BIOLOGY AND NATURAL HISTORY 149 



optimum temperature for functioning of the gills is believed to be between 25° 

 and 30° C. (77° to 86° F.), Through these gills water passes continuously 

 when the shells are open; oxygen is absorbed; carbon dioxide diffuses out; 

 and food is strained from the water. Above 30° C. (86° F.) the rate of 

 pumping begins to decrease and ceases at about 40° C. (104° F.). Since these 

 results were obtained experimentally in the laboratory, it is possible that in 

 nature the normal activities may cease or decrease within different tempera- 

 ture ranges. During extremely hot weather oysters are believed to go into 

 estivation — that is, summering, the opposite of hibernation or wintering. 

 From observations in North Carolina waters it appears that growth rates are 

 greater in the spring and fall. 



SALINITY. Oysters are found growing in a wide range of salinity. Churchill 

 (1920) gives the range from about 2.5 to 33 parts of salt per thousand and 

 states that oysters cannot withstand salinities below 9 parts per thousand for 

 prolonged periods. He places the optimum salinity between 14 and 28 parts 

 per thousand. Hopkins (1936) has shown the effects of significant changes 

 in salinity upon the feeding mechanism. He places the optimum salinity 

 above 20 parts per thousand and possibly as high as 30 to 35 p.p.t. The rate 

 of water pumping is generally higher at salinities of about 28 parts per thou- 

 sand. The effects of salinity changes depend upon the degree of change and 

 the conditions to which the animal has become accustomed through genera- 

 tions. From the point of view of the industry the minimum range of salinity 

 is perhaps the most important. 



TYPES OF BOTTOM. Although oysters are found growing on various types of 

 bottom ranging from mud to hard sand, they survive best upon a firm bottom. 

 Since oysters are immotile in the adult stage and cannot move about as clams 

 and scallops do, bottoms which tend to shift or are of too soft a consistency 

 might result in the smothering of the oysters. Oysters are occasionally found 

 partly buried, with but a small portion of the shell extending above the 

 bottom. This small portion is sufficient to allow the passage of water currents 

 between the shells and enables the oyster to feed and grow. The organic and 

 chemical constituents of the bottom may contribute necessary elements for 

 growth of various food organisms. 



FOOD AND FEEDING. The problem of what an oyster can utilize as food has 

 attracted interest for over fifty years. Martin (1923) states that three 

 theories exist regarding the nature of the food of oysters : namely, that plank- 

 ton organisms constitute the principal source of nutriment; that nourishment 

 is derived from detritus, the finely divided constituents of plant and animal 

 cells; and that dissolved organic matter may be directly utilized. Of the three 

 theories the first two are still prevalent, and possibly both are important. 

 Many factors are involved in the physiology of feeding, such as the size of 

 the particles, concentration of food, tj^es of organisms, and proportion of 



