Predator-induced behavioral defense and its ecological consequences for two calanoid copepods.

We used an automated technique for the observation and quantification of zooplankton swimming behavior to study the behavioral responses of two congeneric, herbivorous, freshwater copepod prey to a copepod predator (Limnocalanus macrurus). One prey, Diaptomus sicilis, often co-occurs with Limnocalanus, while previous studies indicated that the zoogeographic distribution of the second prey, Diaptomus oregonensis, was independent of the predator. We found that in the presence of Limnocalanus, D. sicilis swims more slowly and with less hopping and jumping than D. oregonensis. Diaptomus sicilis is also attacked and consumed by the predator Limnocalanus macrurus less frequently than D. oregonensis. We suggest that the faster, "noisier" swimming of D. oregonensis increases its vulnerability to Limnocalanus. The behavioral defenses to both prey are induced by the presence of the predator, and may represent two different anti-predator strategies, "crypsis" and "avoidance" for D. sicilis and D. oregonensis respectively. In a zoogeographical analysis D. oregonensis occurs at densities below D. sicilis in lakes where Limnocalanus is at elevated abundances, while in "low-predator" lakes the opposite is true. This distribution pattern supports our experimental results, and suggests that D. sicilis is adapted to survive with Limnocalanus, while D. oregonensis is not.