Border habitats such as interfaces and ecotones are promising research targets being likely areas of high species richness and genetic and phenotypic diversity. Springs are intriguing habitats exploited by both subterranean and surface species. For subterranean species, springs can provide higher trophic resources but can be risky in terms of predation and UV radiation, while for surface species, springs can be safer but less productive environments. We coupled field surveys and laboratory experiments to understand how predation risk and physical constraints, like light occurrence, affect spring exploitation by both a subterranean (Niphargus thuringius) and a surface crustacean amphipod species (Echinogammarus stammeri). From March to May 2021, we surveyed multiple springs and evaluated the activity (both during day and night) of the amphipods and of their predators. Furthermore, in a subterranean laboratory, we reared 80 N. thuringius and 80 E. stammeri under safe and risky conditions with both constant darkness and diel light variation assessing their activity and survival. Risky conditions were represented by the occurrence of meso-predators alone or coupled with the presence of a top predator. In the field, N. thuringius activity was negatively related to the density of predators, while laboratory experiments revealed a main role played by light treatments and night period. E. stammeri activity in the field was higher close to surface while in laboratory conditions decreased during time. In laboratory conditions, predation risk negatively affected survival of both amphipods. Our findings reveal that physical constraints play a key role in affecting the exploitation of ecotones and can mediate antipredator responses, thus providing selective pressures for the exploitation of border environments. Significance statement Understanding environmental pressures acting on ecotones is a key point to verify if new adaptations may occur at the border between two distinct habitats. Using both field and laboratory approaches, we show that, in springs, the behaviour of subterranean invertebrates is affected by surface physical constraints which can mediate the effects of predation risk. Behavioural strategies to avoid predation, such as nocturnal activity, may promote spring ecotone exploitation by groundwater animals, such as amphipod crustaceans.
Stygobiont species show common, typical traits derived from their adaptation to subterranean life. Due to the general absence of light in cave environments, the majority of them are eyeless. Although the absence of eyes generally does not allow them to perceive luminous stimuli, some stygobionts still present phototaxis. Previous studies determined that different species of the eyeless amphipod crustaceans of the genus Niphargus are able to react to light; this has been interpreted as an adaptation to avoid dangerous surface habitats, even if recent studies suggest that this could also be an adaptation to exploit them when a situation is less dangerous (i.e., during the night). Niphargus thuringius is a stygobiont amphipod that can also be observed in spring environments despite possessing all the main morphological features of subterranean organisms, such as depigmentation and a lack of eyes. In the present study, we test how the species respond to light stimuli according to the light cycle and predation risk experienced during a conditioning period. We assessed the reactions to light stimuli of adult individuals of N. thuringius after 30 days of rearing in microcosms with different conditions of light occurrence (total darkness or a light/darkness daily cycle) and predation risk (without predators, with one predator, and with two predators). Both light stimuli during the test and rearing conditions affected the behavior of Niphargus thuringius. With light stimuli, individuals presented a strong photophobic response. Moreover, individuals reared in conditions of high predation risk preferred a more sheltered environment during behavioral tests than individuals reared in safe conditions. Our results add a new species to those of stygobiont amphipods known to display negative phototaxis, confirming that this pattern is widespread and conserved in the field. N. thuringius could be a good candidate model to perform further studies aiming to assess if differences occur between spring populations and populations present in deeper groundwater.
Cave-dwelling animals might be regarded as dead-end points when it comes to their evolution and habitat exploitation. However, in the past different observations of the olms’ typical troglomorphic populations have been reported for springs of Venetia Giulia, along with a community rich in stygobionts (Bressi et al. 1999; Stoch 2017). The aim of this work is to point out the non-random active use of surface habitats by this community, providing a comparison with the occurrence observed in caves and performing an assessment of factors favouring ecotone habitats exploitation. Since 2020 we started multiple day and night surveys of olms in both 71 springs and 11 caves. Each spring and cave habitat monitored has been characterised by respect to abiotic and biotic features, including planktonic and benthic prey availability. Additionally, cave and spring populations of Troglocaris sp. were tested to determine whether they showed any behavioral difference regarding their response to light stimuli and predatory cues, as potential adaptations to the different conditions found in the two habitats. We used 34 individuals sampled from three springs and 24 from two caves of the Italian karst. We detected the olm at least once in 12 springs, with a maximum of 9 individuals occurring together. Detection probability in springs and caves was similar. Spring habitats provided higher density of potential prey available. Olms seem to prefer springs without predator fish and temporary hydroperiod. We recorded in one spring a larva of 3.5 cm which could be the smallest ever recorded in the field. Preliminary results of the behavioral experiments show no susceptibility to light neither for spring nor cave populations of Troglocaris sp., as for chemical cues we expect a difference in reactivity depending on the habitat of origin of the shrimp, consistently with the top predator present. We suggest that epigean habitats and borders with surface may have an underestimated importance for animals adapted to subterranean environments (Manenti and Piazza 2021), including the olm. Further studies focusing on the ipogean community which actively exploits spring habitats are underway, with the aim of determining the diel occupation of these complex ecotones.
Border habitats such as interfaces and ecotones promise research targets from an evolutionary and zoological point of view. Springs are typical ecotones that border two strongly distinct environments: surface and underground. They are exploited by both subterranean and surface species for which they may provide specific environmental pressures promoting phenotypic plasticity and local adaptations. The aim of this study is to understand how the landscape of fear (LOF) and physical constraints, like light occurrence, affect springs' exploitation by both a subterranean (Niphargus thuringius) and a surface crustacean amphipod species (Echinogammarus stammeri). From March to May 2021, we surveyed 15 springs, divided into 25 plots according to their distance to the border, and both day and night, we recorded amphipods activity and LOF levels for them. In a subterranean laboratory, we also reared 80 N. thuringius and 80 E. stammeri in safe and risky conditions with constant darkness and diel light variation assessing their activity and survival for 30 days. Risky conditions were represented by meso-predators (four fire salamander larvae) alone or with a top-predator (a dragonfly larva of the species Cordulegaster boltonii). While in field conditions, the activity of N. thuringius seemed negatively affected by the number of active predators, in laboratory experiments, the main role was played by the light treatment; activity was significantly higher in constant darkness conditions. E. stammeri activity in the field was higher in surface plots, while in laboratory conditions was affected by LOF. Predation risk negatively affected the survival of both amphipods. Our findings reveal that while light conditions seem to shape activity patterns of stygobionts strongly, predators have a lower effect on activity, even though predators have negative effects on survival. Moreover, physical constraints, such as light exposure, can affect antipredator responses of subterranean organisms, thus representing selective pressures for the exploitation of surface environments.
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