Reckless males, rational females: Dynamic trade-off between food and shelter in the marine isopod Idotea balthica

Lack of anti-predator recognition in a marine isopod under the threat of an invasive predatory crab

The prey naïveté hypothesis suggests that the failure of prey to recognize novel predators as a threat is caused by a lack of anti-predator adaptations. We tested this hypothesis in a unique natural setting, where the isopod Idotea balthica encountered the rapidly spreading invasive crab, Rhithropanopeus harrissii. Earlier research had indicated high mortality of the isopods during exposure to R. harrissii. The isopod exerted no co-evolutionary history with any littoral crabs and thus the strong impact could be caused by lack of pre-adaptations towards the new predator species. We tested this hypothesis by studying the anti-predator responses of the isopods with water-born cues of R. harrissii and of the native predatory fish Perca fluviatilis. Compared to control water, the isopods lowered their activity when exposed to the fish cue. Instead crab cue did not induce anti-predator behaviour. We also tested the hypothesis that mortality caused by novel predator, similar to predation by P. fluviatilis, would result in differential selection for the two sexes and contribute to the evolution of personalities. However, we found no differences in anti-predator behaviour nor in mortality between the sexes or personalities of the isopods. The outcomes reveal an interesting evolutionary scenario, where predation by a local predator induce soft selection on prey characteristics, but an invasive species cause hard selection without differentiating between prey individuals. Our study—conducted in the dawn of the population outbreak of R. harrissii—provides an excellent reference point for studies resolving the evolutionary impacts of invasive predators on naïve prey.

Parallel morphological evolution and habitat-dependent sexual dimorphism in cave- vs. surface populations of the Asellus aquaticus (Crustacea: Isopoda: Asellidae) species complex

Studying parallel evolution (repeated, independent evolution of similar phenotypes in similar environments) is a powerful tool to understand environment-dependent selective forces. Surface-dwelling species that repeatedly and independently colonized caves provide unique models for such studies. The primarily surface-dwelling Asellus aquaticus species complex is a good candidate to carry out such research, because it colonized several caves in Europe. By comparing 17 functional morphological traits between six cave and nine surface populations of the A. aquaticus species complex, we investigated population divergence in morphology and sexual dimorphism. We found habitat-dependent population divergence in 10 out of 17 traits, likely reflecting habitat-driven changes in selection acting on sensory systems, feeding, grooming, and antipredator mechanisms. Sexual dimorphism was present in 15 traits, explained by sexual selection acting on male traits important in male-male agonistic behavior or mate guarding and fecundity selection acting on female traits affecting offspring number and nursing. In eight traits, the degree of sexual dimorphism was habitat dependent. We conclude that cave-related morphological changes are highly trait- and function-specific and that the strength of sexual/fecundity selection strongly differs between cave and surface habitats. The considerable population variation within habitat type warrants further studies to reveal cave-specific adaptations besides the parallel patterns.

Population divergence in aggregation and sheltering behaviour in surface- versus cave-adapted Asellus aquaticus (Crustacea: Isopoda)

Aggregation (gathering together) and sheltering (hiding in cover) are basic behaviours that can reduce the risk of predation. However, both behaviours have costs, such as increased competition over resources and high prevalence of contact-spread parasites (aggregation) or lost opportunities for foraging and mating (sheltering). Therefore, variation in these behaviours is expected between populations with varying levels of predation risk. We compared aggregation and sheltering in surface- (various predators) and cave-adapted (no predator) populations of the isopod Asellus aquaticus in a common garden experiment. Given that the cave environment is constantly dark, we also tested for population variation in light-induced behavioural plasticity. Variation in sheltering was explained by habitat type: cave individuals sheltered less than surface individuals. We found high between-population variation in aggregation with or without shelters and their light-induced plasticity, which was not explained by habitat type. Cave individuals decreased (habituation) whereas surface individuals increased sheltering with time (sensitization). We suggest that population variation in sheltering is driven by predation, whereas variation in aggregation must be driven by other, unaccounted environmental factors, in a similar manner to light-induced behavioural plasticity. Based on habituation/sensitization patterns, we suggest that predation-adapted populations are more sensitive to disturbance related to routine laboratory procedures.

Sex and size matters: Selection on personality in natural prey-predator interactions

Optimal life-history strategies are currently considered to be a major driving force for the maintenance of animal personalities. In this experimental study we tested whether naturally occurring predation causes personality-dependent mortality of a marine isopod (Idotea balthica), which could maintain personality variation in nature. Moreover, as isopods are known to have sex-differences in behaviour, we were interested in whether personality-dependent predation was sex-specific. We also hypothesised that predation pressure among personality types could vary according to habitat type, as it has been shown in correlative studies that habitat may influence personality variation. We used natural predator (European perch Perca fluviatilis) of I. balthica and studied relative mortality of males and females with a different personality types in laboratory settings with two different habitats. We found that survival in males was lower than in females for high active individuals. Moreover, survival under predation was linked to body size differently in females and males. This, however, depended on personality class as larger size was advantageous for low-active males and middle- and high-active females. Conversely, smaller size was advantageous for low-active females and middle-active males. Size did not affect survival in high-active males. Our results suggest that predation can encourage life-history differences between sexes leading to different optimal life-history strategies and also maintains consistent activity for both sexes.