Selection from visual predation vs. developmental plasticity of isopod pigmentation
Divergent selection and adaptive phenotypic plasticity can jointly influence phenotypic differentiation within and among populations. Understanding how these processes interact is difficult when the same environmental differences that lead to divergent selection can also promote phenotypic plasticity. Previous work has shown that pigmentation of the freshwater isopod Asellus aquaticus varies with background colour (Hargeby et al. 2004). This is hypothesized to be an evolutionary response to visual predation along a gradient of differently coloured macrophyte-backgrounds.
In an six month long outdoor mesocosm experiment, we investigated how predator density (0, 30, and 60 threespine stickleback per mesocosm) and macrophytes (presence/absence) affected the abundance and pigmentation of isopods. While we found that fish presence strongly reduced isopod density, particularly in the absence of macrophytes, we found no effect on the size or pigmentation of isopods (Lürig et al. 2019). Instead, we detected increased pigmentation in the presence of macrophytes, and across all levels of fish density. A subsequent laboratory rearing experiment indicated that pigmentation of isopods may a developmentally plastic trait (Lürig et al. 2019).
Inspired by the findings of our mesocom experiment, we investigated whether developmental phenotypic plasticity could be responsible divergent patterns of pigmentation. In a laboratory experiment we reared over 1000 individuals from 29 families with two diets of contrasting nutritional quality (low/high protein concentration), and quantified developmental trajectories of body size and pigmentation for every individual for 12 weeks. Overall, dietary protein had strong effects on the developing phenotype, and this influenced the “survival landscape” of juvenile isopods. Specifically, we found that higher dietary protein had strong positive effects on the developmental rate of pigmentation, but led to increased mortality under high growth and pigmentation rates. Building on previous work, suggesting that visual predation has mediated the evolution of A. aquaticus body size and cryptic pigmentation, our study shows how dietary effects on the developmental trajectories of juvenile isopods can have fitness consequences in the absence of predation.
Currently in review
Hargeby, A., J. Johansson, and J. Ahnesjö. 2004. Habitat-specific pigmentation in a freshwater isopod: adaptive evolution over a small spatiotemporal scale. Evolution; international journal of organic evolution 58:81–94.
Lürig, M. D., R. J. Best, M. Svitok, J. Jokela, and B. Matthews. 2019. The role of plasticity in the evolution of cryptic pigmentation in a freshwater isopod. The Journal of animal ecology 88:612–623.