SPAWNING AND SETTING OF OLYMPIA OYSTERS 
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first spat are found. During this period the larvae grow in length from 180/x to 320m, 
or an average rate of about 5 m per day. 
Quantitative observations were not made on the abundance of larvae in the water, 
though plankton samples were frequently taken. In the case of this species it is not 
necessary to estimate the frequency and intensity of spawning from the age and 
abundance of larvae in the plankton, for this was more readily and accurately accom- 
plished by opening oysters periodically, as has already been described. Correlation 
between time of spawning and setting is considered below. 
Stafford (1914) wrote that the largest larvae found in plankton samples, and the 
smallest spat found on shells soon after attachment, had a length of 255m. Hori 
(1933), in a series of experiments on the artificial propagation of the species, stated 
that the length of newly-set spat is 320/z. He worked with oysters imported into 
Japan from Puget Sound, and was able to grow the larvae to maturity in dishes by 
feeding them ground sea lettuce ( Ulva ). Setting under the artificial conditions was 
successfully accomplished and his measurements of the spat are identical with those of 
the writer. A great many spat have been measured during these experiments and 
in no case has an attached spat been seen which was significantly less than 320m long. 
It is of interest also that Stafford’s (1913) measurements of larvae of 0. virginica at 
setting size are different from those of other observers. While he stated that newly-set 
spat are 380 m long, Prytherch (1934) measured them as 330m “across the shell at its 
greatest diameter.” 
Whether these differences are due to error in measurement or to actual differences 
in the size of the setting larvae may not be determined from Stafford’s works. It is 
certainly not impossible that under other environmental conditions the larvae of 
0. lurida may be ready to attach at a smaller size. Stafford’s (1914) figures of older 
larvae and of newly-set spat do not show the umbo of the left valve to be as prominent 
as those of Hori (1933), which are in exact accord with observations of the writer. 
It is a possibility, also, that differentiation may proceed, under some circumstances, 
more rapidly than growth, with the result that the organs of the larvae reach the 
stage of maturity at a size smaller than that observed during the present work. 
There is, in addition, the possibility that the oysters studied by Stafford are of a 
distinct variety, having different characteristics from those of the Olympia oyster, 
though such is hardly likely. However, the fact remains that the measurements of 
Hori and the writer agree perfectly for both larvae and spat. Stafford’s measurements 
are exactly the same only up to the time of swarming. 
SETTING 
As was described in the preceding section, spawning is a long-continued process, 
and not as in some places an occurrence confined to a period of a day or two. It would 
therefore be expected that setting of larvae would also continue during a considerable 
period of time. It is important to know the frequency of attachment of larvae 
throughout the season and the relation of this to the rate of spawning. 
However, before describing the methods employed to analyze the results of setting 
during the different seasons it is necessary to consider the matter of cultch. Various 
questions have frequently arisen as to what constitutes the most favorable cultch 
and why oyster larvae attach as they do to certain surfaces. This phase of the 
investigation is of importance in throwing light upon the results described later. 
