A wide variety of invertebrate animals are known for their capacity to develop different forms depending on the presence or absence of predators. Among vertebrates, crucian carp exemplify such `predator-induced phenotypic plasticity' and exhibit fairly streamlined, torpedo-like body morphology in predator-free environments, in contrast to a substantially more deep-bodied shape when living in waters containing a piscivorous predator such as pike. Previous research has already highlighted the potential benefits of this shape difference in relation to vulnerability to gape-limited predators – pike probably have a hard time getting their jaws around the deeper-bodied morph. However, Paolo Domenici, an expert in fish escape behavior from the CNR-IAMC(National Research Council – Marine and Coastal Environment Institute)in Oristano Italy, along with colleagues Håkan Turesson, Jakob Brodersen and Christer Brönmark from Lund University in Sweden, hypothesized that the shape change might also influence the carp's ability to get away from a predator.
Interspecific studies of fish escape behavior have shown that body shape indeed has a significant impact on performance variables such as acceleration and turning rate during an escape maneuver. Thus, Domenici and colleagues decided to exploit the body form differences between crucian carp morphs to test the relationship between body form and locomotor function within a single species, where fewer potentially confounding factors come into play. Both deep- and shallow-bodied fish were collected from local ponds (with and without predators, respectively) in Lund, Sweden and acclimated to laboratory conditions over several weeks. Animals of each morph were then placed into the testing arena, where a high-speed digital video camera was used to record their startle response following the impact of a small, plastic cylinder dropped onto the water surface from a height of 1.1 m. Each animal's center of mass (marked prior to the trial) and snout-tip positions were digitized to quantify various performance variables such as velocity, acceleration, rate of rotation and the cumulative distance traveled.
Deep-bodied fish taken from ponds with predators perform significantly better during escape behaviors than fish from predator-free waters. Specifically, they move further, faster and exhibit a higher turning rate than their shallow-bodied conspecifics, exhibiting improvements of approximately 15–20%. In addition, carp from predator-filled ponds also possess a significantly higher fraction of muscle mass relative to body mass, which,along with their increased depth, may help to explain their higher levels of performance. Although apparently good for escape, the increase in body depth does come with tradeoffs, particularly in the context of the energetic costs of steady cruising, which are higher in deeper-bodied animals. Nonetheless,this work provides indirect evidence that in crucian carp, shape changes that occur as animals develop in the presence of predators probably improve their likelihood of survival. The authors believe their results may also have more general implications, namely that the evolution of deep bodies in other fish species may reflect past selection on escape performance. Continued work that characterizes escape and/or survival rates during actual interactions between predators and the different carp morphs will be an essential step toward solidifying the importance of phenotypic plasticity and increased body depth for improving escape capacity and ultimately fitness in these animals.