Sex Ratio and Sex Reversal in Two-year-old Class of Oyster, Crassostrea gigas (Bivalvia: Ostreidae)

Abstract

The sex ratio (F:M) in the same population of oyster, Crassostrea gigas at the commencement of the study (2007) was 1:1.0, but changed to 1:2.8 by the end of the study (2008). The sex reversal rate in two-year-old oysters was 40.2%. Specifically, female to male sex reversal rate was 66.1%, which is higher than the male to female sex reversal rate of 21.1%. The sex reversal pattern of C. gigas appears to go from male⇒female⇒male, and as such is determined to be rhythmical hermaphroditism.

Aspects of Reproductive Biology of the Mussel, Brachidontes Pharaonis (Fischer, 1870) (Mytilidae:bivalvia: Mollusca) From the Northwestern Coast of Suez Gulf, Egypt

“…However, these ratios are not stable and showed monthly variations, revealing increasing male numbers in January at both sites and only through June and September at Adabia. On the other hand, female numbers were higher in most months during the course of this study, as well as are higher than those reported by Abou Zied (1991), Gab-Allah, et al (2007), Park et al (2012) on Crassosteria giags during 2007, Kandeel et al (2013), Abdel Razek et al (2014), but lower than that reported by Park et al (20012) on C. gigas collected in 2008. These authors calculated sex ratio very close to 1:1 (males to females) in their studies on different species of bivalves.…”

“…The annual changes in sex ratio were reported by Park et al (2012). They found that, sex ratio had been changed from 1:1 (males to females) during 2007 to 1:2.8 in 2008.…”

Gametogenic cycle of Crassostrea gigas in contrasting Mediterranean habitats: marine (Gulf of Tunis) and continental (Bizert lagoon) culture sites

“…Both C. gigas populations studied showed an, overall, equal distribution of sexes, despite the predominance of males in several months, though these results must be cautionary interpreted as only 20 oysters per month and site have been examined. For the same species populations in Japan, sex ratio has been showed to be affected by the oyster length and age [ 26 ], with a predominance of males in both small, early stage and large, older oysters. These data have been interpreted as evidences of rhythmical hermaphroditism [ 26 ].…”

Direct evidence of bi-directional sex change in natural populations of the oysters <i>Saccostrea kegaki</i> and <i>S. mordax</i>

“…To our knowledge, in natural mollusc populations, this phenomenon has only been reported in the oysters C. gigas (Park et al 2012, Yasuoka & Yusa 2016 and C. virginica (Galtsoff 1937, Haley 1979, Kennedy 1983, in the vermetid gastropod Thylacodes arenarius (Linnaeus, 1758) (Calvo & Templado 2005), and in the patellid limpet Patella vulgata Linnaeus, 1758 (Le Quesne Kuwamura et al 1994, Nakashima et al 1995. In our in situ experiments, the number of individuals experiencing sex change was small and the body size of experimental individuals did not vary much.…”

“…The direct observation of sex change is especially important for detecting bi-directional sex change, because size difference between male and female individuals may be obscured if they change sex bi-directionally. Therefore, the paucity of direct observations might partly explain why bi-directional sex change has rarely been documented in molluscs (Galtsoff 1937, Haley 1979, Kennedy 1983, Park et al 2012, Yasuoka & Yusa 2016. Moreover, direct observations under natural conditions are important for understanding the adaptive significance of sexual systems (Yasuoka & Yusa 2016).…”