To determine whether the preference of predators for basslets was driven by basslet species (fairy and blackcap) or basslet size (small and large: 1.7–2.5 and 3.5–5.2 cm TL, respectively) we presented pairs of basslets in cross-factored combinations of the 2 variables, resulting in the following treatments: (1) small fairy and large fairy, (2) small blackcap and large blackcap, (3) small fairy and small blackcap, (4) large fairy and large blackcap, (5) small fairy and large blackcap, and (6) large fairy and small blackcap. In addition to randomizing the order of basslet treatments presented to each predator, we also randomized the corner of the tank basslets were placed in every time a treatment was presented.

Once the predator and basslets were in their respective sides of the tank, we allowed them to acclimate for 20 min, after which we removed the central barrier and observed the predator’s behavior for 10 min. Observations were performed either in person (74 lionfish trials; 73 graysby trials) or filmed with a digital video camera (16 lionfish trials; 17 graysby trials) positioned outside of the tank. During each 10 min trial, we recorded (1) which basslet the predator hunted first (initial hunting preference); (2) the number of times the predator’s mouth made physical contact with each glass container (number of strikes); and (3) the amount of time the predator hunted each basslet (hunting time). We defined the hunting behavior of lionfish as occurring when an individual directly faced a basslet with flared pectoral fins and/or blew pulsed jets of water towards a basslet (Cure et al. 2012). We characterized graysby hunting behavior as occurring when an individual positioned itself near a basslet (<10 cm in this experiment) while directly facing the basslet (Webster 2004).

At the conclusion of the 10 min trial, we separated the predator from the basslets and placed the central barrier back in the tank. A new combination of basslets were placed in the glass containers, and all fish were allowed to acclimate for 20 min before removing the barrier and observing predator response for another 10 min. This procedure was repeated until all 6 basslet treatments had been presented to each predator in random order.