Predator-Induced Defenses in Crustacea

The capacity of an organism with a given genotype to respond to changing environmental conditions by the expression of an alternative phenotype is a fascinating biological phenomenon. Plasticity enables organisms to cope with environmental challenges by altering their morphology, behavior, physiology, and life history. Especially, predation is a major factor driving plasticity in response to seasonal fluctuations of predator populations. Therefore, many taxa have evolved strategies to adapt to this environmental challenge, including morphological defenses, life history shifts, and behavioral adaptations. The evolution of inducible defenses is dependent on 4 factors: a selective agent, a reliable cue, associated costs, and the resulting benefit. Ecologically, predator-induced defenses are of general importance because they reduce predation rates and hence dampen the dynamics of predator-prey systems to stabilize food webs. We analyze the defensive strategies in many crustacean taxa and describe how they can act in concert to reduce predation risk. Additionally, prey species may perform predation risk assessment and reduce defense expression when conspecifics are dense. With increasing numbers of conspecifics, the individual predation risk is reduced due to prey dilution, predator confusion, and increased handling times. Consequently, the need to develop a strong defense is reduced and costs for the full defenses expression can be saved.

Predator threat assessment in Daphnia magna: the role of kairomones versus conspecific alarm cues

Studying the finely tuned mechanism of predation risk assessment allows for a better understanding of how prey organisms make key decisions under different levels of predation pressure. We studied the relative importance of conspecific alarm cues and fish kairomones as initiators of D. magna antipredator defences. By exposing a clone of D. magna to different infochemicals that simulated the presence of an active fish predator, we observed cue-specific responses in terms of altered feeding behaviour, respiration and life-history traits. Results agreed with the hypothesis that D. magna processes information from alarm cues from macerated conspecifics and from predator kairomones to assess the level of predation risk, adjusting the magnitude of their responses to the different levels of threat perceived. Results support the findings of other investigations and further show that single cues (fish kairomones or alarm cues) triggered feeding reduction and increased oxygen consumption, whereas fish kairomones only elicited D. magna life-history responses. Prey-specific alarm cues can thus modify the response of Daphnia to trout kairomones and this combination of both chemical cues appears to be necessary to trigger the full deployment of antipredator responses and avoid unnecessary costs arising from maladaptive responses.

Anthropogenic noise affects risk assessment and attention: the distracted prey hypothesis

Many studies have focused on the effects of anthropogenic noise on animal communication, but only a few have looked at its effect on other behavioural systems. We designed a playback experiment to test the effect of noise on predation risk assessment. We found that in response to boat motor playback, Caribbean hermit crabs ( Coenobita clypeatus ) allowed a simulated predator to approach closer before they hid. Two hypotheses may explain how boat noise affected risk assessment: it masked an approaching predator's sound; and/or it reallocated some of the crabs' finite attention, effectively distracting them, and thus preventing them from responding to an approaching threat. We found no support for the first hypothesis: a silent looming object still got closer during boat motor playbacks than during silence. However, we found support for the attentional hypothesis: when we added flashing lights to the boat motor noise to further distract the hermit crabs, we were able to approach the crabs more closely than with the noise alone. Anthropogenic sounds may thus distract prey and make them more vulnerable to predation.