BIOLOGY AND NATURAL HISTORY 147 



program. Through trial-and-error methods and observation, oystermen have 

 discovered the most favorable times to plant shells, to secure seed oysters 

 and transplant young oysters. On a long-range basis the activities may coin- 

 cide more or less with the events in the life history of an oyster; but for 

 precise information, scientific observations are necessary each season to 

 achieve the greatest efficiency in the management of public areas. In the 

 brief review of the biology of the oyster, an attempt will be made to point 

 out the importance of knowledge of the life history in regulating the various 

 activities. 



SPAWNING. In the late spring and early summer the reproductive organs 

 or gonads of oysters become greatly distended and milky white in color; 

 such oysters are designated "in milk." The sex of an oyster cannot be 

 distinguished externally or from the extrusion of the sex products, as is 

 often done in some species of fish. It is necessary to examine the sex prod- 

 ucts with the aid of a lens or microscope. Our native eastern oysters are 

 generally considered of separate sexes. 



Spawning occurs when the animals are physiologically ripe. The season 

 of actual spawning varies with the ecological conditions of localities. In 

 general, spawning first occurs in North Carolina in April or May and may 

 continue until November (Grave, 1904). It was believed that spawning is 

 directly correlated with water temperatures; however, in recent years evi- 

 dence has been presented to show that such factors as salinity, tidal cycles, 

 hydrogen-ion concentration, pressure, and phases of the moon may influence 

 spawning activities. Temperatures at which spawning first occurs may vary 

 from 16.4° to 25° C. (61.5° to 77° F.) (Nelson, 1928; Galtsoff, 1938; 

 Loosanoff, 1939-b; Stauber, 1947). 



Active spawning has been described as observed in nature and in the 

 laboratory. During spawning, the shells of the female rhythmically open and 

 shut about once every thirty seconds, expelling a fan shaped cloud of eggs. 

 In the male the valves gape open slightly and a thin stream of sperm issues 

 forth. In confined areas the surrounding waters may soon turn milky. The 

 time of spawning is important, for preparations can be made for planting 

 of shells. Some oysters spawn out completely in the early summer and may 

 develop new sex products to spawn again in the same season, while other 

 individuals may spawn over an extended period throughout the entire 

 summer. 



EARLY DEVELOPMENT. The development of larval oysters from the egg to 

 the setting stage has been described by many workers (Brooks, 1880; 

 Ryder, 1884; J. Nelson, 1910; Stafford, 1913; T. C. Nelson, 192 1 and 

 others). For a detailed account, reference should be made to these publica- 

 tions. The liberated eggs and sperm unite in the water each fertilized egg to 

 form, in about four hours, a microscopic mass of cells capable of locomotion. 



