Phenotypic plasticity and the evolution of an inducible morphological defence in crucian carp

Abstract: In this thesis, I have investigated factors (cues, benefits, and costs) affecting the evolution of an inducible morphological defence in crucian carp (Carassius carassius). Inducible defences are favoured over permanent defences when prey have reliable cues to detect predators, when defences are effective but costly, and when predation pressure is variable and unpredictable, but sometimes strong. In a laboratory experiment, presence of northern pike, Esox lucius, induced an increase in body depth in crucian carp. This effect was mediated by chemical cues from the predator. When European perch, Perca fluviatilis, were fed macroinvertebrates, there was no effect, but when perch were fed fish, crucian carp increased in body depth. The cues responsible for the effect was therefore related to the piscivorous diet. I then investigated how crucian carp respond behaviourally to waterborne cues. Crucian carp with no prior experience of piscivores were capable of distinguishing piscivorous, unfamiliar species (pike, perch) from a non-piscivorous, unfamiliar species (roach, Rutilus rutilus). Crucian carp fright behaviour, e.g. decreases in swimming activity, was influenced by both alarm substance-related (a specific substance indicating a predation event) and general, predator-related cues. Anti-predator behaviour was less pronounced in deep-bodied individuals coexisting with predators. Deep-bodied crucian carp were more difficult to handle for pike, which is a gape-limited predator. Further, when given a choice, pike preferred shallow-bodied individuals. Thus, the increase in body depth benefits crucian carp and can be viewed as a morphological defence. In a field experiment, I found that deep-bodied crucian carp suffered a density-dependent cost, a lower body mass gain relative to shallow-bodied conspecifics at high population densities. I then showed that theoretical drag is consistently higher for deep-bodied individuals, which should lead to higher swimming costs. Using swimming respirometry, I found that energy expenditure during locomotion increased more rapidly for deep-bodied crucian carp. However, these fish had lower standard metabolic rates, and costs of transport at maximum range velocity was similar for the two morphs. Using a modelling approach, I showed that although modifications of standard metabolic rate may partially compensate for hydrodynamically disadvantageous designs, deviations from optimal velocities are more expensive for deep-bodied individuals. Furthermore, shallow-bodied individuals will have competitive advantages when foraging in a patchy environment.