DOI: 10.5061/dryad.mkkwh70zp
title: Long-term change in the parasite burden of shore crabs (Hemigrapsus oregonensis and H. nudus) on the northwestern Pacific coast of North America

description: 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, Hemigrapsus oregonensis and H.
 nudus, 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 Portunion conformis has varied around a
 stable mean for the past fifty years. No change over time was observed for
 larval acanthocephalans. However, larval microphallid trematodes increased
 in prevalence over time among H. oregonensis 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.
description: <i>Contemporary collection</i> We
 strove to reproduce the methods, sites, collection dates, and levels of
 replication of the Pacific Transect Expedition (PacTrEx) as closely as
 possible (Kuris et al. 1980), with some important exceptions noted below.
 To facilitate this process, the leader of the PacTrEx study (AMK) trained
 the data collectors of the present study (JQ, DG, and AG) in all crab
 collection and dissection protocols and was available for consultation as
 questions arose. <i>H. nudus </i>and <i>H. oregonensis
 </i>were collected from three sites in Oregon, six sites in
 Washington, and two sites in British Columbia (<b>Supplementary
 Information Table S1; Figure 1</b>). All historical and contemporary
 crab collections were conducted by hand during low tide at each sampling
 site. Given that parasite burden can vary substantially over small spatial
 scales, we sought to ensure that the exact same locations were sampled
 within each sampling site. To that end, AMK sampled several sites
 alongside JQ and DG, using the detailed field notes from his 1969–1970
 sampling expeditions to ensure that the exact same location was sampled.
 For contemporary sampling trips that AMK was not available to join, the
 team used Google Earth to query AMK about the exact location of sampling.
 The contemporary crabs were kept alive and transported to the laboratory,
 where they were euthanized by freezing. Crabs were stored in a freezer for
 at least 48 hours before dissection. There was one
 important difference between the methods of the historical PacTrEx study
 and the contemporary study. In the historical study, AMK dissected all of
 the crabs collected immediately following euthanasia, without freezing,
 while in the contemporary study, JQ, DG, and AG froze crabs and dissected
 them after freezing. To test whether this difference in methods would
 influence parasite counts, we performed an experiment (see
 <b>Supplementary Information Text S1</b>).
 <i>Crab dissection</i> We
 performed dissections of all sampled crabs in order to identify and count
 their parasites and matched our protocols as closely as possible to those
 used in the PacTrEx study. Dissections were performed identically for both
 <i>H. oregonensis </i>and <i>H. nudus</i>. Crabs
 were retrieved from the freezer and left to thaw in room-temperature sea
 water. The carapace width was measured in millimeters by placing calipers
 between the 2<sup>nd</sup> and 3<sup>rd</sup>
 carapace spikes. Sex was also recorded for each crab.
 Parasitological dissections were performed by lifting the
 carapace of the crab, cutting the digestive tract at the juncture of the
 thorax and the abdomen, and carefully examining the tissues of the crab
 for <i>P. conformis. </i>The remaining digestive tissue of the
 crab was removed and searched for other parasites, such as
 acanthocephalans, which were counted. After the digestive tissue was
 removed and searched, the body cavity of the crab was searched for
 metacercariae. We used the same categorical system to quantify
 metacercariae that was used in the PacTrEx study: “0” for no
 metacercariae, “+” for 1–5, “++” for 6–25, and “+++” for more than
 25.
description: This deposition includes: one raw dataset on the abundance of
 three parasite taxa in the shore crabs <em>Hemigrapsus
 oregonensis</em> and <em>H. nudus</em> (raw_data.csv),
 which includes both historical (1969-1970) and contemporary (2018-2020)
 data, a meta-data file that explains the column names for each column in
 raw_data.csv (raw_data_metadata.csv), scans of the raw datasheets from the
 historical sampling campaign (historical_datasheet_scans.pdf), and a
 metadata file that explains how to interpret the raw datasheets
 (historical_datasheet_scans_metadata.csv).  In addition to scans of the
 sampling sites analyzed in our paper (historical_datasheet_scans.pdf), we
 also provide scans of raw datasheets that reflect data from more southerly
 sites (historical_datasheet_scans_additional_sites.pdf) where we were not
 able to recreate sampling at the contemporary time point.  These more
 southerly sites are not entered into our raw dataset (raw_data.csv), but
 they represent an opportunity for an enterprising parasite ecologist to
 recreate sampling conducted more than half a century ago.
 Data are available in this repository, and are also permanently
 archived in
 GitHub: https://github.com/wood-lab/Quinn_et_al_2021_Proc_B