Ecological mechanisms driving the anti-predator defense response in Crucian carp (Carassius carassius): Variation in morphology, resource use, and life-history strategies along a gradient of predation risk

Predation is one of the main structuring forces of freshwater communities, influencing population dynamics, phenotypic variation, resource use, and life-history traits within and among prey populations. In order to counteract predation risk, prey organisms may display several anti-predator morphological and behavioral adaptations. The crucian carp (Carassius carassius) represents a classic example of predator-induced morphological defense, as it develops a deep body to decrease vulnerability to predation. Very few studies, however, have explored the ecological drivers underlying morphological variation observed among crucian carp populations in the wild. This PhD thesis aimed at revealing the effects of predation risk on morphology, resource use, and life-history traits of crucian carp along a natural gradient of predation risk. The study was performed in fifteen small lakes from southern Norway, which presented no predators or increasingly efficient gape-limited predators: brown trout, perch, or pike. The results show that crucian carp is provided with a fine-tuned morphological defense response against gape-limited predators. Progressively efficient predators determined an increase in crucian carp relative body depth and size, a decrease in population density, rapid growth at young age, larger lifespans, and higher reproductive effort. Predation pressure likely reduced fish abundance, relaxing intraspecific resource competition and favoring individual growth of survivors. High-predation lakes also corresponded to productive systems with high food availability and complex vegetated littoral habitats. Reduced intraspecific competition, larger food availability, and increased habitat complexity associated with predation risk favored energy allocation to both growth and reproduction. Plastic feeding habits and a shift to more energetically rewarding prey with increasing body size may also have supported this energy allocation. Thus, the expression of the defense response in crucian carp was likely a result of the synergistic effects of predation risk and favorable environmental conditions.