Oyster culturing
Oysters (Crassostrea virginica) were cultured as spat-on-shell at the Auburn University Shellfish Laboratory (AUSL) on Dauphin Island, AL starting in late May 2019 using standard techniques (Congrove et al. 2009). Oyster larvae were settled onto sun-bleached oyster shells to create spat-on-shell. After 3 days, when oyster spat were approximately 1.0 millimeters they were exposed to either exudate from predatory blue crabs or empty cage controls in four flow-through holding tanks (length = 2.4 meters, width = 0.9 meters, water depth = 0.4 meters) supplied with unfiltered seawater pumped directly from the Gulf of Mexico. The number of spat per shell varied from ~5 – 40 and we elected to not alter initial density to mimic natural settlement during the induction period. Oysters were suspended above the tank bottom in oyster aquaculture baskets (64 x 23 x 14 centimeters) with 140 spat-covered shells per basket) to prevent sediment buildup from suffocating oysters. Seven oyster baskets were present in each tank (28 total).

Spat were exposed to blue crab predator cues by holding four live caged adult blue crabs (Callinectes sapidus) in two of the tanks (8 crabs total), whereas the remaining two tanks contained empty cages (control) to mimic conditions where oysters regularly experience predator cues or are limited in their exposure from cues. Water volumes and crab densities were informed from established procedures (Belgrad et al. 2021). Crabs in each tank were held in four separate cages (32 centimeters x 23 centimeters x 14 centimeters) to prevent crabs from consuming the experimental oysters or each other. Every crab was fed one adult oyster daily (approximately 5.0 centimeters in length) to maximize predator cue intensity as experimental oysters would be exposed to exudates from predators and damaged conspecifics. This ensured that oysters were exposed to the most natural set of cues indicative of a predation event, which produces a strong response in oysters (Scherer et al. 2016). Crabs were replaced during the experiment as needed due to mortality. Experimental oyster baskets were rotated around the crab cages daily to reduce differences in oyster growth due to proximity to predator cues, and no differences among cages were found. The induction period was 2 months.

Oyster survival experiment
We performed a field experiment on degraded, low-quality oyster reefs near Dauphin Island, AL to determine if the ecological relationships produced from induced defenses remain consistent across habitat quality and remained similar with laboratory experiments. Oyster spat taken from the culturing period described above (also used in the mesocosm experiment) was planted on three small (5 - 10 meters long x 3 meters wide) oyster reefs in early September 2019. Oyster reefs were within 50 meters of each other and composed of dead shell hash on a bed of sand with no mud crabs found among the hash (30°16'42.6" N; 88°07'14.7" W). No live oyster clusters were found within at least 500 meters of the site. Spat-covered shells were scraped to 10 individuals per shell to standardize predation risk. Forty-four pairs of induced and control shells were zip tied to poles and set in 11 transects total (3 – 5 transects per reef depending on its size). Transects contained four poles, with a 1-meter separation between each pole, planted in the following locations: the upper tidal zone of the reef, the lower tidal zone of the reef, the reef edge, and in the bare substrate with at least a 1-meter distance from the reef. Individual survival of all oysters was checked 48 hours after planting (n = 880 spat total; 110 spat per treatment).