Disease-associated aggregation of Dactylopleustes yoshimurai on sea urchins: host-level and lesion-level processes

Masafumi Kodama , Ryoga Yamazaki, Ko Tomikawa, Gen Kume, Toru Kobari, Jun Hayakawa

Amphipods of the genus Dactylopleustes are specialized symbionts of sea urchins, and in some species aggregations on host lesions have been reported; however, the behavioural mechanisms underlying such lesion-associated aggregation remain poorly understood. We investigated host-level and within-host processes underlying lesion aggregation in the symbiotic amphipod Dactylopleustes yoshimurai on the short-spined sea urchin Strongylocentrotus intermedius. In paired host-selection trials with freely moving urchins, amphipods accumulated more on diseased than on healthy hosts. When hosts were held apart in net cylinders, this bias toward diseased hosts disappeared, and individuals that had settled on a host rarely switched hosts, suggesting that host-to-host transfer occurs mainly when urchins approach one another. In a time-series aggregation experiment on diseased hosts, most amphipods attached soon after introduction and gradually concentrated on the lesion surface within approximately 6 h. Qualitative observations suggested that repeated contacts with host pedicellariae may promote stepwise movements across the test, whereas amphipods that reached pedicellariae-inaccessible microhabitats, including long-spine tips or the lesion surface, became largely stationary. Finally, on otherwise healthy hosts, both experimental pedicellariae removal and surface wounding induced amphipod accumulation, indicating that lesion-like microhabitats can form via reduced defence, disturbance cues, or both. Together, our results support a stepwise model in which D. yoshimurai first attaches with limited host-level discrimination, switches hosts primarily via short-range transfer, and then is retained in lesion microhabitats on the host surface. This two-scale framework links experimental behaviour to field patterns and highlights host contact rates and lesion availability as key determinants of symbiont distribution.