Caged oysters still get scared: Predator presence and density influence growth in oysters, but only at very close ranges

Two common forms of variation that may influence consumptive and non-consumptive effects differently are how the biomass of predators is allocated among individual predators (e. g. several small vs. few large predators) and how predators are spaced throughout a community. We analyzed how varying the presence, biomass (density, size, and total biomass), and distance to crown conchs Melongena corona impacted growth in eastern oysters Crassostrea virginica grown in field conditions. The presence of predators decreased growth (new shell added and mass) and increased shell thickness in a 58 d experiment. Although these effects were more pronounced as predator density increased, total predator biomass and predator size had limited impact on the strength of non-consumptive effects. The allocation of total oyster mass between shell and tissue was also not impacted by predator treatments. Results from a 96 d study examining the range of these effects indicated that they may exist only over short distances or change as oysters grow, as oysters at varying distances from a caged predator showed no differences in growth traits. These results show that non-consumptive interactions in oyster reef communities may be highly non-linear in regards to predator community structure and exposure distance and indicate these factors may be important in determining the impact of non-consumptive effects in other communities. Our growth data also show that non-consumptive effects may have major impacts on oyster growth under normal aquaculture conditions and suggest that these effects may need to be considered in management efforts.