to develop into females the following reproductive 

 season. 



The conclusions may be considered only tenta- 

 tive since they are based on a small number of 

 experiments which need to be repeated on a larger 

 and more comprehensive scale. Sex alternation 

 in oysters offers fascinating possibilities for further 

 research on this fundamental biological problem. 



The spawning reactions of sex-reversed oysters, 

 as reflected in the type of shell movements and 

 in the manner of dispersal of sex cells, are in every 

 respect identical to those of the reactions of true 

 males and females (fig. 286). In some sex-re- 

 versed oysters the change from male to female 

 behavior is however delayed. Examination of 

 records shows that in several instances females 

 which were males during the preceding year 

 spawned at the beginning of the reproductive 

 season in a male fashion, discharging the eggs 

 through the cloaca. Full female reaction in- 

 volving rhythmical contractions of the adductor 

 muscle was fully developed toward the end of the 

 spawning season (fig. 287). It can be inferred 

 from these observations that the mechanism of 

 female sexual behavior develops at a slower rate 

 than the morphological changes in the gonads. 

 There was no retention of female behavior in 

 oysters which returned to the male phase. The 

 male reaction was apparent in them at the be- 

 ginning of the season. 



First summer Typical male spawning 



Following summer —female 



j.jjjjjii!il^^ 



First summer — femole 



Following summer — mole 



Following summer, June2l Eggs discharged through clooca 

 I month loter.July 20  Typical female reoction 



Figure 287. — Spawning of the sex-reversed male C. 

 virginica. First line — spawning at the male phase. 

 Second line — sex of tlie gonad changed to female; 

 spawning proceeds in the male fashion. Third line — 

 typical female spawning. Oyster was 5 years old during 

 the "first" summer. Time interval, 1 minute. 



In one instance the spawning of an hermaphro- 

 ditic oyster was recorded (Galtsoff, 1961). Both 

 eggs and sperm were discharged simultaneously 

 through the cloaca, and the rhythmical contrac- 

 tions of the adductor muscle were not fully 

 developed (fig. 288). A small portion of the gonad 

 of this oyster is shown in fig. 289. Microscopic 

 examination of the tissue revealed the presence of 

 relatively few mature eggs in the follicles occupied 

 by spermatozoa. Spawning of this oyster was 

 induced by raising the water temperature. Eggs 

 removed from the spawning tank were found to 

 be fertilized and their development traced to 

 trochophore stage was normal. 



LUNAR PERIODICITY 



The modern zoologist may disregard the early 

 popular beliefs and superstitions which endowed 

 the moon with mysterious effects on human 

 affairs, on animals, and plants; nevertheless, he is 

 confronted with several undeniable instances of 

 lunar periodicity in the reproduction of marine 

 invertebrates. Probably the most famous and 

 generally known examples are the swarming and 

 breeding of the Palolo worm {Eunice viridis Gray) 

 of the South Pacific at the moon's last quarter of 



Figures 286. — Kymograph records of sex-reversed male 

 (two upper lines) and sex-reversed female (two lower 

 lines) C. virginica recorded at two consecutive breeding 

 seasons. Both oysters were 5 years old at the "first" 

 summer. In both instances eggs were dispersed through 

 the gills, the sperm through the cloaca. Time interval, 

 1 minute. 



Figure 288. — Shell movements of a spawning hermaphro- 

 ditic C I'zVguMca. Temperature 24.5° C. Time interval, 

 1 minute. 



318 



FISH AND WILDLIFE SERVICE 



