SEX RATIO, HERMAPHRODITISM, 

 AND SEX CHANGE 



The oviparous species of oysters of the genus 

 Crassostrea usually are not hermaphroditic; speci- 

 mens in which functional eggs and sperm are 

 found together are relatively rare. This condition 

 exists in C. virginica, C. gigas, C. angidata, C. 

 rhizophorae, and is probably common to all 

 members of the genus. The frequency of her- 

 maphroditism in Crassostrea oysters varies with age 

 and envu-onment. The earliest record was made 

 by Kellogg (1892), who found one hermaphrodite 

 among the many adult C. virginica he kept in 

 breeding tanks. Burkenroad (1931) reported 

 that about 1 percent of the oyster population on 

 the coast of Louisiana were hermaphrodites. 

 Needier (1932a, 1932b) found only four her- 

 maphrodites (less than 0.4 percent) among the 

 1,044 oysters of various ages growing on beds in 

 the waters off Prince Edward Island. The 

 hermaphrodites were found only among the 

 2-, 3-, and 4-year-old oysters; none were en- 

 countered in oysters from 5 to 8 years old. In 

 the course of my studies I found only two her- 

 maphrodites among several thousand se.xually ripe 

 oysters from 5 to 7 years old. 



Amemiya (1929) reported only one l)ermaphr(>- 

 ditic specimen among 120 sexually mature C. 

 r;i>a.s (0.8 percent). The percentage is appa.-ently 

 higher in the Bombay oyster, 0. cucuUata, for 

 Awati and Rai (1931) reported 23 hermaphroditic 

 specimens (2.9 percent) among the 794 oysters they 

 examined 



The larviparous oysters of the genus (Mrea {0. 

 edulis, 0. luriila, 0. eqiie.^tri.% and others) are as a 

 rule ambisexual, i.e., they undergo rhythmical 

 changes in sexuality. The initial phase in these 

 species is usually nuxle, followed by alternating 

 female and mde phases. 



Orton (1927) distinguishes several arbitrary 

 categories of sexual changes in 0. edulis from pure 

 male or female to hermaphrodites which contain 

 an equal abundance of ripe spermatozoa and ova. 

 Different transitional pliases of sex changes which 

 take place during the life of the European oyster 

 are discussed later in this chapter. 



Oysters have no secondary sexual cliaracters, 

 and their sex can be recognized only during tlie 

 reproductive periods by microscopic examination 

 of gonads. Sperm suspension, which can be forced 

 "Ut by gentle pressure on the surface of the gonad, 

 is viscous and white; the suspension of eggs is 



314 



creamy and has a granular appearance. Sex 

 determination made with the naked eye should be 

 verified by microscopic examination of smears. 



In many species of bivalves sex is unstable, and 

 hermaphroditism and alternation of sex are 

 common. With respect to sex change oysters fall 

 into two groups: oysters in which sexual phases 

 cliange regidarly in a definite rhythm, as in 0. 

 edulis, 0. lurida; and tliose belonging to the 

 Cras.sostrea type, in which the sexes of the adults 

 are separate, as in C. iirginica, C. gigas, C. 

 angidata, and 0. cucullata. The gonads of the 

 oysters of the first group contain functional ova 

 and spermatozoa simultaneously. These oysters 

 are hermaphrodites. In the second group her- 

 maphroditic individuals are relatively rare. 



The difference between the two groups is not as 

 explicit as it appears since the primary gonad of 

 Crassostrea is bisexual, i.e., it contains the germinal 

 cells of both sexes. 



As early as 1882 the outstanding Dutch natural- 

 ist, Hoek (1883), in his studies of 0. edidis made 

 the important observation that "at the time when 

 an oyster is sexually mature, it always functionates 

 as a male as well as a female; it is, therefore, 

 physiologically dioecious." The significance of 

 this important discovery was appreciated nearly 

 half a century later after Orton (1927, 1933) 

 showed experimentally that maleness developed 

 in 97.3 percent of young or adult females which 

 carried eggs, embryos, or larvae. He further 

 established the fact that the earlier state of male- 

 ness was always found in the more recently 

 spawned females. Great advances in the under- 

 standing of sex changes in 0. edulis and other 

 species were made by the works of Stafford (1913) 

 on 0. lurida, by experimental research conducted 

 by Sparck (1925), and p<articularly by observa- 

 tions on the American species made by Coe 

 (1932-41). It was clearly established by these 

 investigations that the young oysters of the larvip- 

 arous species {0. edulis, O. lurida) become se.xually 

 mature first as males then gradually change into 

 functional females; later they become males again, 

 and such alternation with some modification con- 

 tinues tliroughout life. Comparable phases of 

 sex changes occur in the Crassostrea species al- 

 though the rhythm of sex alternation is different. 

 At the age of 12 to 16 weeks the primary gonad 

 of C. virginica is bisexual (ambisexual, according 

 to Coe's terminology) since both ovogonia and 

 spermatogonia are found in the same follicles 



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