3. But outstanding peiformance of Maine hybrids 

 as larvae and early postselting spat. 



4. A hybrid growth rate of the spat in the first year 

 slower than that of the local controls, but betterthan 

 that of the nonlocal controls. 



5. Extremely poor growth of the older spat of 

 Maine hybrids and of the Maine x Maine controls 

 with total mortality before the second year. Maine 

 hybrids did better though than the Maine x Maine 

 control. 



Full fertility was found in some geographic hy- 

 brids of C. i,'/>rt,s. along with fertility of their Fi (Imai 

 and Sakai. 1961). Generally speaking, the 

 crossbreds had a higher degree of hardiness, as com- 

 pared to the inbred strains, and a greater adaptation 

 to environmental conditions. Morphological charac- 

 teristics fell between those of the strains crossed as 

 did growth, weight, index of meat weight, and 

 glycogen content. 



Some years ago H. C. Davis of the Milford 

 Laboratory crossed C. viri>inica with C. ^igas 

 (Davis, 1950). Fertilization took place readily, cell 

 division was normal, and early veliger larvae were 

 obtained. However, they all died before reaching 

 the umbo stage. Imai and Sakai (1961) obtained the 

 same result. They further found both crossing bar- 

 riers and hybrid inviability in crosses of C. gigas 

 with C riviilaris and C. echinatu. More recently, 

 Menzel (1968a) reported obtaining a few spat from 

 the hybrid cross of C. virgiiiica with C. gigas, and 

 some spat from crosses of other species with C 

 virginica. Interspecies crosses, at least those in- 

 volving C. virginica. in general, though appear dif- 

 ficult to obtain and difficult to culture successfully 

 (Menzel, 1967, 1971). Recently, a cross of C. vir- 

 ginica with C. angulata. the Portuguese oyster, re- 

 sulted in 209? of the eggs developing to the 

 straight-hinge larval stage (Stiles. 1973). All of the 

 larvae, most of which were abnormal, died shortly 

 after this. Cytogenetic examination of this inter- 

 species fertilization revealed 35% of the eggs to be 

 unfertilized. Another I09f had a very delayed fer- 

 tilization. Polyspermy occurred in 35% of the eggs. 

 Sperm nuclei were abnormally large and irregular in 

 259f of the eggs. Cleavage was irregular in y'r . 

 Nuclei of the early larval tissues were abnormally 

 vacuolated and highly irregular in shape. Adult oys- 

 ters were reared from the hybrid C. gigas x C. 

 angulata made by Imai and Sakai (1961). 



Difficulties in obtaining species hybrids of oysters 

 should not be viewed altogether pessimistically. 



Some of the methods used in plants and other ani- 

 mals to break down gamete cross incompatibility 

 barriers, as in inbreeding, would, no doubt, be useful 

 in accomplishing interspecific fertilization in the 

 oyster. The problem of hybrid inviability might be 

 overcome by crossing large enough numbers of indi- 

 viduals, carrying larger than usual cultures, by cross- 

 ing different races of the species involved, or with 

 mutagenic agents, if direct hybrids between the de- 

 sired species are not possible even then orare sterile, 

 the use of a third "bridging"" species can sometimes 

 circumvent the barrier. 



Currently the only reliable way to achieve pure 

 hybridization in the oyster without any contaminat- 

 ing nonhybrids is to make single crosses of individual 

 oysters spawned separately, a tedious process not 

 commercially practical. This is so since the oyster 

 cannot be sexed until spawning and because the 

 oyster once sexed can reverse its sex. One male 

 spawning in a group of intended female hybrid par- 

 ents would fertilize all the eggs before the desired 

 hybrid cross could be achieved. This problem could 

 possibly be solved through the use of some sperm 

 inhibiting agents in the mass-spawning population 

 intended for use as the pool of female parents in the 

 mass hybridization. 



EFFECTS OF IONIZING IRRADIATION 

 ON C. VIRGINICA 



Mutation breeding has been scarcely attempted in 

 economically useful farm animals. This is because of 

 the high cost of culling out the large numbers of 

 individuals carrying the great numbers of lethal or 

 subvital mutations due to the low reproductive rate 

 of mammals. Because of the oyster's tremendously 

 high reproductive rate and the insignificant worth of 

 a single oyster, there would be no such limitations on 

 mutation studies, or breeding with mutations in the 

 oyster. 



Some irradiation-mutation studies were mitiated 

 inC. virginica for the basic information that could be 

 derived, and to determine the radiosensitivity of this 

 mollusk. a member of a group about which there is 

 relatively little such information (Fig. 9) (Longwell. 

 1969; l.ongwell and Stiles. 1970: I ongwell and 

 Stiles, unpublished data).' There is. of course, the 

 probability of practical application of such informa- 

 tion in the future. For example, irradiation might 



■' Irradiation was carried oul at the Brookhaven National 

 Laboratory with the assistance and advice of A. H. Sparrow. 



84 



