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

The Chilean oyster, Ostrea chilensis, is one of the most prized eating oyst ers and, consequently, it has been transferred to many locations around the world for aquaculture development. The most extensive wild beds of this oy ster are in Foveaux Strait, New Zealand. From 1985, most of the oysters in these beds were destroyed by a protozoan parasite, Bonamia sp. Subsequent s tudies showed that the developmental cycle of the parasite was closely rela ted to the reproductive cycle of the host oyster. The recovery of stored sa mples of oysters taken from four sites in Foveaux Strait nearly 30 years ag o 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 t he population structure that followed. Analysis of these samples revealed t hat the oysters were protandrous, maturing first as males by 20 mm in shell height. Beyond 50 mm, most oysters developed ova while continuing to produ ce sperm, although oysters did not begin brooding larvae until 60 mm. Consi derable quantities of ova were present in oysters throughout the year, but only a very small proportion of oysters spawned ova from July to December w ith a peak in October. Oysters commonly contained and released sperm throug hout the year, although peak spawning was from November to March. The phago cytosis 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 Bonam ia sp. in infected oysters from this population. However, the findings of t his study suggested that the infection of oysters with the parasite Bonamia sp. may not be related to the sexuality of the oysters as previously thoug ht. This study improves the understanding of the reproductive biology of th is oyster and will help to elucidate the relationship between the disease a nd the host oyster. It can also allow for further comparisons with data tak en from this population whilst later extensively infected with Bonamia sp. (C) 2000 Elsevier Science B.V. All rights reserved.