Laboratory Settlement of the Eastern Oyster Crassostrea virginica Influenced by Substratum Concavity, Orientation, and Tertiary Arrangement

To examine the role of substratum concavity in determining oyster [Crassostrea virginica (Gmelin, 1791)] settlement success, laboratory settlement was examined on natural and artificial substrata of various concavities, defined by width-to-depth (W:D) ratio. Experiments were conducted in 204-l capacity laboratory flumes. Concavities tested include W:D ratios of 2, 5, and 16, plus a flat surface for control comparisons. Active settlement, passive settlement, and flow around these concavity treatments were examined. In addition, settlement on oyster shell cultch presented in different surface orientations, and either zip-tied to open mats, or packed in mesh bags, was examined. Results indicate that concavity can affect settlement success (W:D ratio of 5 resulted in the highest settlement success under this study's conditions). Examination of flow rates and patterns suggests that water movement over W:D 5 treatments delivers larvae to the settlement surface, but active attachment behavior is required to explain W:D 5's higher settlement densities relative to W:D 2. The highest settlement occurred with open mats on surfaces oriented horizontally and facing downward. Comparisons of mat, bag, and orientation results suggest that optimal current range for settlement is between 0.3 and 2.0 cm/sec. Too little flow results in supply failure, while flow exceeding 2 cm/sec inhibits searching and attachment behaviors.