Reproductive activity in a pre-epizootic wild population of the Chilean oyster, Ostrea chilensis, from southern New Zealand

Abstract

The Chilean oyster, Ostrea chilensis, is one of the most prized eating oysters and, consequently, it has been transferred to many locations around the world for aquaculture development. The most extensive wild beds of this oyster are in Foveaux Strait, New Zealand. From 1985, most of the oysters in these beds were destroyed by a protozoan parasite, Bonamia sp. Subsequent studies showed that the developmental cycle of the parasite was closely related to the reproductive cycle of the host oyster. The recovery of stored samples of oysters taken from four sites in Foveaux Strait nearly 30 years ago provided a unique opportunity to assess the reproductive cycle of oysters 15 years prior to the onset of the epizootic and the major disruption to the population structure that followed. Analysis of these samples revealed that the oysters were protandrous, maturing first as males by 20 mm in shell height. Beyond 50 mm, most oysters developed ova while continuing to produce sperm, although oysters did not begin brooding larvae until 60 mm. Considerable quantities of ova were present in oysters throughout the year, but only a very small proportion of oysters spawned ova from July to December with a peak in October. Oysters commonly contained and released sperm throughout the year, although peak spawning was from November to March. The phagocytosis of reproductive material from the follicles of oysters was present in a small proportion of oysters throughout the year. However, it was much more common from January to March amongst both male and female reproductive material, including smaller (<50 mm), solely-male oysters. This period of extensive phagocytosis has been associated with the proliferation of Bonamia sp. in infected oysters from this population. However, the findings of this study suggested that the infection of oysters with the parasite Bonamia sp. may not be related to the sexuality of the oysters as previously thought. This study improves the understanding of the reproductive biology of this oyster and will help to elucidate the relationship between the disease and the host oyster. It can also allow for further comparisons with data taken from this population whilst later extensively infected with Bonamia sp.