The Pacific oyster, Magallana gigas, has been present along the Western Swedish coastline since 2006. However, sea-based cultivation is prohibited to prevent further growth of wild populations. Understanding the reproductive cycle of M. gigas in Sweden is the first step towards changing legislation and building management strategies for cultivated oysters. A model based on cumulative heat exposure (Mann, 1979), predicts the progression of gametogenesis in M. gigas. It postulates that surpassing a threshold temperature (t0) of 10.55˚C will initiate gametogenesis and 592-thermal degree days must be accumulated before spawning can occur. This study aims to validate this model for accurate prediction of maturation and spawning in M. gigas in Sweden.
Materials & Methods
Over 18 months, 15 wild-collected oysters were harvested every 2 weeks from a submerged culture system. Biometric data, histology, and image analysis were conducted on each oyster to classify the reproductive stage and quantify gonadal material for identification of spawning.
Two consecutive reproductive cycles of M. gigas were described for the first time in Swedish waters, with two spawning events occurring in the first cycle. Gametogenesis was initiated before the previous cycle ended and continued throughout the winter. Subsequently, the threshold temperature was adjusted to represent the proliferation period when there was a significant increase in gonad fullness. Evaluation of the temperature maturation model found that using t0=10.55˚C correctly predicted full maturity after 592-degree days in both years.
This study provides fundamental knowledge for developing an M. gigas cultivation industry in Sweden. Predicting oyster spawning will enable strategic management such as pre-spawning removal of cultivated oysters and cultivation in suitable temperature zones. It could also inform timely collection of wild spat for hatcheries. Further in-situ validation studies are needed to refine the threshold temperature in relation to continuous gametogenesis and proliferation.
Mann, R. 1979. Some biochemical and physiological aspects of growth and gametogenesis in Crassostrea gigas and Ostrea edulis grown at sustained elevated temperatures. Journal of the Marine Biological Association of the United Kingdom, 59: 95–110.