BIOLOGY AND NATURAL HISTORY 163 



they burrow in the sand and are said to "set." Since setting frequently 

 occurs below the low tide level, "sets^' often pass unnoticed. There is a 

 need for the development of commercial methods for collecting clam sets 

 during these periods. 



In the adult stage the hard clam differs markedly from the oyster in that 

 the clam retains power of locomotion. The adult clam possesses a well 

 developed foot which it uses to burrow, to crawl for short distances, and to 

 turn over. Although hard clams can crawl, there is little migration of the 

 clams from the areas where they set. Belding (1912) shows that the 

 average distance traveled in 38 days was two inches, with a maximum of 

 six inches, from the spot where the clams were first bedded. 



The adult clams do not bury themselves very deep, for they feed through 

 siphons which extend to the surface. The siphons are short (about one 

 inch), in comparison to those of the soft clam (Mya), which extend about 

 six inches. Hard clams rarely bury themselves to a depth greater than the 

 length of the shell. The depth to which clams burrow appears to be cor- 

 related with periods of feeding, water temperatures, and tidal cycles. 



ECOLOGY 



CURRENTS. Many of the early investigators of oysters and clams (Lotsy, 

 1895; Moore, 1898; Kellogg, 1900-b, 1903; Grave, 1904; Belding, 1909, 

 1912) emphasized the importance of water currents, which they correlated 

 with feeding. Presumably the greater the current, the more food is avail- 

 able to the animal. Nelson (1947), however, has recently shown the 

 importance of the bottom microorganisms in the food supply. Currents do 

 play an important role in replenishing the oxygen supply and in carrying 

 away the metabolic waste products. 



DEPTH OF WATER. Belding (1912) reports that hard clams live as well 

 in the intertidal zone as at depths up to 50 feet. They are most frequently 

 found on the flats in relatively shoal waters, a few feet below the low tide 

 zone. 



SOIL. Clams grow in a variety of bottoms ranging from sand to gravel 

 and rocks. A bottom consisting of an equal mixture of mud and sand is 

 considered the most favorable. Such a bottom not only makes it easier for 

 the clam to burrow but greatly facilitates harvesting of the crop. Soils that 

 contain organic acids may cause corrosion of the shell and retard the 

 growth of clams. 



SALINITY. There appears to be a rough correlation in North Carolina 

 between the distribution of clams and salinity. Clams are confined to areas 

 of higher salinities than the oyster and are rarely found in the western and 

 northern Pamlico Sound areas where the salinities range from 8 to 20 parts 

 per thousand. Belding (1912) states that salinity has little effect on clams. 



