The relative importance of metabolic rate and body size to space use behavior in aquatic invertebrates

Elucidating the underlying mechanisms behind variations of animal space and resource use is crucial to pinpoint relevant ecological phenomena. Organism's traits related to its energy requirements might be central in explaining behavioral variation, as the ultimate goal of a forager is to fulfill its energy requirements. However, it has remained poorly understood how energy requirements and behavioral patterns are functionally connected. Here we aimed to assess how body mass and standard metabolic rate (SMR) influence behavioral patterns in terms of cumulative space use and time spent in an experimental patchy environment, both within species and among individuals irrespective of species identity. We measured the behavioral patterns and SMR of two invertebrate species, that is, amphipod Gammarus insensibilis, and isopod Lekanesphaera monodi, individually across a range of body masses. We found that species of G. insensibilis have higher SMR level, in addition to cumulatively exploring a larger space than L. monodi. Cumulative space use scaled allometrically with body mass, and it scaled isometrically with SMR in both species. While time spent similarly in both species was characterized by negative body mass and SMR dependence, it was observed that L. monodi individuals tended to stay longer in resource patches compared to G. insensibilis individuals. Our results further showed that within species, body mass and metabolic rate explained a similar amount of variation in behavior modes. However, among individuals, regardless of species identity, SMR had stronger predictive power for behavioral modes compared to body mass. This suggests that SMR might offer a more generalized and holistic description of behavioral patterns that extend beyond species identity. Our study on the metabolic and body mass scaling of space and resource use behavior sheds light on higher-order ecological processes such as species' competitive coexistence along the spatial and trophic dimensions.

Among demons and killers: current and future potential distribution of two hyper successful invasive gammarids

Biological invasions represent one of the main contemporary pressures facing freshwater ecosystems, and a better understanding of invasive species potential distributions is essential to prepare for future stressors. Crustacean invaders contribute significantly to global invasions with the Ponto-Caspian region being one of the primary donor areas for the Palearctic. The amphipods Dikerogammarus villosus and Dikerogammarus haemobaphes, popularly known as “killer” and “demon” shrimps, are emblematic of successful Ponto-Caspian invaders of European freshwaters. However, the geographical areas in which the abiotic environment is potentially suitable for them have not been investigated. To address this gap, current and future potential distributions were studied for the European Western Palearctic considering two scenarios and time periods (2050 and 2070) as well as the association between anthropogenic activities and individual species habitat suitability. Results show large areas of central-western Europe are currently suitable for both species and indicate some potential for range expansion within colder European areas. In particular, D. haemobaphes has the potential to expand its range further west and within southern parts of Europe. Scenarios of future climate change don’t provide evidence for further range expansion compared to the current conditions and suggest a reduction of range overlap within the most suitable areas. Results reveal lowland areas are at greatest risk of colonisation as well as a significant association with anthropogenic activities for both amphipods. The outcomes of the research could be used by resource managers for preparing and managing future changes of both species distributions and facilitate decision-making for monitoring and control.

What doesn’t kill you doesn’t make you stronger: Parasites modify interference competition between two invasive amphipods

We used a freshwater amphipod-microsporidian model (Ponto-Caspian hosts: Dikerogammarus villosus and D. haemobaphes, parasite: Cucumispora dikerogammari) to check whether parasites affect biological invasions by modulating behaviour and intra- and interspecific interactions between the invaders. We tested competition for shelter in conspecific and heterospecific male pairs (one or both individuals infected or non-infected). In general, amphipods of both species increased their shelter occupancy time when accompanied by infected rather than non-infected conspecifics and heterospecifics. Infected amphipods faced lower aggression from non-infected conspecifics. Moreover, D. villosus was more aggressive than D. haemobaphes and more aggressive towards conspecifics vs. heterospecifics. In summary, infection reduced the intra- and interspecific competitivity of amphipods, which became less capable of defending their shelters, despite their unchanged need for shelter occupancy. Dikerogammarus haemobaphes, commonly considered as a weaker competitor, displaced by D. villosus from co-occupied locations, was able to compete efficiently for the shelter with D. villosus when microsporidian infections appeared on the scene. This suggests that parasites may be important mediators of biological invasions, facilitating the existence of large intra- and interspecific assemblages of invasive alien amphipods.

Might the interspecies interaction between fish and shrimps change the pattern of their avoidance response to contamination?

Contamination seems to exert a crucial role in the spatial distribution of some organisms, such as shrimps and fish. Both, especially the freshwater fish Danio rerio and the shrimp Atyaephyra desmarestii, have been tested experimentally for their avoidance response and have showed the ability to escape from toxic effects. As the behavior of avoiding or not the contamination might be altered in the presence of other factors, the aim of the current study was to verify whether the avoidance response of both species, when exposed jointly (multispecies tests), to a copper gradient is different from the avoidance response observed in monospecies tests. The avoidance was assessed in a multi-compartmented exposure system, in which a copper gradient was simulated. Organisms were tested individually and together. Both species avoided potentially toxic copper concentrations; however, shrimps were slightly more sensitive in the monospecies tests: AC₅₀ (avoidance concentration for 50% of the population) of 60 (53-68) μg/L for the zebrafish and 50 (45-56) μg/L for the shrimp. In the multispecies tests, the sensitivity pattern changed: the avoidance response by the fish [AC₅₀: 30 (14-46) μg/L] was greater than by the shrimps [AC₅₀: 70 (22-141) μg/L]. Although the AC₅₀ values are in the same order of magnitude, a slight trend to change the avoidance pattern was observed in the shrimps during multispecies test: the avoidance was lower and time-delayed. This behavioral change could be linked to the stress caused by the zebrafish sharing the space with the shrimps, perhaps increasing the territorialism of the fish, or a delay in the shrimps detecting the risk of contamination.

