The light environment experienced by individuals can vary markedly within and across populations. Studies describing spectral sensitivities of individuals from a few species have shown that visual systems can respond to diverse environments through phenotypic plasticity, presumably optimizing sensory performance in a given ecological context. Sensory organs can exact a large energetic cost on the animal however, and other non-sensory environmental factors potentially limit the nature of the phenotypic response. Daphnia, a freshwater crustacean, possess a relatively large, single compound eye, and inhabit environments that vary widely both in the light and resource environment. Compound eye size is a general indicator of visual capabilities, where larger eyes are more sensitive to light and often have higher visual resolving power. In this study, we tested the plastic response of compound eye size in Daphnia, under contrasting, but ecologically relevant light and resource environments. We reared Daphnia in 4 environments: high light (HL) x low resource (LR), HL x high resource (HR), dim light (DL) x LR, and DL x HR. Eye size scales allometrically with body size in Daphnia, therefore both absolute and body size adjusted eye size were measured.
Results/Conclusions
We found that resources, but not light, affect eye size. Daphnia reared on low resources have smaller absolute eye sizes but also smaller body sizes. However, Daphnia reared on low resources also have proportionally smaller eye sizes than those reared on high resources. Our results demonstrate that Daphnia exhibit plasticity of eye size with respect to resource environment. Daphnia invest less in their compound eye when resources are limiting, potentially redirecting resources to other somatic or reproductive tissues. In turn, reduced investment in overall eye size may negatively affect light sensitivity of the smaller-eyed Daphnia. Overall, we show that other non-sensory environmental factors can influence the expression of the compound eye phenotype.