Sit-and-wait foraging is not enough in food-deprived environments: evidence from groundwater and salamanders

Foraging strategies are fundamental traits that characterize predators, with strong differences between sit-and-wait predators and active-searching predators. Optimal foraging theory predicts that environmental conditions affect the efficiency of these strategies, with active predators being favoured when prey are scarce and difficult to detect. Subterranean habitats are ideal models to study the effectiveness of foraging strategies. Laboratory studies on fish and salamander predators showed that active foraging often characterizes cave-adapted species, but field studies demonstrating the advantages of active foraging for growth and survival are lacking. In this study, we assessed how predators displaying a sit-and-wait strategy can cope with the variable costs of foraging under different ecological contexts, such as cave and surface environments. We performed a cross-environment experiment that was repeated in 3 years by rearing salamander, Salamandra salamandra, larvae from caves and surface streams in cages placed in both surface and cave environments. We measured larval growth (weight and total length) repeatedly every 10–20 days, from March to July, and assessed water temperature variation, prey availability and metamorphosis achievement in the rearing sites. Larvae in stream cages grew larger than larvae in subterranean cages, which showed negative growth. Our results suggest that the sit-and-wait strategy does not provide enough prey for development in cave environments, irrespective of larvae origin. In food-deprived environments, active foraging is necessary to obtain the energy required for the basic functions of the organisms exploiting them.