Reproductive development of the threatened giant grouper Epinephelus lanceolatus

Abstract

Abstract The giant grouper is presumed to follow the reproductive pattern of most Epinephelus species, characterized by protogynous hermaphroditism wherein male maturation is attained through sex reversal of a functional female. This hypothesis, however, has not been verified due to lack of biological data. The present study addresses this gap by investigating the reproductive development of giant groupers from juvenile stage through sexual maturity. Gonad histological analysis of hatchery-bred juvenile giant grouper from Queensland, Australia (0.8–5.2\u202fkg, n\u202f=\u202f43) have shown earliest occurrence of primary oocytes (i.e. ovarian differentiation) in 47.8\u202fcm and 2.5\u202fkg fish. Monitoring of sexual maturity by gonadal biopsy was performed in a stock of wild-caught giant groupers (2–52\u202fkg) held in sea cages in the Philippines and Vietnam from 2015 to 2017. Onset of female sexual maturity was at 96.9\u202f±\u202f1.6\u202fcm and 23.5\u202f±\u202f1.5\u202fkg in the Philippines, and 103.0\u202f±\u202f4.1\u202fcm and 33.5\u202f±\u202f2.5\u202fkg in Vietnam. In both locations, development of primary males was observed wherein fish produced milt (or spermiated) without passing through a functional female phase. The ratio of primary males to females in both locations was about 1:2. Size at maturity of primary males is 86.5\u202f±\u202f4.8\u202fcm and 17.1\u202f±\u202f2.1\u202fkg in the Philippines, and 97.3\u202f±\u202f1.3\u202fcm and 34.3\u202f±\u202f0.9\u202fkg in Vietnam. To aid in the monitoring of female maturation, we developed a non-invasive method based on immunoassay of vitellogenin in skin mucus and this was shown to be effective in detecting female maturation 9\u202f±\u202f2\u202fmonths prior to first observation of oocytes through gonadal biopsy. Our findings suggest that giant grouper is a diandric protogynous hermaphrodite. This study provides novel information on the reproductive biology of giant grouper, an economically important and vulnerable species.