The interaction between suction feeding performance and prey escape response determines feeding success in larval fish.

The survival of larval marine fishes during early development depends on their ability to feed before depleting their yolk reserves. Most larval fish capture prey by expanding their mouth, generating a 'suction flow' that draws the prey into it. These larvae dwell in a hydrodynamic environment that impedes their ability to capture even non-evasive prey; however, the marine environment is characterized by an abundance of evasive prey, predominantly copepods. Copepods sense the hydrodynamic disturbance created by approaching predators and perform high-acceleration escape maneuvers. Using a 3D high-speed video system, we characterized the interaction between Sparus aurata larvae and prey from a natural zooplankton assemblage that contained evasive prey, and assessed the factors that determine the outcome of these interactions. At 8-33 days post hatching, larvae preferentially attacked large prey that was moving prior to the initialization of the strike; however, feeding success was lower for larger, more evasive prey. Thus, larvae were challenged in capturing their preferred prey. Larval feeding success increased with increasing Reynolds numbers, but decreased sharply when the prey performed an escape maneuver. The kinematics of successful strikes resulted in a shorter response time but higher hydrodynamic signature available for the prey, suggesting that strike success in our experiments was determined by brevity rather than stealth: executing a fast strike eliminated a potential escape response by the prey. Our observations of prey selectivity reveal that larval performance, rather than preferences, determines their diet during early development.