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Some Studies on the New Zealand Oysters

Abstract

Abstract

Two species of Ostrea Linnaeus are present in New Zealand: Ostrea lutaria Hutton (the mud-oyster) and Ostrea heffordi Finlay (the southern rock-oyster). The Auckland rock-oyster, currently referred to Saxostrea glomerata Gould is a species of Crassostrea, probably Crassostrea commercialis Iredale and Roughley (the Sydney rock-oyster). Studies on Ostrea lutaria show that it is distinct from the South Australian mud-oyster, Ostrea angasi Sowerby, but has Ostrea eharlottae Finlay as a synonym. Ostrea lutaria is larviparous and hermaphroditic, spawning for at least eight months of the year between August and March at temperatures as low as 10.0°C, it is probably protandric with regular alternating female and male phases, and it becomes hermaphroditic after the second or third breeding season. Fecundity is about 1,000,000, egg size 200μ–270μ. Larvae are incubated for about 20 days; are released when fully developed and settle when they measure approximately 0.44mm × 0.34mm. No free-swimming oyster larvae have been found in plankton collected from Evans Bay, Wellington. The large size of the veliger and the absence of larvae from the plankton suggests that the free swimming phase is extremely short, probably only a few days. The settling larva exhibits an exploratory phase; does not appear to be photosensitive and when approximately 5.5mm in length has all the gills present.