Data from: Duration of the parasitic phase determines subsequent performance in juvenile freshwater pearl mussels (Margaritifera margaritifera)

Host–parasite systems have been useful in understanding coevolutionary patterns in sympatric species. Based on the exceptional interaction of the long-lived and highly host-specific freshwater pearl mussel (FPM; Margaritifera margaritifera) with its much shorter-lived host fish (Salmo trutta or Salmo salar), we tested the hypotheses that a longer duration of the parasitic phase increases fitness-related performance of mussels in their subsequent post parasitic phase, and that temperature is the main factor governing the duration of the parasitic phase. We collected juvenile mussels from naturally and artificially infested fish from eight rivers in Norway. Excysted juvenile mussels were maintained separately for each collection day, under similar temperature and food regimes, for up to 56 days. We recorded size at excystment, post excystment growth, and survival as indicators of juvenile fitness in relation to the duration of the parasitic phase. We also recorded the daily average temperatures for the entire excystment period. We observed strong positive relationships between the length of the parasitic phase and the post parasitic growth rate, size at excystment and post parasitic survival. Temperature was identified as an important factor governing excystment, with higher temperatures decreasing the duration of the parasitic phase. Our results indicate that juvenile mussels with the longest parasitic phase have better resources (larger size and better growth rate) to start their benthic developmental phase and therefore to survive their first winter. Consequently, the parasitic phase is crucial in determining subsequent survival. The temperature dependence of this interaction suggests that climate change may affect the sensitive relationship between endangered FPMs and their fish hosts.

宿主-寄生系统是解析同域物种协同进化模式的重要研究模型。本研究依托长寿且高度宿主特异性的淡水珍珠蚌（Freshwater Pearl Mussel, FPM；学名*Margaritifera margaritifera*）与其寿命更短的宿主鱼类（褐鳟*Salmo trutta*或大西洋鲑*Salmo salar*）之间的独特互作关系，对两个假说开展验证：其一，寄生阶段时长的延长可提升蚌类在寄生后期与适合度相关的表现；其二，温度是调控寄生阶段时长的核心影响因素。我们从挪威8条河流的自然感染与人工感染宿主鱼体内采集幼龄珍珠蚌，将不同采集日期获得的脱胞幼蚌按批次单独饲养，在统一的温度与食物供给条件下饲养至多56天。我们以脱胞时的体型、脱胞后生长情况及存活率作为幼蚌适合度的评价指标，分析其与寄生阶段时长的关联；同时记录了整个脱胞阶段的日平均温度。研究结果显示，寄生阶段时长与寄生后期生长速率、脱胞时体型及寄生后期存活率之间存在显著正相关关系。温度被证实为调控脱胞过程的关键因素，温度升高会缩短寄生阶段时长。本研究表明，寄生阶段时长最长的幼蚌具备更优的资源储备（更大的初始体型与更快的生长速率）以开启底栖发育阶段，因此更易顺利度过首个越冬期。由此可见，寄生阶段是决定幼蚌后续存活的关键环节。该互作的温度依赖性提示，气候变化可能会威胁濒危淡水珍珠蚌与其鱼类宿主之间的脆弱共生关系。