Predator species related adaptive changes in larval growth and digestive physiology.

Prey species are often non-randomly distributed along predator gradients but according to how they trade off growth against predation risk. The foraging-mediated growth/predation risk trade-off is well established, with increased foraging accelerating growth but also increasing predator induced mortality. While adaptations in digestive physiology may partly modify the relationship between foraging and growth in response to predation risk, studies exploring the impact of digestive physiology on growth in prey subjected to predation risk are still scarce. Larvae of the dragonfly genus Leucorrhinia segregate at the species level between lakes either being dominated by predatory fish (fish-lakes) or predatory invertebrates (dragonfly-lakes). Predators of these two lake types differ dramatically in their hunting style like searching and pursuing mode causing different selection pressure on prey traits including foraging. In a laboratory experiment we estimated growth rate, digestive physiology (ingested food, growth efficiency, assimilation efficiency, conversion efficiency) and metabolic rate (oxygen consumption) in the presence and absence of predator cues. Whereas fish-lake and dragonfly-lake Leucorrhinia species did not differ in growth rate, they evolved different pathways of digestive physiology to achieve similar growth rate. Because fish-lake species expressed a higher metabolic rate than dragonfly-lake species, we assume energy to be differently allocated and used for metabolic demands between species of both predator environments. Further, growth rate, but not digestive physiology was plastic in response to the presence of predator cues. Our results highlight the impact of digestive physiology in shaping the foraging-mediated growth/predation risk trade-off, with digestive physiology contributing to species distribution patterns along predator gradients.