Living at the Interface: Behavioral, Evolutionary and Ecological Insights of Spring Use by Highly Mobile Stygobiont Crustaceans, <i>Troglocaris planinensis</i> (Decapoda: Atyidae)

ABSTRACT Springs represent ecotones between groundwater and surface freshwater habitats. Recent research suggested that springs can be more important than expected for stygobiont (i.e., adapted to live in groundwater) species, still information on habitat exploitation and activity of stygobionts in springs is far from complete. The aims of this study are: (i) to identify environmental factors promoting the exploitation of ecotone habitats by the stygobiont shrimp Troglocaris planinensis , commonly found in spring environments in northeastern Italy; and (ii) to experimentally evaluate whether this species exhibits differential behavioral responses to light and to subterranean and surface predator cues based on its habitat of origin (spring versus cave). From June 2020 to January 2025, we started multiple day and night surveys of T. planinensis in 64 springs of the Classic Karst (NE‐Italy). Each site has been characterized with respect to abiotic and biotic features. In the laboratory, shrimps from both cave and spring populations were tested to assess behavioral differences in response to light stimuli and predatory cues, as potential adaptations to the contrasting conditions of their respective habitats. In springs, T. planinensis density reached up to 116 shrimps/m 2 , with significantly higher counts at night and lower densities at sites with greater fish predator abundance. Laboratory tests showed that predator cues, but not light exposure, influenced shrimp behavior regardless of their cave or spring origin. This study suggests that stygobiont crustaceans can represent a significant portion of biomass in surface waters and exploit these environments in response to changes in abiotic and biotic conditions and stimuli. However, further research is necessary to determine how stygobionts perceive surface conditions and how ecotonal pressures may drive adaptive shifts in typically groundwater‐dwelling animals.