female cells in 

 the primary bisexual gonad. These proposals 

 require corroboration. Since it is known that the 

 growth rate of young oysters is accelerated by 

 keeping them suspended above the bottom, there 

 apparently would be no difficulties in conducting 

 a comparative test using slow and fast growing 

 oysters selected from a single population. 



After spawning the gonad of C. virginica retains 

 its bisexual potencies and its sex may alternate in 

 either direction. Needier (1932a, 1932b) was the 

 first to demonstrate that such a change actually 

 occurs among adult C. virginica. She found that 

 out of 24 siu-viving oysters which were known to 

 be males during one simamer, 5 became functional 

 females the following year, and out of 12 females 

 5 changed to males. Among the 57 C. virginica 

 studied by Needier (1942) for a period of 4 years 

 there was a high proportion of males which re- 

 mained unchanged while other oysters changed 

 sex at least once. Some of the individuals 

 changed sex every year. Needier suggested, 

 without providing corroborative evidence, that 

 the sex of the males which remained unchanged 

 was geneticallv determined and that the other 



oysters in which sex alternation occurred air 

 random were hermaphrodites. 



Adidt Japanese oysters, C. gigas, may change 

 their sex during the interval between the two 

 breeding seasons. Amemiya (1929), who estab- 

 lished this fact, found that the rate of change was 

 higher in the males (60 percent) than in the 

 females (25 percent). Sex change occurs also in 

 C. commercialis; 95 percent of the very young 

 oysters of this species were found by Roughley 

 (1933) to be males, but among the adult specimens 

 of large size the females predominated at the rate 

 of 270 to 100 males. 



The sex of the oysters used in the investigations 

 by Needier and Amemiya was determined by 

 drilling a hole in the shell and pinching off a piece 

 of gonad for microscopy. The injury caused by 

 the operation constitutes a factor which may affect 

 the mistable gonad and influence its sex change. 

 By removing about one-third of all gill lamellae 

 in adult C. gigas at the time when the gonad was at 

 the indifferent phase after spawning, Amemiya 

 (1936) demonstrated that the percentage of males 

 in the mutilated group in all cases was larger than 

 those in the control. Removal of the gill tissue may 

 have indirectly influenced the development of male 

 sex by reducing the rate of feeding and growth. 

 This assumption also needs further corroboration. 



Injuiy to the oyster used for observation on sex 

 change can be avoided by inducing spawning in 

 each individual oyster, obtaining kymograph 

 tracings of muscular contractions, and examining 

 the discharged sex cells. This technique was em- 

 ployed in the Woods Hole laboratory, fn a test 

 which continued for 5 consecutive years, 4-year- 

 old oysters were obtained from one of the private 

 oyster beds near Onset, Mass. During the first 

 summer 202 oysters were induced to spawn, their 

 sex was recorded, and an identifying number was 

 engraved on the right (upper) valve. Upon com- 

 pletion of the tests the oysters were returned to 

 outdoor tanks or were placed in the harbor and 

 remained there until the next reproductive season. 

 The testing was repeated every summer (Galtsoff , 

 1961). 



Because the males respond to spawning stimuli 

 more readily than the females, their number at the 

 beginning of the experiment was greater than that 

 of the females. The disparity does not represent 

 an actual sex ratio of the popidation of 4-year-oId 

 oysters which was found to be about 1 to 1. The 

 mortality, especially among the 7-year-old oysters 



316 



FISH AND WILDLIFE SERVICE 



