Implications of the feeding current structure of Euchaeta rimana, a carnivorous pelagic copepod, on the spatial orientation of their prey

Many marine planktonic organisms create water currents to entrain and capture food items. Rheotactic prey entrained within these feeding currents often exhibit escape reactions. If the direction of escape is away from the feeding current, the prey may successfully deter predation. If the escape is towards the center of the feeding current, the prey will be re-entrained towards its predator and remain at risk of predation. The direction of escape is dependent on (i) the ability of the prey to escape in a direction different than its pre-escape orientation and (ii) the orientation caused by the interaction of the prey's body with the moving fluid. In this study, the change in orientation of Acartia hudsonica nauplii as a result of entrainment within the feeding current of Euchaeta rimana, a planktonic predatory copepod, was examined, When escaping in still water, A.hudsonica nauplii were able to vary their pre-escape direction by only 10 degrees. This allows only a limited ability to escape in a direction different than their pre-escape orientation. Analyses of the feeding current of E.rimana show the flow speed to be most rapid in the central region with an exponential decrease in speed distally. In contrast, flow vorticity is minimal in the center of the feeding current and maximal at 1.75 mm along the antennae. As a result, the degree of rotation of the prey towards the center of the feeding current shows a strong dependency on the prey's location within the feeding current. The feeding current of E.rimana rotated the prey 14 degrees when near the center of the flow field and up to 160 degrees when located more distal in the feeding current. Since the prey's escape abilities cannot compensate for the rotation due to the flow, this mechanism will maintain the escaping prey within the feeding current of their predator. Therefore, the feeding current facilitates predatory copepods in capturing prey by (i) increasing the amount of water which passes over their sensors and through their feeding appendages and (ii) controlling the spatial orientation of their prey prior to escape.