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.