ADAPTATION OF OYSTER TO CHANGES IN SALINITY 
361 
innumerable factors, both internal and external, influence the activity. Lack of 
constancy in the rate of pumping of the gills may be traced to contraction or relaxa- 
tion of the gill musculature, changes in size of the ostia caused by variations in 
diameter of blood vessels of the gill filaments (Elsey, 1935), secretion of mucus 
which impedes activity of the cilia, etc. However, in spite of such uncontrolled 
factors which render exact analysis difficult, the results are of sufficient unifoimity 
to warrant definite conclusions. 
The experimental results described above suggest that an important factor in- 
fluencing the quality of oyster meats grown on different grounds is the salinity 
conditions. The rate at which the gills pump water, from which food particles are 
filtered, depends upon the salinity of the medium or, more accurately stated, upon 
the frequency, amplitude, and duration of changes in salinity. It is well understood 
that rate of pumping of water does not alone determine the rate of feeding, for the 
abundance of food material in the water is a variable, depending upon other factors, 
such as the availability of nitrates and phosphates for utilization by the microscopic 
plant life on which oysters feed. In some instances land drainage may bring fer- 
tilizing materials which cause prolific development of food organisms, at the same 
time so diluting the sea water that oysters are not able to pump sufficient water and 
to feed effectively. 
Any significant change in salinity causes an immediate slowing or cessation in 
the rate of pumping. Recovery depends upon the amplitude of the change and upon 
whether it is to a higher or lower level. While only a few hours may be required for 
adaptation following a rise in salinity, several days may be necessary for recovery 
following the same change in the opposite direction. It was thought possible that 
this difference was due to the fact that the specimens had been grown in water 
ranging generally about 25 to 29 parts per mille and any change from this level 
would produce a marked effect, while restoration of this salinity resulted merely in 
the resumption of normal activity. However, adaptation to a further rise in salinity 
to 36 or 39 parts per mille, higher than oysters encounter in nature, is also rapid, 
but when a salinity of about 28 parts per mille is restored adaptation is very slow. 
Rate of adaptation following a change in salinity is also proportional to the extent 
of the change. Further, as the salinity becomes lower the sensitivity of the oyster 
to small changes increases, very much as was found (Hopkins, 1931a) with regard 
to temperature. It was shown that a small drop in temperature caused closure of 
the shell if the temperature was well below the optimum but produced no effect if 
near the optimum. 
It appears likely that endosmosis following a reduction in salinity swells the 
tissues and blood vessels, such as those in the gills, resulting in decrease in the size 
of the pores, or ostia, through which water is forced by ciliary action. An increase 
in salinity, on the other hand, would involve extraction of water and consequent 
increase in size of the ostia. The adductor muscle is probably also affected by swell- 
ing and shrinking. The slow rate of adaptation to a lowering of salinity is in most 
cases clearly traceable to diminished activity of the gills, while the valves soon open 
as wide as, or wider than, before the change. On the other hand, a rise in salinity 
involves a slow accommodation in degree of openness of the shells and an almost 
immediate adaptation of the gill mechanism. 
& In certain cases there occurred what appeared to be an actual stimulation of 
gill activity following rise in salinity, whereby the specimen pumped more rapidly 
