EFFECTS OF PREY ESCAPE ABILITY, FLOW SPEED, AND PREDATOR FEEDING MODEON ZOOPLANKTON CAPTURE BY BARNACLES

Experimental studies of feeding on zooplankton often involve the use o f non-evasive Artemia spp. to represent zooplanktonic prey. Some zoopl ankton, however, such as copepods, are potentially evasive due to poss ession of effective predator-avoidance mechanisms such as high-speed e scape swimming. In the present study, we compared the efficiencies wit h which non-evasive (A. salina) and evasive (copepods) zooplankton wer e captured by a sessile, suspension feeder, the coral-inhabiting barna cle Nobia grandis (Crustacea, Cirripedia). M. grandis specimens and zo oplankton used in the present study were collected near Eilat, Israel in 1993. The effect of different flow speeds (from 0 to 14 cm s(-1)) o n captures of the two preys was also investigated. Additionally, we ex amined the effect of a flow-induced barnacle behavioral switch from ac tive to passive suspension feeding, on zooplankton capture. Two video cameras were used to make close-up, three dimensional recordings of pr edator-prey encounters in a computer-controlled flow tank. Frame-by-fr ame video analysis revealed a highly significant difference (P<0.001) in the efficiency with which A, salina and copepods were caught (A. sa lina being much more readily captured than copepods). After an encount er with cirri of feeding barnacles, copepods were usually able to swim out of the barnacles capture zone within one video frame (40 ms), by accelerating from a slow swimming speed (approximately 1.85 cm s(-1)) to a mean escape swimming speed of 18.11 cm s(-1) (ca. 360 body length s s(-1)). This was not the case for A. salina nauplii, which usually r emained in contact with cirri before being transferred to the mouth an d ingested. Thus, experimental studies addressing the methodology of o rganisms feeding on zooplankton should consider that slow-swimming pre y like Artemia sp. nauplii may only represent the non-evasive fraction of natural mesozooplankton assemblages.