Reproductive Biology and Early Life History of the Chilean Oyster, with Special Reference to Populations in Northern New Zealand.

Abstract

The Chilean oyster, Tiostrea chilensis, is a commercially important species that is native to New Zealand and the Pacific Coast of South America. The description of the variability in life history characteristics among populations of the Chilean oyster is fundamental to understanding its biology and may help in solving some of the problems encountered in culturing this species. Research presented in this thesis describes some aspects of the reproductive biology of the Chilean oyster from four populations, mainly in northern New Zealand, and compares the results with previous studies from elsewhere. Similar patterns of gametogenesis were found among three study populations at Manukau Harbour, Hauraki Gulf and Foveaux Strait. Oysters were protandrous, maturing firstly as males and later also producing ova. In all three populations the majority of the contents of all the gonads were male reproductive products. There was no evidence that the sexuality of oysters alternated rhythmically as has been reported in other species of larviparous oysters. For spawning females, the often concurrent release of ripe sperm indicated the possible presence of self fertilisation. In samples of oysters examined from Foveaux strait the sexuality of oysters was found to change markedly with size. This suggested that the infection of oysters by the parasite Bonamia may not be related to their sexuality as was previously thought. At two northern populations (Hauraki Gulf and Manukau Harbour) larvae were produced from young, small oysters, and a much larger proportion of the population was brooding larvae each year than has been reported elsewhere. In both populations, larvae were being brooded, released and were settling at all times of the year, unlike other populations. The mean fertility of the Manukau Harbour oysters was the highest so far reported for any population of this species. The size of larvae in all three northern populations were smaller than has been reported for all other locations in New Zealand and chile and is thought to be related to differences in water temperatures. Overall, the results highlight the importance of investigations into populations of molluscs across their geographical range for revealing variation in life history characteristics which may be of benefit for aquaculture.