Predatory Suspension-Feeding in Diaptomus: Prey Defenses and the Avoidance of Cannibalism

High-speed (500 frames·s⁻¹) 16-mm film analysis was used to examine the predatory suspension-feeding behavior of Diaptomus pallidus. Prior high-speed film analysis of Diaptomus feeding on algae revealed a transition from passive to active captures as cell size increased, where the transition from passive to active captures was distinguished by the additional use of the swimming legs and maxillipeds to aid in capture. In the current study we found that when feeding on microzooplankton, Diaptomus may also employ its first antennae and a more vigorous flap of its swimming legs in an actual attack or orientation response to facilitate prey capture. Diaptomus responded to different prey species at distances that varied with prey type. Many of the microzooplankton which are potentially vulnerable to predation by suspension-feeding diaptomids have rheotactic capabilities which permit them to detect the feeding currents of Diaptomus before body contact and avoid predation through a rapid escape response. The most effective rheotactic escape response is exhibited by the nauplii of Diaptomus. Some experiments with CO₂-anesthetized nauplii demonstrate that nauplii are highly palatable to the omnivorous adults and that the rheotactic capabilities of the nauplii aid in reducing cannibalism.