Abstract
<jats:p>Host-associated symbionts are often described by a single host species, yet host use may be partitioned among life stages and host conditions. The sea urchin-associated amphipod Dactylopleustes yoshimurai commonly occurs on Strongylocentrotus intermedius in Otsuchi Bay, Japan, where juveniles frequently aggregate on disease lesions, whereas ovigerous females are rarely found. This pattern suggests that juvenile recruitment and reproduction may depend on different host resources. We tested whether the larger co-occurring sea urchin Mesocentrotus nudus functions as a reproductive host by combining field sampling during the reproductive season with paired-choice host assays. Field samples showed contrasting population structures among host categories: diseased S. intermedius was dominated by juveniles, whereas M. nudus harboured more large individuals and ovigerous or mature females. These results support a stage-dependent multi-host life-history model in which reproduction and late development occur mainly on M. nudus, while recruitment and early growth are concentrated on S. intermedius, particularly within lesion microhabitats. In laboratory assays with freely moving hosts, both large and small amphipods settled preferentially on M. nudus when offered either healthy or diseased S. intermedius as an alternative, although this tendency appeared less pronounced when the alternative host was diseased. Overall, our results show how host specificity in a marine symbiont can emerge from a multi-host life cycle linking reproductive habitat, disease-mediated nursery habitat, and post-settlement retention.</jats:p>