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Spawning and Fecundity

Spawning in the oyster involves the discharge of eggs and sperm from the gonads into the sea. The sperm pass down the suprabrancial chamber and enter the sea by way of the exhalent chamber. The eggs, however, pass immediately from the suprabranchial chamber through the spaces of the gills and the gill ostia into the inhalent chamber. The eggs lie on the gill plates, just posterior to the labial palps; the inhalent chamber thus acts as a brood chamber. The length of time during which the eggs are retained is dependent on the larvae reaching a certain stage of development and on external factors such as temperature.

Korringa (1941) refers to the liberation of larvae from the inhalent chamber to the sea as "swarming" and considers that this is in effect only the delayed completion of the spawning act. The release of eggs from the gonads in incubatory oysters has never been observed, observations being confined to the liberation of developed larvae.

The spawning season of an oyster varies in length according to local conditions and can only be observed in incubatory oysters by a regular examination of adults throughout the summer season. The first oysters examined from Wellington Harbour were collected on 18/12/60, the water temperature being 11.0°C. The gonads of all the oysters were undeveloped and appeared to be in an "overwintering" or resting period. However, from this date, the water temperature rose steadily to 18.5°C. by 28/12/60 when approximately 36 oysters were collected from Evans Bay; two of these oysters liberated larvae on the following day. Further examination of the adults showed that spawning continued until early March. The last collection of oysters that contained larvae was on 1/3/61, at a water temperature of 16.0°C. Examination of over 100 oysters during the following weeks failed to produce any incubating oysters or even oysters with well developed gonads.

Examination of adults was recommenced on 22/8/61 when the water temperature was as low as 10.0°C. This collection of oysters followed a succession of severe southerly storms that lasted for several weeks. The average water temperature during this cold weather was 9.4°C. as already mentioned above. Of the 31 oysters examined from this collection, one only was carrying larvae. The appearance of the gonads of the remaining oysters suggested various stages of development; some ripe, others less so. This is notable for two reasons: (1) the gonads were actually capable of maturation at temperatures as low as 10.0°C. and (2) oysters were spawning for at least eight months of the year, that is, from August to March.

It would seem possible then, since the water temperature rarely goes very far below 10.0°C. in Wellington Harbour (lowest record was 8.8°C. on 11/8/61) that spawning takes place throughout the year. This does not mean that spawning takes place with the same frequency and regularity throughout the year, but rather will there be periods of more active spawning during the summer months when there are higher water temperatures. Before it is possible to state with certainty that O. lutaria spawns throughout the year, it will be necessary to examine oysters collected from April to July. It is equally possible that any of these months and maybe all of them will be found to be periods of over-wintering, but maturation of the gonads and spawning would be possible during warmer months.

A spawning season of eight months is considerably longer than recorded for other incubatory oysters, except where oysters occur 'in warmer water (i.e., the Mediterranean) than in New Zealand and spawning takes place over the whole year.

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Korringa (1941, p. 93) reviews the little that has been written on the number of larvae that may be spawned by incubatory and non-incubatory species of oysters. There have been no estimates of the number of eggs or larvae produced by O. lutaria until the present account but marketable oysters (approximately four to five years old), produce between one million and two million larvae while oysters of two or three years produce about 500,000. At the end of the breeding season, oysters of three and four years may produce as few as 2,000 larvae. It is possible that these oysters were spawning at the time of collection and that consequently only the remaining larvae were spawned in the laboratory. In general however, there is a tendency for oysters to produce fewer larvae at the end of the breeding season.