Comparative vulnerability to predators, and induced defense responses, of eastern oysters Crassostrea virginica and non-native Crassostrea ariakensis oysters in Chesapeake Bay

Management agencies are considering introducing the Suminoe oyster Crassostrea ariakensis into Chesapeake Bay, USA. It is unknown if the growth of feral populations of this non- native oyster would be regulated by the same predators that once controlled the abundance of the native eastern oyster C. virginica. In laboratory studies, we compared the relative susceptibility of juvenile diploids ( shell height < 25 mm) of both oyster species to invertebrate predators of eastern oyster juveniles. Predators included four species of mud crabs [ Rhithropanopeus harrisii ( carapace width 7 - 11 mm), Eurypanopeus depressus ( 6 - 21 mm), Dyspanopeus sayi ( 8 - 20 mm), and Panopeus herbstii ( 9 - 29 mm)], the blue crab Callinectes sapidus ( 35 65 mm), and two sizes of polyclad flatworms ( Stylochus ellipticus and possibly Euplana gracilis; planar area <= 5 mm(2) and similar to 14 to 88 mm(2)). All four species of mud crab and the blue crab preyed significantly ( ANOVA, P <= 0.05) more on C. ariakensis than on C. virginica, but predation by flatworms of both sizes did not differ significantly between oyster species. The greater susceptibility of C. ariakensis to crab predation was likely due to its shell compression strength being 64% lower than that of C. virginica ( P = 0.005). To test for predator- induced enhancement of shell strength, we held oysters of both species for 54 days in the presence of, but protected from, C. sapidus and R. harrisii. Crabs were fed congeneric oysters twice weekly within each aquarium. Compared to controls, shell