Stable Differences in Growth Rates of Juvenile Triploid Oysters Crassostrea gigas Thunberg (Osteidae)

Three groups of mollusks of the triploid Pacific oyster Crassostrea gigas differing in morphometric parameters-fast-growing (FG) with a shell height of over 40 mm; medium-growing (MG), 15 mm < H < 40 mm; and slow-growing (SG), H < 15 mm-have been studied from the Donuzlav estuary (Black Sea). Stable differences have been revealed in the growth rates of juveniles of the same age. Mollusks of the FG group are dominant in weight gain and linear dimensions throughout the study. The average daily increase in the shell height of the studied mollusks ranges from 0.1 to 0.35 mm/day, with maximum values in June and September. The weight increases with different rates: on average, 0.051 g/day in the SG groups, 0.168 g/day in the MG group, and 0.287 g/day in the FG group. The peaks of this parameter have been recorded in August and September, reaching 0.12, 0.26, and 0.43 g/day, respectively. The shell height of slow-growing polyploid oysters has a negative allometry (b = 2.17), while the other two groups are characterized by a clear positive allometry (b = 3.23 for MG and 3.80 for FG); i.e., the increase in weight is faster than the linear growth in juveniles of the species. It has been suggested that polyploidy (triploidy) determines the differences in the growth rates of the same-aged mollusks. The allometry index b can be used to identify growth features at the early stages of oyster development.