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.

Contemporary collection

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. H. nudus and H. oregonensis were collected from three sites in Oregon, six sites in Washington, and two sites in British Columbia (Supplementary Information Table S1; Figure 1). 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 Supplementary Information Text S1).

Crab dissection

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 H. oregonensis and H. nudus. 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^nd and 3^rd 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 P. conformis. 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.

This deposition includes: one raw dataset on the abundance of three parasite taxa in the shore crabs Hemigrapsus oregonensis and H. nudus (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