THE RELATIONSHIP BETWEEN THE GAMETOGENIC CYCLE OF THE PACIFIC OYSTER, CRASSOSTREA GIGAS, AND THE SUMMER MORTALITY PHENOMENON IN STRAINS OF SELECTIVELY BRED OYSTERS (WASHINGTON)

The gametogenic cycle of the Pacific oyster, Crassostrea gigas, was investigated to determine what relationship it had with annual summer mortalities of the Pacific oyster in Washington state. The oysters used were experimental families produced in a selective breeding program since 1974 at the University of Washington. This program was designed to breed oysters resistant to the summer mortality phenomenon. Comparisons were made between families that exhibited low mortalities and those that exhibited high mortalities in the same environments (bays).;The reproductive effort exhibited by C. gigas in this study was dramatically greater than other oyster species within the genus Crassostrea. This fact, combined with the finding that females were selectively killed during mortality episodes suggested a proximal cause related to the reabsorption of this unusually large volume of eggs produced in food-rich (i.e., eutrophicated) environments.;The ultimate cause of the summer mortality phenomenon of the Pacific oyster was theorized to be the end result of a basic life cycle strategy in which C. gigas might be classified as a "reckless reproducer" as defined by Calow (1981). In Calow's definition r-selected iteroparous (repeat breeding of parents) species such as C. gigas become semelparous (parental death after breeding) under conditions of increased food levels due to a dynamic reproductive strategy whereby reproductive effort is increased dramatically.;In a related study (Chapter II) it was shown that carbohydrate content could be increased in Pacific oysters through selective breeding. These findings suggested that the success of the breeding program to select families resistant to summer mortality might be due to the alteration (through selective breeding) of a trait related to the gametogenic cycle. In addition, the results indicated a method for improving the quality of Pacific oysters because high carbohydrate content is associated with better tasting oysters, especially during summer months.;It was found that annual mortalities in these groups of oysters occurred in August at a point just after peak gonadal development. Families that spawned in July or early August escaped heavy mortalities. Experimental families that exhibited reduced reproductive effort showed good survival during mortality episodes.