Effects of copepod foraging behavior on predation risk: An experimental study of the predatory copepod Pareuchaeta nowegica feeding on Acartia clausi and A. tonsa (Copepoda)

The effect of foraging behavior on predation risk was studied by exposing the two small Calanoid copepods Acartia clausi and Acartia tonsa to 0 or 1 ppm (1,500 cells ml-l) of the diatom Thalassiosira weissfogii and to presence of the predator Pureuchaeta norvegica. In filtered water, predation rate was the same on the two species. In algal suspension, predation rate on A. clausi was half that in filtered water and half that on A. tonsa. Video observations revealed distinct differences in motility of Acartia depending on algal concentration. Both species performed frequent short feeding bouts in algal suspension; nonfeeding copepods in filtered water alternately sank or adjusted their vertical distribution by stronger jumps. Jump frequency nearly doubled for A. clausi in filtered water, but no significant difference was observed for A. tonsa. To explain the predation, assuming that P. norvegica is a rheotactic predator, WC developed a model of potential hydrodynamic disturbance associated with each foraging behavior. Increased encounter rate with P. nowegica caused by frequent strong jumps by A. clausi in the absence of algae could explain >40% of the observed increase in predation rate. For A. tonsa, jump frequencies and predation rates were similar in both food treatments, which is in accordance with the model. Like most animals, planktonic copepods must gather food and at the same time avoid predation. In many models, behavior results as a conflict between these two objectives; for example, food collection is often associated with a higher risk of being eaten (Lima and Dill 1990; McNamara and Houston I 992). Refuges from predators are not obvious in the pelagic environment, but vertical migration from the food-rich surface waters to dark and deeper waters during the day is commonly thought to be a response to the presence of visual predators (Gliwicz 1986; Bollens and Frost 199 I ). Plankton exposed to nonvisual predators may hide by reducing the transmission of chemical or hydrodynamic signals used for prey detection by many predators. Reduced motility is often suggested as a means to reduce both encounter and detection and in a few cases this has been clearly documented (Kerfoot 1978; Meng and Orsi 1991). The theories of foraging strategies often assume a cost of food search related to predation risk (Abrahams and Dill 1989; Newman 1991). The energetic cost of motility is small for most copepods (Kiorboe et al. 1985), and the varying predation risk associated with different swimming behaviors likely dominates the cost-benefit situation. Feeding activity of planktonic copepods often results in specific swimming patterns because feeding appendages are also used for propulsion. Provided that the hydrodynamic signals associated with these patterns are important for prey detection, behavioral changes should translate to changes in predation rate. Adaptations that allow efficient foraging at low predation risk would obviously be highly advantageous and selected for. In jimnetic environments, predation is given much attention and is thought to structure many lake ecosystems (Neil1 1994). In contrast, behavioral studies on marine copepods have traditionally focused on adaptations to food concentration (e.g. Buskey 1984) and prey size distribution (e.g. Greene 1988), largely ignoring predation risk. We exposed Acartia tonsa and Acartia clausi to the predator Pamuchaetu nowegica to assess how prey swimming behavior affects the predation rates. For A. tonsa, two welldefined swimming behaviors can be induced by varying the food concentration (Saiz et al. 1993). We could therefore present the nonvisual predator with hydrodynamically different signals that were related only to motility and not to size or shape of the prey. In addition to direct mortality of the prey, we monitored their feeding activity, gut fluorescence, a.nd egg production during the incubation in an attempt tc, reveal any sublethal effects of predator presence. The study consisted of two types of experiments: bottle incubations of A. clausi Giesbrecht and A. tonsa Dana with the predator P. nowegica (Boeck), and video filming of behavior elf the two Acartia species under various food concentrations. The bottle experiments were carried out at Dr@bak Ma:rine Laboratory, Norway, in September 1995. We collected P. nowegica in the Oslofjord on 13 and I5 September by repeated vertical hauls from the bottom (200 m) to 80 m with a 500-pm net. Upon retrieval, animals were immedia.tely transferred with a widebore pipette to a thermos filled with water from the collection depth. The animals were kept in the laboratory at 10°C in I O-liter jars for 5-7 d without food before the experiments. We collected A. clausi on 17, 19, and 21 September, also in the Oslofjord, by horizontal tows with a 200-pm plankton net. A. clausi was kept in a 4-liter jar without food for 2 d before each experiment. A. tonsa females were obtained from the laboratory culture at the Danish Institute for Fisheries and Marine Research (DIFMAR) in Charlottenlund, Denmark. Salinity was 30%0 in all experiments. Prosome lengths (LSD) were: P. nowegica 5.9OkO.23 mm, A. clausi 0.83kO.027 mm, A. tonsa 0.83+ 0.04 mm. At 12.-15 h before the start of each experiment, 20 adult females of A. clausi or A. tonsa were transferred to each of