Long-term change in the parasite burden of shore crabs (Hemigrapsus oregonensis and H. nudus) on the northwestern Pacific coast of North America

<b>Abstract</b><br/><p>The abundances of free-living species have changed dramatically in recent decades, but little is known about change in the abundance of parasitic species. We investigated whether populations of several parasites have shifted over time in two shore crab hosts, <i>Hemigrapsus oregonensis </i>and <i>H. nudus,</i> by comparing the prevalence and abundance of three parasite taxa in a historical dataset (1969–1970) to contemporary parasite abundance (2018–2020) for hosts collected from 11 intertidal sites located from Oregon, USA to British Columbia, Canada. Our data suggest that the abundance of the parasitic isopod <i>Portunion conformis </i>has varied around a stable mean for the past fifty years. No change over time was observed for larval acanthocephalans.<i> </i>However, larval microphallid trematodes increased in prevalence over time among <i>H. oregonensis </i>hosts, from a mean of 4.7% to 61.8% between the historical and contemporary time points. The substantial increase in the prevalence of larval microphallid trematodes could be due to increased abundances of their bird final hosts, increased production of parasite infective stages by snail intermediate hosts, or both. Our study highlights the variability among parasite species in their temporal trajectories of change.</p>