Host-plant adaptation in an herbivorous marine amphipod: Genetic potentialnot realized in field populations

Evolutionary responses of herbivores to their, host plants depend not only on selection from plants, but also on the genetic basis of traits relating to host use. The genetic basis of such traits has been investigated extensi vely among terrestrial insect herbivores, but has received almost no attent ion among marine herbivores. We tested whether performance traits in the he rbivorous marine amphipod Peramphithoe parmerong display heritable variatio n and, for the first time for a marine herbivore, whether selection has res ulted in local adaptation to host plants on two spatial scales. Peramphitho e parmerong displayed heritable genetic variation for survival on two host macroalgae, the high-quality Sargassum linearifolium and the poor-quality P adina crassa, and for growth on S. linearifolium. Differences in performanc e on different hosts thus have the potential to select for differential use of hosts by this amphipod. Despite this potential, there was no evidence a mong field populations of local adaptation to host algae on either scale re sted: between hosts within a site or among sites differing in algal species composition. Within a site, amphipods were not more likely to prefer or pe rform better on the host on which they were collected. Similarly, amphipods collected from sites in which P. crassa was present were not more likely t o perform well on this host than amphipods collected from sites where this alga was not found. Ecological factors that may explain the persistence of P. parmerong on P. crassa and the possibility of phylogenetic constraints o n host use by P. parmerong are discussed.