Duration of free swimming stage.

If it is assumed that under normal conditions the larvae of O. lutaria are liberated complete with well-developed foot and pigment spots, then it is apparent that the duration of the free swimming stage will be reduced accordingly. During the course of this study only three oysters of the 565 held from time to time in the laboratory liberated fully developed larvae which subsequently became attached. The free swimming larvae of these three oysters became attached as follows: one, five days after a temperature rise from 15.5°C. to 18.5°C.; the second, three days after a temperature rise from 18.0°C. to 20.0°C.; and the third, two days after a temperature change from 16.0°C. to 19.5°C. The temperature change was caused by removing oysters from Evans Bay to warmer sea water in the laboratory. In all cases mentioned, the rise in temperature was sufficient to induce oysters to liberate their larvae prematurely. In other instances, the successful attachment of the larvae depends on their state of development at the time of liberation.

During experiments some larvae were held at 21.5°C. Of these, a few became attached within two hours. The majority died and about 20 per cent were still in an exploratory phase 18 days later. This indicates that the larvae are capable of postponing attachment for some considerable length of time, but the experimental temperature of 21.5°C. is higher than that recorded in Evans Bay so that it is quite possible that the larvae behaved abnormally.

In conclusion, it seems reasonable to assume that any pelagic free swimming stage of the larvae of O. lutaria if it does exist, lasts a few days only, but that attachment may be postponed if suitable conditions are not present.