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Some Studies on the New Zealand Oysters

[Introduction]

Ostrea lutaria reproduces for at least eight months of the year, between August and March and possibly for longer when favourable conditions are present. Sections of the gonads show that sexual products are present all the year and that a variety of sexual phases are represented including hermaphrodite individuals. The first functional phase appears to be that of the male while the female phase does not develop until the third summer. When ovulation occurs, the eggs are shed from the gonad and retained in the inhalent mantle chamber where fertilisation and development take place. The New Zealand mud-oyster is thus an incubatory species.

During the present study, all stages of incubation were observed in oysters held in the laboratory which had been induced to liberate their larvae by a rise in sea water temperature. The larvae are liberated when they are provided with a shell and pigment spots and are capable of swimming. A study of the plankton was made to find free swimming larvae. This investigation was carried out over a period of one year, including two summers and resulted in the identification of the larvae of two species of mussel and four other bivalve larvae, but no oyster larvae. The process of settling and metamorphosis were also observed in the laboratory.

The larvae were studied in the first instance as live material and these observations were supplemented by whole mounts stained with acetic acid-alum-carmine. Attempts to section the embryo were unsuccessful.