Population dynamics and disease-linked host use of the sea urchin symbiont Dactylopleustes yoshimurai (Crustacea: Amphipoda: Pleustidae) on Strongylocentrotus intermedius

Dactylopleustes yoshimurai is an echinoid-associated amphipod that frequently aggregates on disease lesions of the short-spined sea urchin Strongylocentrotus intermedius in Otsuchi Bay, northeastern Japan. However, its life history and use of diseased hosts remain poorly understood. We combined four years of monthly SCUBA surveys (from January 2020 to January 2024) with quantitative sampling of diseased and healthy sea urchins (from January 2021 to January 2024) to examine how host disease status structures the population dynamics of D. yoshimurai. The amphipod occurred on ～10–80% of sea urchins and was consistently more frequent and much denser on diseased than on healthy hosts, with winter peaks that broadly coincided with maxima in disease prevalence. Size–frequency distributions on diseased sea urchins were strongly dominated by juveniles (<2.5 mm), especially in winter to early spring, whereas healthy sea urchins supported low densities but a relatively higher proportion of adults. Ovigerous females were rare but occurred mainly in late summer and autumn on both host types, and juvenile pulses in winter, together with the year-round presence of small juveniles, indicate an extended reproductive period with a seasonal peak in late summer–autumn and recruitment continuing into winter. These results show that host disease status and amphipod life history jointly shape the population dynamics of D. yoshimurai and highlight sea urchin lesions as seasonally dynamic habitat patches that are particularly important for juvenile stages.