Abstract
The flat oyster, Ostrea chilensis, is an important cultural and commercial species in New Zealand fishing and aquaculture industries. However, natural populations and commercial operations are threatened by haplosporidian parasites in the genus Bonamia. Mass mortality from a Bonamia exitiosa infection epidemic during 1986-1992 led to a period of closure in New Zealand’s largest flat oyster fishery at Foveaux Strait, and recent infections by a new incursion of Bonamia ostreae have forced the destocking of oyster farms nationwide. Recruitment issues have also been identified as key drivers of these commercial populations, and there is a need to improve the understanding of the reproductive biology for O. chilensis. Furthermore, B. exitiosa proliferation is thought to be controlled by O. chilensis gametogenic cycle. With these key issues in mind, the present study aimed to improve the understanding of aspects of O. chilensis reproduction, patterns of B. exitiosa infection, and to compare gametogenesis processes with those observed in Foveaux Strait before the 1986 epizootic.
The gametogenic cycle, biochemical composition, and the intensity of B. exitiosa infection in Foveaux Strait O. chilensis was examined monthly at one location from July 2017 to September 2018, with spatial variation being investigated seasonally from spring 2017 to spring 2018, across five sites. Monthly histological examination of O. chilensis gonad tissue revealed that the gametogenic cycle was relatively similar to that described before bonamiasis epidemics, with the onset of a continuous spawning event beginning in spring 2017. However, there was evidence for a more female skewed sex ratio, with less than 5% purely female oysters overall described previously, while almost one quarter of all samples were exclusively female in the present study. In addition, the development of female reproductive features at smaller sizes was observed, as well as an increased component of gonadal inactivity.
Monthly examination of the total percentage protein, carbohydrate and lipid content of oysters revealed consistent seasonal patterns in the utilisation and accumulation of reserves. Carbohydrates were depleted over the winter and spring, while high concentrations persisted through summer and autumn. While proteins and lipids exhibited the opposite trend, and were accumulated through winter, peaking in spring, with extensive utilisation occurring over summer and autumn. Oysters in western Foveaux Strait may have exhibited more conservative elements in the utilisation and accumulation of biochemical components, relying more on reserves for spawning. While eastern populations were more opportunistic, relying less onbiochemical reserves during spawning, and conserving these constituents for utilisation during lean autumn and winter months.
This may have been related to differences in B. exitiosa infection, as qPCR assays of a subset of samples suggested a higher prevalence in western Foveaux Strait, and biochemical reserves were lower in infected oysters. Overall B. exitiosa prevalence was low, at 2.2%, however, the parasite tended to exhibit a preference for female oysters weakened by spawning, or by concurrent infection of the trematode Bucephalus longicornutus. This means that gametogenesis may remain largely unaffected by B. exitiosa infection, as proliferation may occur after spawning. The present study provides insights into the relationship between B. exitiosa infection and O. chilensis gonadic condition, as well as the first description of the seasonal cycle of the biochemical composition.