The role of plasticity in the evolution of cryptic pigmentation in a freshwater isopod

Cryptic pigmentation of prey is often thought to evolve in response to predator-mediated selection, but pigmentation traits can also be plastic, and change with respect to both abiotic and biotic environmental conditions. In such cases, identifying the presence of, and drivers of trait plasticity is useful for understanding the evolution of crypsis. Previous work suggests that cryptic pigmentation of freshwater isopods (Asellus aquaticus) has evolved in response to predation pressure by fish in habitats with varying macrophyte cover and coloration. However, macrophytes can potentially influence the distribution of pigmentation by altering not only habitat-specific predation susceptibility, but also dietary resources and abiotic conditions. The goals of this study were to experimentally test how two putative agents of selection, namely macrophytes and fish, affect the pigmentation of A.aquaticus, and to assess whether pigmentation is plastic, using a diet manipulation in a common garden. We performed two experiments: (a) in an outdoor mesocosm experiment, we investigated how different densities of predatory fish (0/30/60 three-spined stickleback [Gasterosteus aculeatus] per mesocosm) and macrophytes (presence/absence) affected the abundance, pigmentation and body size structure of isopod populations. (b) In a subsequent laboratory experiment, we reared isopods in a common garden experiment on two different food sources (high/low protein content) to test whether variation in pigmentation of isopods can be explained by diet-based developmental plasticity. We found that fish presence strongly reduced isopod densities, particularly in the absence of macrophytes, but had no effect on pigmentation or size structure of the populations. However, we found that isopods showed consistently higher pigmentation in the presence of macrophytes, regardless of fish presence or absence. Our laboratory experiment, in which we manipulated the protein content of the isopods' diet, revealed strong plasticity of pigmentation and weak plasticity of growth rate. The combined results of both experiments suggest that pigmentation of A.aquaticus is a developmentally plastic trait and that multiple environmental factors (e.g. macrophytes, diet and predation) might jointly influence the evolution of cryptic pigmentation of A.aquaticus in nature on relatively short time-scales.