The fitness of herbivores that have limited mobility as juveniles or larvae may often depend on the host choice behavior of adults. For such herbivores, selection should favor adults that choose plants that maximize the performance of their offspring, resulting in positive correlations across host plants among adult preferences, offspring performance (growth, survival, etc.), and for herbivores that are restricted to living on host plants, population‐level parameters such as abundance on different hosts. We tested this hypothesis for the marine, nest‐building amphipod Peramphithoe parmerong, using 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 Zonaria diesingiana consistently avoided. Juvenile amphipods were relatively immobile and, thus, mostly constrained to the host alga selected by their mother. Differences in the growth and survival of juvenile amphipods raised on single‐species diets were consistent with adult nest‐building preferences among algae, with the best performance on the two high‐preference species of Sargassum. Thus, adult preferences for host plants largely determined juvenile performance due to (1) restricted movement by juveniles and (2) differences among algal species in their effects on growth, survival, and onset of reproduction. In contrast to host plant preferences by adults, feeding rates on different algae were not as clearly correlated with juvenile performance. In particular, the low‐preference C. peregrina was consumed at a high rate, but survivorship on this alga was relatively poor. Differences in abundances of P. parmerong on different host algae in the field were consistent with laboratory preferences and not related to relative algal abundance or epiphyte loads, supporting the hypothesis that there were population‐level consequences of host plant choice in this species. With regard to mechanisms underlying host use, amphipod preferences were closely (negatively) correlated with the presence of nonpolar secondary metabolites. Nonpolar crude extracts from nonhost species, all of which contain secondary metabolites, affected amphipod host acceptance behavior, whereas extracts from host species, all of which lack such metabolites, did not. Overall, host plant use was unrelated to the nutritional value of the algal species, but one low‐preference host, C. peregrina, was notably nutritionally poor. These observations, and the correlations among adult preferences, offspring performance, and field densities for P. parmerong, are consistent with intrinsic host plant qualities determining host plant range for this species. This contrasts with previous studies that emphasized the importance of extrinsic factors, particularly predation, in determining amphipod host use.

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Marine amphipods commonly use other organisms as both habitat and/or food, offering an opportunity to contrast host associations in marine environments with that of terrestrial invertebrates. A few species of amphipods have been found to specialise on their hosts for habitat or food; however, their host associations remain poorly described and previous studies have confounded habitat and feeding specificity. We examined the patterns of host use for amphipods associated with macroalgae and sponges. For macroalgae, the analysis was limited to strictly herbivorous species in order to address feeding specificity only. On both hosts, amphipods displayed a range of host specificity, from more generalised to more specialised species. Even when not specialised, amphipod species had disjunct distributions, resulting in a distinct assemblage of amphipods associated with each host species. This was true for both herbivorous amphipods inhabiting algae, where host specificity relates to feeding specificity, and for amphipods on sponges, where host specificity is likely to relate to habitat specificity only. For most species of herbivorous amphipods, patterns of host use were largely unrelated to sampling time (day vs night), remained distinct across seasons, and were unaffected by the presence of alternative food sources (epiphytes).

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Seven species of ampithoid amphipods are reported from subtidal macroalgae living in Port Jackson, Sydney, Australia. In the genus Ampithoe the Indo–West Pacific species Ampithoe kava Myers, 1985, is recorded for the first time from Australia, and two new species, A. caddi, sp. nov. and A. ngana, sp. nov., are described. One new species ofCymadusa,C. munnu, sp. nov., is described. The genera Exampithoe (Melanesius), Peramphithoe and Plumithoe are recorded for the first time from Australian waters. The new species E. (M.) kutti, sp. nov. and Peramphithoe parmerong, sp. nov. are described, and Plumithoe quadrimana (Haswell, 1879b), comb. nov. is redescribed and a neotype is selected. New ecological and behavioural information is presented for these species. A new key and diagnoses for all known genera of Ampithoidae are presented. Paradusa Ruffo, 1969 is synonomised with Cymadusa Savigny, 1816.Exampithoe (Melanesius) gracilipes Ledoyer, 1984 is transferred to Exampithoe (Exampithoe) and Cymadusa uncinata Stout, 1912 and C. variata Sheard, 1936 are transferred to Paragrubia.