morphological reflection of release of enzymes into the medium. Degranulation 

 does not involve damage to the plasma membrane of granulocytes. The function 

 of lysozyme in serum of M. mercenaria and other mollusks remains uncertain. 

 Degranulation of granulocytes of M. mercenaries in vitro has been observed in 

 absence of bacteria, but the process is increased when bacteria are present. 

 Thus, it is assumed that release of enzymes from hemolymph cells is a normal 

 process, which is enhanced during phagocytosis. - J.L.M. 



336 



Chesapeake Biological Laboratory. 1953. 



The commercial fisheries of Maryland. Md. Board Nat. Res., Dept. Research 

 Educ. , Educ. Ser. 30, 45 p. 



Hard clam grows in commercial abundance above the Virginia line in Chesapeake 

 Bay only in a very limited area off Crisfield. It is restricted to saltier 

 waters. In coastal bays on the ocean side, Chincoteague, Sinepuxent, and 

 Isle of Wight, hard clams abound. - J.L.M. 



337 



Chestnut, Alphonse F. 1951. 



The oyster and other mollusks in North Carolina. In Survey of Marine 

 Fisheries of North Carolina, by Harden F. Taylor and a staff of associates. 

 Univ. N.C. Press, Chapel Hill, p. 141-190. 



Twelve species of Venus along the southeastern coast, eight of which are 

 reported from waters of N.C. Species mercenaries the only one of commercial 

 importance. Literature is meager. Little has been published since pioneer 

 works of Kellogg and Belding in first 2 decades of 20th century. Sexes 

 separate, but clams can change sex. Nearly all develop first as males, 

 approximately 50% become female the 2nd year. Soon after spawning ceases 

 in fall, sex products develop, often reaching maturity by midwinter. Cold 

 water prevents spawning until next spring. Ovaries and testes usually white, 

 but take on reddish or yellow tint with age. Spawning believed directly 

 correlated with temperature, but other factors may operate. Spawns in Mass. 

 from mid-June to mid-August, commencing at 76° to 77°F., in Long Island Sound 

 at about 73°, beginning earlier and continuing longer. Some clammers 

 believe they spawn year-round in N.C. Young hard clams may spawn at an age 

 of three or four months, but most spawn after they reach their first year. 

 Others have concluded that clams must be two years old before spawning. 

 One observer stated that littlenecks and large chowders, although they may 

 contain sex products, do not spawn. Sex cells are extruded into the mantle 

 cavity and pass out through the excurrent siphon. Within 36 hours a shelled 

 veliger larva is formed. Free swimming ceases 6 to 12 days later. The clam 

 attaches to some surface with a byssus thread secreted by a gland in the 

 foot. The primary purpose of attachment appears to be protective, to prevent 

 involuntary transport by waves or currents. Soon the young clam burrows into 

 the bottom. At all stages, to the adult, hard clams retain considerable powers 

 of locomotion. The well-developed foot is used to burrow, crawl for short 

 distances, or to turn over. Lateral movement is limited, however, and the 

 average distance travelled in 38 days was 2 inches, and a maximum of 6 

 inches, from the place where clams were first bedded. Hard clams rarely 

 burrow to a depth greater than the length of the shell. Burrowing depth 

 appears to be related to periods of feeding, water temperature, and tides. 

 Some investigators have emphasized the importance of currents to hard clam, 

 assuming that the stronger the current the greater the food supply. Currents 

 also provide oxygen and carry away metabolic products. Bottom microorganisms, 

 as well as plankton, are an important part of the food supply. Hard clams 

 live as well in the intertidal zone as they do at 50 feet, but are usually 

 found on flats in relatively shallow waters a few feet below low tide. An 

 equal mixture of mud and sand is considered to be the best bottom type, but 

 clams grow in a variety of bottoms, ranging from sand to gravel to rocks. 

 Organic acids in the bottom can erode the shell and retard growth. Natural 

 clam beds can be found at salinities from 10 to 28°/°°, and clams can 

 tolerate wide fluctuations of salinity for short periods. A hurricane in 

 1933 ruined oyster beds in some NX. waters. Soon after, hard clams became 

 very abundant in areas where they had never existed in commercial 

 quantities. It was concluded that increased salinity, provided by new 



94 



