2 



Centimeters 



Figure 393. — T. lamellosa Gmelin, a native species of the Pacific coast of the United States. 1 — apertural view; 



2 — abapertural view. 



The Japanese species, Ocenebra japonica, is far 

 more dangerous than the native snail. Mortal- 

 ities due to devastation by this snail are estimatetl 

 at 15.4 to 22.6 percent. The first specimens of 

 0. japonica were introduced into the waters of 

 Puget Sound with the planting of Japanese seed 

 oysters, a practice which began in 1902 and 1903 

 and wliich reached considerable proportions by 

 1922 when from 1,500 to 4,000 boxes, each con- 

 taining about 5,000 seed, were planted annually. 

 In October 1928 while examining the oyster beds 

 in Samish Bay, Wash., I found a number of 

 0. japonica Dunker (Galtsoff, 1929, 19.32), and 

 warned oystermen and state officials of the possible 

 damages that could result if the practite of bringing 

 infested seed oysters from Japan was continued. 

 The warning received no attention. In the late 

 1940's Ocenebra was well established in the waters 

 of Puget Sound and became a serious menace to 

 the native oysters. When given a choice of food, 

 Ocenebra prefers 0. lurida and Manila clams, 

 Venerupif! japonica, to C. gigas (Chew, 1960). It 

 drills holes in the shell by combined cheniical and 

 mechanical action (Carriker, 1961a). The fertil- 

 ity of the species is high, the female laying an 



average of 25 egg cases, each containing about 

 1,500 eggs. The egg cases are often found in the 

 inaccessible crevices of the concrete walls of dikes 

 surrounding the Olympia oyster beds. Salinity of 

 18°/oo adversely affects Ocenebra, and brackish 

 water of less than 12°/oo salinity is lethal. 



Large conchs or whelks, Bu..<^ycon carica Gmelin 

 and B. canaliculatum Linne, are common in the 

 shallow water of the Atlantic coast and occa- 

 sionally attack oysters and open them by inserting 

 the edge of the shell between the valves and 

 forcing them apart (Colton, 1908). Carriker 

 (1951) reinvestigated the problem and found that 

 penetration of shells of oysters and clams is a 

 purely mechanical process which consists of chip- 

 ping by the edge of the conch's shell combined with 

 rasping of the radula. The shell edge of an oyster 

 destroyed by these conchs bears the marks of the 

 attack (fig. 395). In the northern part of Cape 

 Cod, Busycon seems to attack the oyster in prefer- 

 ence to other mollusks, annelids, or dead fish, 

 which they are known to constmie. Local depre- 

 dations on oysters observed in the Cape Cod area 

 (Shaw, 1960, 1962) may be severe enough to war- 

 rant trapping of conchs during their reproductive 



FACTORS AFFECTING OYSTER POPULATIONS 



435 



