362 
BULLETIN OF THE BUREAU OF FISHERIES 
than it had previously pumped in water of the same salinity. With respect to the 
above interpretation this would possibly be expected, for the ostia would be much 
wider open than normally after the osmotic pressure of the blood has reached equi- 
librium with the medium. 
Living, as oysters do, in a highly changeable environment, subject to seasonal 
weather conditions which may cause variations in salinity between almost pure fresh 
water and undiluted sea water, they must necessarily have a remarkable tolerance for 
salinity changes. It is well known that in time of freshet oyster beds may be covered 
for weeks with almost fresh water without resulting in any considerable mortality. 
That they are able to maintain life under such conditions is probably largely due to 
their ability to remain closed for long periods of time (Hopkins, 1931b), protecting 
the actual tissues. However, the present results indicate that pumping of water and 
feeding probably do not go on under such extreme conditions, so that starvation 
would eventually take place if the low salinity were of too great duration. 
It was shown above that there is a minimum limit of salinity below which pump- 
ing does not go on and a further, more lasting effect produced which renders the oyster 
capable of only very slow recovery after being returned to water of higher salinity. 
This limit appears to lie between 10.5 and 13 parts per mille. At the latter salinity 
only a small amount of water was pumped, but recovery was readily accomplished 
in a high salinity. These results compare favorably with Nelson’s (1921) estimate 
of a salinity of about 10.42 parts per mille as the lowest at which 0. virginica can 
feed, although his conclusion was based upon whether or not the shell remained open. 
Pumping activity was approximately the same at salinity ranging from about 
25 to about 39 parts per mille, although the latter value is considerably h'gher than 
that of pure ocean water. A salinity as high as 56 parts per mille is obviously un- 
favorable, and, although pumping some water, the specimen gaped and would 
probably have soon died. From an ecological standpoint it is the lower limit that is 
of importance, for seldom are oysters in nature forced to adjust themselves to a 
salinity higher than that of the ocean. 
The oyster appears to be increasingly sensitive to changes in salinity below about 
20 parts per mille, possibly indicating that the optimum is higher. The results do 
not permit one to state the optimum salinity, but it would appear to be well above 
20 parts per mille, possibly as high as 30 or 35 parts per mille. It may be that 
the optimum depends upon the medium to which the particular oysters have been 
accustomed for generations, that specimen's from beds in relatively fresh bays would 
react differently from those grown in more saline water, as suggested by Nelson 
(1923a). Most likely, however, little difference of this nature is to be expected, save 
that a longer time would be required for adaptation of specimens from the water of 
higher salinity. 
SUMMARY 
1. Adaptation of the feeding mechanism of the oyster to changes in salinity was 
studied by recording on the kymograph the degree of openness of the valves and the 
relative rate of flow of water pumped by the gills. 
2. Both the activity of the gills and that of the adductor muscle, which by con- 
trolling the position of the valves determines the size of the inhalent and exhalent 
apertures, are markedly affected by any considerable change in salinity. The initial 
effect of such a change is to cause partial or complete contraction of the adductor 
muscle and slowing or cessation of the flow of water. 
