Preference-performance relationships and effects of host plant choice in an herbivorous marine amphipod

The fitness of herbivores that have limited mobility as juveniles or larvae may often depend on the host choice behavior of adults. For such herbivore s, selection should favor adults that choose plants that maximize the perfo rmance of their offspring, resulting in positive correlations across host p lants among adult preferences, offspring performance (growth, survival, etc .), and for herbivores that are restricted to living on host plants, popula tion-level parameters such as abundance on different hosts. We tested this hypothesis for the marine, nest-building amphipod Peramphithoe parmerong, u sing a series of behavioral and performance assays and relevant field data. Adults displayed strong preferences among eight species of brown algae in habitat choice assays, with Sargassum linearifolium and S. vestitum highly preferred; Colpomenia peregrina and Padina crassa of lower preference; and Dictyopteris acrostichoides, Dictyota dichotoma, Dilophus marginata, and Zo naria diesingiana consistently avoided. Juvenile amphipods were relatively immobile and, thus, mostly constrained to the host alga selected by their m other. Differences in the growth and survival of juvenile amphipods raised on single-species diets were consistent with adult nest-building preference s among algae, with the best performance on the two high-preference species of Sargassum. Thus, adult preferences for host plants largely determined j uvenile performance due to (1) restricted movement by juveniles and (2) dif ferences among algal species in their effects on growth, survival, and onse t of reproduction. In contrast to host plant preferences by adults, feeding rates on different algae were not as clearly correlated with juvenile perf ormance. In particular the low-preference C. peregrina was consumed at a hi gh rate, but survivorship on this alga was relatively poor. Differences in abundances of P. parmerong on different host algae in the field were consis tent with laboratory preferences and not related to relative algal abundanc e or epiphyte loads, supporting the hypothesis that there were population-l evel consequences of host plant choice in this species. With regard to mech anisms underlying host use, amphipod preferences were closely (negatively) correlated with the presence of nonpolar secondary metabolites. Nonpolar cr ude extracts from nonhost species, all of which contain secondary metabolit es, affected amphipod host acceptance behavior, whereas extracts from host species, all of which lack such metabolites, did not. Overall, host plant u se was unrelated to the nutritional value of the algal species, but one low -preference host, C. peregrina, was notably nutritionally poor. These obser vations, and the correlations among adult preferences, offspring performanc e, and held densities for P, parmerong, are consistent with intrinsic host plant qualities determining host plant range for this species. This contras ts with previous studies that emphasized the importance of extrinsic factor s, particularly predation, in determining amphipod host use.