August 12, 1922] 



NA TURE 



was found on July 3 to be carrying thousands of 

 young oyster embryos. This same individual was 

 taken to Plymouth and kept alive ; on July 18 it 

 was again examined and found to be practically 

 ready to spawn as a male. 



It is, therefore, clear that even in England, in such 

 suitable circumstances as occurred in 192 1, oysters may 

 become mature as females in the first year of growth, 

 and further that a one-year-old oyster which had 

 spawned as a female in the summer following that in 

 which it was born changed immediately after spawning 

 into a male. An examination of about 300 young 

 oysters from the oyster-beds, estimated as one-year- 

 oid oysters from comparison with the known one- 

 year-olds, gave the same result as that obtained 

 from the examination of the spat known definitely 

 to have settled in 1921, namely, that all the smaller 

 ones were males but that some of the larger ones 

 were either females or had already spawned as 

 females and were changing or had changed into 

 males. From these results the conclusion is drawn 

 that all oysters are born as males, but may or may 

 not change into females at an age of one year. The 

 proofs for this statement are not yet sufficient to 

 establish it as a fact, and indeed actual proof could 

 only be obtained in the most fortuitous circum- 

 stances. The kind of result required to amount 

 to proof would be one which gave 1000 individuals 

 all males out of 1000 individuals examined, but as 

 oyster larvae settle at different intervals over a 

 period of several months, a heterogeneous population 

 with regard to age — apart from other causes — results 

 unless very .special precautions are taken. Since, 

 however, sex-change may occur very rapidly, a 

 difference in age of a few months in young oysters 

 is sufficient to give time for sex-change in a collection 

 of rapidly growing oysters whose greatest age is not 

 more than one year, hence the difficulty. In spite 

 of the difficulties, however, the knowledge of the 

 conditions of sex-change mentioned above will help 

 towards designing an experiment to lead to a definite 

 result. 



An extremely interesting result follows the observa- 

 tion that an oyster may function as a female at an 

 age of one year, namely, that Gerbe's work in 1876 

 (Revue et Magasin de Zoologie pur et applique, 3 

 serie, iv.) can be regarded as confirmed. Gerbe 

 examined 435 one-year-old oysters and found 35 

 with spawn in the gill, 127 with eggs in the gonad, 

 189 with sperm in the gonad, and presumably 84 

 with the gonad undifferentiated. An additional 

 observation supporting these was also obtained from 

 the Blackwater experiment mentioned above. A 

 few Portuguese oysters (0. angulata) settled on the 

 shells at the same time as the native oysters. By 

 good fortune one ripe male and one ripe female were 

 obtained. An artificial fertilisation made from these 

 two individuals gave a very good proportion of 

 swimming oyster larva? and quite as good as a control 

 gave on adult Portuguese oysters. 



Thus Gerbe's results — although nearly 50 years 

 old — may be accepted and taken into account 

 definitely in discussions on sex in the European 

 oyster. There is nothing in his results at variance 

 with the observations described above. The view 

 one naturally took of Gerbe's results — prior to the 

 writer's observations given above — was that European 

 oysters at birth developed in approximately equal 

 numbers into males and females, and in view of sex- 

 change afterwards taking place from female l to male 

 and almost certainly also from male to female, the 

 sex-phenomena in this species appeared to be unique. 

 The rapidity of sex - change in oysters must now, 



; of authenticated change from female to male have 

 NO, 2754, VOL I IO 



however, be taken into consideration, and the writer's 

 view of the sex-changes in the young oyster found 

 to be carrying embryos this year may be recapitulated. 

 That oyster settled some time in 192 1 after June 9, 

 it is predicated that this oyster became sexually 

 mature as a male and "spawned as a male in the 

 summer of 1921 (see Nature, December 15, 1921). 

 On July 3 this year this same individual had spawned 

 as a female and was carrying thousands of young, 

 and on July 18 this same individual was again 

 sexually mature and practically ready to spawn as 

 a male. Thus this oyster has already had two and 

 probably three experiences of sex alternating from 

 male (?) to female and from female back to male 

 within about one year. It is hoped to follow further 

 sex-changes in this particular oyster, which is still 

 alive, for its last condition of sex, namely as a male, 

 was determined by tapping the gonad through a 

 boring in the shell. It has been found that if the 

 boring and tapping operations be carefully performed 

 an oyster can easily and quickly recover and cement 

 over the boring on the internal face of the shell. 



The definite information obtained from this one 

 oyster is corroborated by sex-conditions in other 

 young oysters taken from the grounds whose age 

 could be determined as one-year-old oysters with 

 practical but not with absolute certainty. As 

 indicated above, however, the conditions in 1921, 

 namely the unusually long warm summer, were 

 highly abnormal, and it is not to be expected that 

 one-year-old female oysters will often occur in 

 British waters. 



The rapidity of the sex-change in oysters is 

 paralleled by a similar observation on sex-change in 

 the American slipper-limpet (Cvepidula fomicata) in 

 the same series of experiments in 1921-22. Crepidula, 

 moreover, has undoubtedly a similar span of life 

 to that of the European oyster. Sex-change from male 

 to female occurred in isolated slipper-limpets (up to 

 26-5 mm. long) in the same period as that observed 

 in the oyster, but unfortunately none of the sex- 

 changed limpets were actually in spawn at the time 

 of examination although the gonad in several cases 

 was full of either ripe or fairly large ova, and the 

 penis had been reduced to a mere discoloured trace. 

 Individuals of a similar age settled on other slipper- 

 limpets had the normal beautifully - shaped and 

 well - developed healthy black penis of the young 

 male. (See Orton, Proc. Roy. Soc, vol. 81, B, 

 1909, p. 469.) 



The conditions under which sex-change occurs in 

 the oyster and the slipper-limpet are of much general 

 interest. In the oyster development of both the 

 male and female sex-elements can occur in the winter 

 and spring period. We also know that male-elements 

 can begin and attain full development in the summer 

 and autumn, but as yet we have no definite informa- 

 tion about the development of the female sex- 

 elements in the summer and autumn. Experiments 

 have been started to obtain information on the 

 latter problem, but until that information is obtained 

 one is not in a position to discuss the possible causes of 

 sex-change in the oyster. It would appear, however, 

 that the factor for sex-causation is within the control 

 of the organism and not in external conditions, but 

 it will be more profitable to await further information 

 on sex - phenomena before discussing the question 

 fully. In Crepidula there is no doubt that sex-change 

 occurs in young forms when the young males cannot 

 function as males ; on the other hand, if young males 

 settle down on females, they undoubtedly remain 

 males and function as such for a variable time, which 

 may be as long as five or six years or as short as one 

 year, but again the underlying factor appears to be 

 that of opportunity to function as a male. The 



