Abstract:
Two species of Ostrea Linnaeus are present in New Zealand: O. lutaria Hutton (the mud-oyster) and O. heffordi Finlay (the southern rock-oyster). Studies on O. lutaria show that it is distinct from the South Australian mud-oyster, O. angasi Sowerby but has O. charlottae Finlay as a synonym. O. lutaria is typical of the genus Ostrea in anatomy; it is larviparous and hermaphroditic, spawning for at least eight months of the year between August and March, at temperatures as low as 10'C. ; is probably protandric with regular alternating female and male phases; becomes hermaphroditic after the second or third breeding season. Fecundity is about 1,000,000; egg size at liberation 170u. - 200u. Larvae are incubated for about 20 days; are ready to settle when they measure 0.44mm. x 0.34mm. A study of the pelagic bivalve larvae of Evans Bay has produced no free swimming oyster larvae ; the large size of the veliger and the absence of larvae in the plankton suggests that the free swimming phase is extremely short, probably less than a week. The settling larva exhibits an exploratory phase; the spat doubles its size in 24 hours and all the gills are present in a spat measuring 5.5mm. in length.
