SPAWNING AND SETTING OF OLYMPIA OYSTERS 
461 
oysters. The variation in water temperature on the oj^ster grounds during winter 
and summer and at different stages of tide has been described in table 1 and figures 
5 and 6. 
Although he appeared to be uncertain of the specific factor involved in stimulating 
the beginning of spawning, Orton (1926) concluded that spawning of 0. edulis takes 
place primarily during the full-moon tidal period. Prytherch’s (1929) work indicated 
that the rise in water temperature, due to warming of the tide lands during the 
extreme low tides, was responsible for stimulating reproductive activity. He stated 
that in Milford Harbor, Conn., “the majority of the oysters spawned at the end of the 
July full-moon tidal period, when the water was brought to a favorable spawning 
temperature.” Nelson (1928 a, b) concluded that there is a definite relationship 
between the rapidity of the rise in temperature after the high-tide temperature reaches 
20° C. and the time required for the initiation of spawning in 0. virginica. Pie 
found during several seasons that spawning started from 52 to 94 hours after the 
temperature of 20° was reached, depending upon the rapidity of the subsequent rise. 
His observation is in accord with Galtsoff’s (1930b, 1932) experimental finding that a 
sharp rise in temperature will induce discharge of germ cells. 
In the case of Olympia oysters, however, the grounds are diked and are all between 
low- and high-tide levels. The variation in salinity and pH at different stages of the 
tide is usually not very great (fig. 12) but the temperature is subject to wide fluctua- 
tions. When the tide is low the oysters are covered by only a few inches of water 
which quickly responds to weather conditions. During the day low tides in March, 
when the weather is favorable, the temperature may rise to 20° or 25° C., probably 
causing the maturation of eggs and sperms. Actual spawning does not usually begin 
until late in April or some time thereafter. 
During each season oysters were opened S 3 7 stematically on representative grounds 
in both Oyster Bay and Mud Bay to determine when spawning started and the 
number of adults bearing larvae throughout the season. Sampling was begun well 
in advance of the spawning period and consisted in the opening of 100 oysters on each 
of the test beds, 2 or 3 times a week. When a gravid individual was found the 
larvae were placed in a vial and preserved with formalin for examination in the 
laboratory. The method is described in greater detail with reference to the rate of 
larval development, and it is necessary here only to state that graphs of the results 
were made showing the percentage of oysters bearing larvae throughout each season. 
From these results and the thermograph records it is possible to correlate spawning 
activity and water temperature. 
Four graphs (figs. 15-18) are reproduced showing the percentage of gravid oysters 
on different days and the daily average and minimum temperature. Three of the 
figures refer to Oyster Bay, one to Mud Bay, but all agree with respect to the influence 
of temperature upon spawning. In 1932 (fig. 15) no specimens bearing larvae were 
found until May 17, when 12 out of 100 bore very young embryos. The temperature 
record shows a sudden rise at this time. For some days the average temperature had 
varied from 13° to 15° C., but the minimum, or high-tide temperature had been 
relatively stable. On the 16tli the minimum temperature rose from about 12° to 
over 13° C., and was followed by the sudden onset of spawning. For several weeks 
afterward, wlfile both minimum and average temperature steadily increased, spawn- 
ing was quite prolific. On the same graph the daily range of tide is plotted to indicate 
possible correlation with tidal periods, as described by Orton (1926). This is dis- 
cussed below. 
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