Vertebrate growth plasticity in response to variation in a mutualistic interaction

Abstract

A fundamental question of evolutionary ecology is, what determines body size? In general, the body size of vertebrates is thought to be relatively inflexible, a product of their genes, food, environment, and stress. However, vertebrate growth can be plastic in response to population interactions such as predator-prey and competition. While these relationships can elicit plasticity of vertebrate growth, mutualistic relationships have yet to be investigated. An iconic example of mutualism involving a vertebrate is the relationship between anemone and anemonefish. In this interaction anemonefish size is often positively correlated with anemone size. Here, I test the hypothesis that anemonefish growth is a plastic response to variation in anemone size. Juvenile clownfish (Amphiprion percula) of relatively uniform size were paired with sea anemones (Entacmaea quadricolor) of variable size and monitored over three months. The average anemone size over the course of the month was then used to predict the fish growth each month. Mixed model analyses verified that anemone area is significantly associated with both change in fish standard length and change in fish body depth. Fish in larger anemones grew more than did fish in small anemones. Remarkably, individuals in large anemones achieved this despite receiving the same amount of food as individuals in small anemones. This clownfish growth plasticity in response to anemone size might be adaptive if anemone area is a good indicator of resource availability in the wild, because it would enable the fish to maximize their reproduction without compromising survival. This study extends the understanding of how plasticity of vertebrate species can be influenced by a wide variety of population interactions.