Predation constrains host choice for a marine mesograzer

For small sedentary herbivores that inhabit seaweeds, choosing a host that provides adequate nutrition and refuge should be favored by natural selection. Yet, the relative importance of seaweed nutritional value versus habitat quality in driving mesograzer host choice remains poorly understood for most herbivores. Previous work in coastal North Carolina, USA, and 2 tropical locations suggests that amphipods often utilize host seaweeds that offer superior refuge from both omnivorous and carnivorous consumers. Our study was conducted in New Zealand, where carnivores alone are the major consumers of seaweed-dwelling amphipods. We show that the herbivorous amphipod Aora typica preferentially utilizes the dictyotalean seaweed Dictyota kunthii over a dominant canopy-forming kelp, Ecklonia radiata, and that this preferred seaweed host provides a superior refuge from predators in both laboratory and field experiments. There was no difference in A. typica feeding preference between D. kunthii and E. radiata, but A. typica grew faster when reared on E. radiata. These results suggest that seaweed refuge quality, not nutritional quality, drives A. typica host preference and distribution in this system.