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

AbstractThe 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. MethodsContemporary 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 2nd and 3rd 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. Usage notesThis 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...摘要 近几十年来，自由生活物种的丰度发生了显著变化，但人们对寄生生物丰度的变化却知之甚少。本研究以两种滨蟹（Hemigrapsus oregonensis和H. nudus）为宿主，对比了1969-1970年历史数据集与2018-2020年当代采样中三类寄生虫类群的感染率和丰度，探究了这两种宿主体内多种寄生虫种群随时间的变化趋势。采样宿主采集自美国俄勒冈州至加拿大不列颠哥伦比亚省的11个潮间带站点。 研究数据显示，寄生等足类（parasitic isopod）*Portunion conformis*的丰度在过去五十年间围绕稳定平均值波动。棘头虫幼虫（acanthocephalans）未出现随时间的显著变化。然而，微茎科吸虫（microphallid trematodes）幼虫在H. oregonensis宿主中的感染率随时间显著升高，从历史采样时期的平均4.7%升至当代采样时期的61.8%。微茎科吸虫幼虫感染率的大幅升高，可能与其终末宿主鸟类的丰度增加、中间宿主螺类产生的寄生虫感染阶段增多，或两者共同作用有关。本研究揭示了不同寄生虫物种随时间变化的轨迹存在显著差异。 方法 当代采样 我们尽可能复刻了太平洋样带科考（Pacific Transect Expedition, PacTrEx）的采样方法、站点、采集时间与重复设置（Kuris等，1980），仅在以下关键环节存在差异。为确保复刻准确性，PacTrEx研究负责人AMK对本研究的数据采集人员JQ、DG与AG开展了所有滨蟹采集与解剖流程的培训，并在后续过程中随时提供咨询支持。本研究采集的H. nudus与H. oregonensis分别来自俄勒冈州3个站点、华盛顿州6个站点以及不列颠哥伦比亚省2个站点（详见补充材料表S1与图1）。所有历史与当代的滨蟹采集均在退潮时分于各采样站点徒手完成。考虑到寄生虫负荷在微小空间尺度下即可存在显著差异，本研究力求确保每个采样站点内的取样位置与历史采样完全一致。为此，AMK陪同JQ与DG对部分站点进行了预采样，结合其1969-1970年采样的详细野外记录，确保取样位置完全匹配。对于AMK无法参与的当代采样行程，研究团队通过谷歌地球（Google Earth）结合AMK的确认，明确具体采样位置。当代采集的滨蟹被活体运输至实验室，通过冷冻方式实施安乐死，随后置于冷冻冰柜中保存至少48小时后再进行解剖。本研究与历史PacTrEx研究的方法存在一处关键差异：历史研究中，AMK在滨蟹安乐死后立即进行解剖，未进行冷冻；而本研究中JQ、DG与AG先将滨蟹冷冻，后再行解剖。为验证该方法差异是否会影响寄生虫计数结果，本研究开展了对照实验（详见补充材料文本S1）。 滨蟹解剖 本研究对所有采集到的滨蟹进行了解剖，以鉴定并计数其体内寄生虫，且解剖流程尽可能匹配PacTrEx研究的方法，两种滨蟹的解剖流程完全一致。从冷冻柜中取出滨蟹后，将其置于室温海水中解冻。使用游标卡尺在第二与第三背甲棘之间测量背甲宽度（单位：毫米），同时记录每只滨蟹的性别。寄生虫学解剖流程为：掀开滨蟹背甲，在胸部与腹部连接处切断消化道，仔细检查组织以寻找*Portunion conformis*。随后移除剩余消化组织并排查其他寄生虫（如棘头虫）并计数。移除并排查消化组织后，检查滨蟹体腔以寻找囊蚴（metacercariae）。本研究采用与PacTrEx研究一致的分类系统对囊蚴进行定量："0"代表无囊蚴，"+"代表1-5个，"++"代表6-25个，"+++"代表25个以上。 使用说明 本数据集存档包含以下内容：一份关于滨蟹（Hemigrapsus oregonensis与H. nudus）体内三类寄生虫类群丰度的原始数据集（raw_data.csv），涵盖1969-1970年历史数据与2018-2020年当代数据；一份用于说明raw_data.csv各列字段含义的元数据文件（raw_data_metadata.csv）；历史采样活动原始记录单的扫描件（historical_datasheet_scans.pdf）；以及一份用于说明如何解读原始记录单扫描件的元数据文件（historical_datasheet_scans_metadata.csv）。除本论文中分析的采样站点扫描件外……