Sex-Determined Growth of Yearling Pacific Oysters (Crassostrea gigas)

ABSTRACT The sexes of sequentially hermaphroditic cupped oysters are both environmentally and genetically determined. Sex is also associated with growth or body size, females being larger than males of the same age on average. Whether sex determines size or size determines sex has not been resolved. To examine the relationship between growth and sex, 1,360 tagged individuals from six F2 families of Pacific oyster Crassostrea gigas were reared through their second summer in Thorndyke Bay, Washington; individual live weights were recorded for 5 consecutive months, starting in June. At the last weighing, microscopic examination of gonadal tissue revealed four “sexes” in the cohort, females (43% of cohort), males (29%), hermaphrodites (2%), and sex-undetermined individuals with no gametes (26%). Live weights were fit to various sigmoidal growth curves (logistic, Gompertz, and Richards) with fixed and random parameters to account for individual variation. The best fitting curves suggested that growth of the cohort was exponential until late August, when temperatures declined and more than 70% of final weight had been gained. Individual live weights, growth-curve parameters, and growth measures were fit to general linear models with family and sex classification variables. Family-by-sex interaction was significant in many such analyses. Females were larger than males on average and appeared to attain their size advantage from superior growth characteristics rather than from initial size. Males were strikingly either like females or like the smaller sex-undetermined oysters with respect to growth comparisons within family. Sex-undetermined oysters may have been “true” males that were too small to mature as yearlings, though a size-threshold for maturation was not apparent. Genetically determined sex and family genetics appeared to control growth.