SPAWNING AND SETTING OP OYSTEES 
77 
the salts. Analyses of Galveston Bay water showed similar and more striking 
variations in some of the salts. These data will be published at a later date. It is 
possible that oyster larvae require a high concentration of some salt which in time of 
freshets is in concentration too low to permit the completion of larval development. 
If this is the case such a salt would be expected to be in high concentration in the 
pure Gulf water and lower concentration in the diluted bay water. 
Prytherch 3 recently found that mature oyster larvae may be caused to set by 
dissolving a small amount of a copper salt in the water. His evidence indicates that 
the copper in solution in the river waters emptying into Long Island Sound is instru- 
mental in causing larvae to set. The fresh water from the rivers, according to this 
worker, must be present in a high enough proportion to allow setting to take place; 
that is, a certain dilution, due to the river water, is conducive to setting. This is 
directly the opposite of the condition found to obtain in Galveston Bay, where setting- 
appeared not to take place when the salinity was below about 20 parts per thousand. 
It may be that optimum conditions for setting are to be found when the sea water 
is diluted by land drainage only to a limited degree, and that when dilution is greater 
some necessary substance other than copper becomes reduced below the minimal 
required concentration. No conclusion can be drawn with respect to the actual cause 
of the phenomena observed in Galveston Bay, save that in some manner setting of 
oyster larvae seems to be dependent either directly or indirectly upon salinity. 
ABUNDANCE OF DIATOMS 
The food of the oyster consists of the microscopic or semimicroscopic organisms 
which float in the water, particularly the diatoms and other algae of comparable size 
and form. The richness of this plankton at different periods of the year determines 
in large measure the quality of the oysters, for without sufficient food oysters obviously 
can not fatten. It is important, therefore, to estimate the relative abundance of 
diatoms in the water near oyster grounds. 
Plankton collections were made in the vicinity of Galveston during the spring 
and summer months, and these results show an interesting fluctuation in the relative 
abundance of diatoms during this time. Collections were made with a plankton net 
of No. 20 bolting silk, and hauls were made at the surface of the water. While the 
collections were made primarily for obtaining oyster larvae, they served as well foi 
diatoms. Samples were taken only infrequently during April and the first part of 
May, but after this they were taken almost every day. 
The relative abundance of diatoms was estimated as in the case of oyster larvae. 
Numbers from 0 to 6 were used to designate the relative number of diatoms from 
none to the maximum ever taken. The method is not strictly quantitative but 
serves well enough to show roughly the seasonal variation in abundance of diatoms. 
The numbers representing abundance were defined as follows: 0, diatoms totally 
absent, although this was never the case, for in every collection there would be 
at least one or two diatoms; 1, very few diatoms, representing scarcely enough 
to form a coating on the bottom of the bottle; 2, few; 3, fair number; 4, many; 5, 
very many; and 6, extremely abundant, so that the plankton net became clogged 
and the sample bottle was as full as possible of diatoms. The gradations in between 
the extremes are largely arbitrary, but since all estimates were made over a period 
of a few days, while the last of the samples were being taken, and by the author only, 
they represent a reasonable degree of accuracy. 
* Unpublished manuscript. 
