mum conditions by 10. Tlie values between are 

 based on the percentage of time the oyster popula- 

 tion is effected by unfavorable salinities. 



TEMPERATURE 



A great difference in climatic conditions exists 

 within the range of distribution of C. virginica. 

 The water temperature under which the species 

 lives varies from a minimum of about 1 ° C. during 

 the winter in northern states to a maximum of 

 about 36° C, which occasionally has been ob- 

 served in Texas, Florida, and Louisiana. The 

 temperature of oysters exposed to the sun at low 

 tide on the flat registers 46° to 49° C. measured 

 by inserting a small thermometer between the 

 slightly opened valves. Normally oysters of the 

 tidal zone remain exposed for 2 to 3 hours at the 

 maximum. Occasionally strong offshore winds 

 drive the waters away and oysters beds in shallow 

 places remain out of water for several days. Such 

 instances occur along the coast of Texas where as 

 a rule the low stage of water is caused by strong 

 northern winds. The exposed population may 

 perish either from excessive warming or from 

 freezing temperatures brought from the north by 

 cold fronts. 



The temperature regime affects the life of the 

 oyster by controlling the rate of water transport, 

 feeding, respiration, gonad formation, and spawn- 

 ing. C. virginica ceases feeding at a temperature 

 of 6° to 7° C The maximum rate of ciliary 

 activity responsible for the transport of water is 

 at about 25° to 26° C; above 32° C. ciliary 

 movement rapidly declines. Nearly all functions 

 of the body cease or are reduced to a minimum at 

 about 42° C Using the seasonal fluctuations of 

 temperature, it is easy to determine the percentage 

 of time dm'ing which oysters in any given locality 

 continue to feed and reproduce. Similar observa- 

 tions may be made on the growth of shells and 

 calcification. Two curves in figure 368 show the 

 seasonal changes in mean monthly temperatures 

 in two localities separated by about 11.5 degrees 

 of latitude. The northern location of oyster 

 grounds of Long Island Sound is at about lat. 41 ° 

 30' N.; the southern location is that of Apalachi- 

 cola Bay, Fla., at about lat. 30° N. The two 

 cui-ves, upper for Apalachicola and lower for Long 

 Island Sound, parallel each other but are at two 

 distinct levels. The difference is greatest during 

 the winter and early spring and is smallest during 

 the fall. The two temperature levels indicated 



a: 



3 



< 

 a. 



a. 



5 



< 



5 



2 

 O 



Figure 368. — Mean monthly temperature in °C. of water 

 in Apalachicola Bay (upper curve) and Long Island 

 Sound (lower curve). 



by broken lines mark the periods of successful 

 mass spawning and setting of oysters at tempera- 

 tures of 20° C. and above and the inhibition of 

 feeding and growth of the oysters below 8° C, 

 often called hibernation. Apalachicola oysters 

 continue to feed and grow throughout the year, 

 and then- reproductive season may last for 7 

 months or 58 percent of the year, whereas the 

 period of feeding and growth of the northern 

 oysters is limited to about 6K months, or 56 per- 

 cent of the time and the reproductive season is 

 restricted to 2 summer months, or about 16 

 percent of the time. The use of monthly means 

 based on several years of observations gives a 

 pictm-e of a general condition not unduly in- 

 fluenced by short-term fluctuations which may 

 differ from year to year. 



Little is known about the prolonged effect of 

 temperatures above 32° to 34° C. on oyster popu- 

 lations. From a few physiological observations 

 it may be inferred that long continued exposure 

 to high temperature is unfavorable and impedes 

 the normal rate of water transport by the gUls. 



FACTORS AFFECTING OYSTER POPULATIONS 



407 



