Dataset: Results of risk-addition experiment (adult oyster condition index)

As part of a study that manipulatively “pressed” risk cues onto oyster prey, a field experiment was conducted on oyster reefs in the Guana Tolomato Matanzas National Estuarine Research Reserve (Ponte Vedra Beach, Florida) from June to November 2012. Three areas within the southern areas of the GTM NERR (south of Matanzas inlet) were used in the experiment: Summer Island North (SIN), Marine Land (ML) and Pellicer Flats (PF). The SIN site occurred closest to the inlet (farthest from freshwater input), the PF site occurred farthest from inlet and closest to freshwater input, while the ML site occurs between the inlet and the freshwater input. Oyster survival, growth and recruitment were checked monthly. At the mid point and conclusion of the experiment, individual oysters were also destructively sampled to quantify differences in oyster traits (shell versus tissue mass) as a function of experimental treatment and location. This dataset concerns the survival of adult oysters.

Each experimental unit consisted of a focal oyster cage with dimensions: 18 centimeters (cm) × 13 centimeters (cm) × 18 centimeters (cm) centered between two opposing smaller cages for predators (13 cm × 13 cm × 13 cm). One predator was placed in each of the two smaller cages, which were held flush to the exterior of the central focal cage with cable ties, so that oysters in the central cage were exposed to predator cues from two directions, but were protected from being eaten. All cages were constructed with PVC-coated wire mesh (6-millimeter mesh openings) and were sewn shut as well as together with Maxi Edge trimmer line (0.17 cm diameter). To mimic the turbulent dispersion of water-borne cues on natural oyster reefs, we placed four sun-bleached oyster shells in each predator cage to disperse water flow. At each site, we established four transects (6-meter in length) parallel to the shoreline and separated by 3 meters on a mudflat. Along each transect, six experimental units were deployed at 1-meter intervals. In this estuary, settlement of larval oysters to the benthos primarily occurs in two large pulses in the spring and fall of each year. The experiment began in June 2012, after the spring recruitment pulse, and ended in November 2012, after the fall recruitment pulse.

Within each site, experimental units were randomly assigned among four levels of the cue factor: no cue, mud crab (Panopeous herbstii) cue, crown conch (Melongena corona) cue, and multiple predator cue (mud crab cue as juveniles, then crown conch cue as adults). There were 24 total experimental units per site (n = 6 for each cue treatment). The mud crabs had a mean carapace width of 38 millimeters (mm), and the crown conchs had a mean shell length of 83 mm. Predators were replaced weekly with new animals collected from nearby oyster reefs. Each replicate of a cue treatment contained two mud crabs or two conchs. This density of experimental predators per unit area is within the range of natural predator density on oyster reefs throughout the Matanzas River Estuarine system (MRE).

Within each central oyster-holding cage, we installed 12 ceramic tiles (8 cm × 8 cm) by drilling a hole into the top of the tiles and fastening the tiles to the inner wall with cable ties. Prior to the installation of tiles, juvenile oysters of equal age and size (6–8 mm shell length) were produced in a local hatchery (Research Aquaculture Inc., Jupiter, FL). Ten of these juvenile oysters were attached to each tile with superglue (Loctite gel).

Within each predator cue treatment, we randomly assigned the 12 tiles among four levels of simulated consumptive effect: none, juvenile stage only, adult stage only, or both stages. This produced 6 x 3 = 18 replicates for each combination of predator cue and simulated consumptive effect (CE) at each site. Simulated predation rates were based on oyster survival curves from previous field experiments in this estuary. On a weekly basis, we manually culled juvenile oyster density according to a log-transformed survival curve for juvenile oysters (log-survival = -0.0072´days + 0.074, approximately 5 percent removal per week). This prescribed culling was applied until the oysters grew to 20–25 millimeters (mm), which is when they become less vulnerable to mud crabs and approach sexual maturity. Once oysters reached 25 mm in length, we manually culled oysters on a weekly basis in accordance to a log-transformed survival curve for adult oysters (log-survival = -0.029´days + 0.049, approximately 19 percent removal per week). After four months (124 days), we harvested all experimental units to quantify adult oyster survival, growth, and condition index. In addition, we quantified the abundance of larval recruits on each tile to estimate how site, predator cue, and CE influenced the natural colonization of oysters.

The specific methods for quantifying the condition index first involved placing each of the harvested oysters in a drying oven at 70 degrees C for 24 hours and then weighing each individual oyster to estimate the combined weight of tissue and shell mass. Next, oysters were placed in a muffle furnace at 525 degrees C for two hours and re-weighed to estimate shell mass. By subtracting an oyster’s shell mass (second weight) from the combined mass of its tissue and shell (first weight), we estimated oyster tissue mass. The condition index of an oyster was calculated as its ratio of tissue to shell mass.