The Braveheart amphipod: a review of responses of invasive Dikerogammarus villosus to predation signals

Predator pressure is a fundamental force driving changes at all levels of the community structure. It may protect native ecosystems from alien species. Therefore, resistance to diverse predators resulting from a universal anti-predator strategy seems crucial for invasion success. We present a comprehensive review of the responses of an invasive amphipod Dikerogammarus villosus to sympatric and allopatric predator signals. We summarize diverse aspects of the gammarid anti-predator strategy, including predator identification, morphological and behavioural adaptations, effectiveness of shelter use and resistance to indirect predator effects. The response of D. villosus is independent of predator species (including totally allopatric taxa), which assures the high flexibility of its predator recognition system. It has a harder exoskeleton and better capability of utilizing shelters compared to other gammarids, resulting in relatively high resistance to predators. Therefore, it can use predator kairomones as indirect food signals (sharing the diet with the predator) and follow the predator scent. This resistance may allow D. villosus to reduce the costs of its physiological responses to predators and sustain growth in their presence. This might facilitate invasion success by increasing its competitive advantage.

Combined effects of predator cues and competition define habitat choice and food consumption of amphipod mesograzers

Predation has direct impact on prey populations by reducing prey abundance. In addition, predator presence alone can also have non-consumptive effects on prey species, potentially influencing their interspecific interactions and thus the structure of entire assemblages. The performance of potential prey species may, therefore, depend on both the presence of predators and competitors. We studied habitat use and food consumption of a marine mesograzer, the amphipod Echinogammarus marinus, in the presence/absence of a fish mesopredator and/or an amphipod competitor. The presence of the predator affected both habitat choice and food consumption of the grazer, indicating a trade-off between the use of predator-free space and food acquisition. Without the predator, E. marinus were distributed equally over different microhabitats, whereas in the presence of the predator, most individuals chose a sheltered microhabitat and reduced their food consumption. Furthermore, habitat choice of the amphipods changed in the presence of interspecific competitors, also resulting in reduced feeding rates. The performance of E. marinus is apparently driven by trait-mediated direct and indirect effects caused by the interplay of predator avoidance and competition. This highlights the importance of potential non-consumptive impacts of predators on their prey organisms. The flexible responses of small invertebrate consumers to the combined effects of predation and competition potentially lead to changes in the structure of coastal ecosystems and the multiple species interactions therein.

Habitat use of coexisting Microtus vole species under competition and predation risk

Competing species and predators can alter the habitat use of animals, but both factors are rarely simultaneously controlled. We studied in experimental enclosures how closely related species, the sibling vole (Microtus levis Miller, 1908) and the field vole (Microtus agrestis (Linnaeus, 1761)), adjust their habitat use when facing either the competing species or simultaneously competition and predation risk. The species responded differently in their proportional use of two habitat types, a low cover (productive but riskier) and a high cover (safer but poorer). When alone, field voles used the low-cover habitat according to availability at low densities, but decreased its use with increasing density. Sibling voles, however, avoided the low-cover habitat in single-species populations. Under interspecific competition, the habitat-use patterns switched between species: sibling voles used the low-cover habitat according to availability, with decreasing use as densities increased. Sibling voles responded to predation risk by showing a stronger density-dependent decrease in the use of low-cover habitat. Field voles, initially using mostly high cover, did not change behaviour under risk of predation. Our results highlight the importance of considering both predation risk and interspecific competition when interpreting patterns of habitat selection among coexisting species.

Keep calm and don't stop growing: Non-consumptive effects of a sympatric predator on two invasive Ponto-Caspian gammarids Dikerogammarus villosus and Pontogammarus robustoides

Predators shape prey populations by elimination of individuals (consumptive effects) and by inducing modifications in prey behaviour, physiology or morphology (NCE—non-consumptive effects). Due to the resource allocation to defence, decreased feeding and higher stress, the costs of predator NCEs can be considerable. Therefore, the resistance to NCEs may be crucial for population growth and interspecific competition. We tested the resistance of Ponto-Caspian gammarids Dikerogammarus villosus and Pontogammarus robustoides to NCEs imposed by their predator, the racer goby Babka gymnotrachelus. As D. villosus is often avoided by predators in the presence of alternative food, we hypothesised that it would bear lower behavioural and physiological costs of anti-predator responses. We tested gammarid feeding in short-time experiments (2–4 h) with food (chironomid larvae) located at various distances from the stony shelter (to enforce food searching, Experiment I) or in the direct gammarid proximity (no searching needed, Experiment II). Moreover, we checked the predator effect on gammarid growth in a 2-week Experiment III. Both gammarids exposed to predators reduced feeding efficiency outside the shelter (Experiment I). Contrary to our expectations, the response of D. villosus was stronger. When food was provided in their direct proximity (Experiment II), the feeding of both species was unaffected by predators, indicating that a shelter supplied with food can reduce predator NCEs. The growth of P. robustoides was reduced in the presence of predators (Experiment III), whereas that of D. villosus was unaffected. Although D. villosus has a more effective defence strategy than P. robustoides, it bears similar or even higher behavioural costs of NCEs. However, it exhibits the higher resistance to the long-term predator presence, sustaining its growth rate under such conditions. This may be one of the factors contributing to the great invasion success of D. villosus, currently taking place in European fresh waters.