Data from: Is the continental life of the European eel Anguilla anguilla affected by the parasitic invader Anguillicoloides crassus?

Quantifying the fitness cost that parasites impose on wild hosts is a challenging task because the epidemiological history of field-sampled hosts is often unknown. In this study we used an internal marker of the parasite pressure on individual hosts to evaluate the costs of parasitism with respect to host body condition, size increase and reproductive potential of field-collected animals for which we also determined individual age. In our investigated system, the European eel Anguilla anguilla and the parasitic invader Anguillicoloides crassus, high virulence and severe impacts are expected because the host lacks an adaptive immune response. We demonstrated a nonlinear relationship between the severity of damage to the affected organ (i.e. the swimbladder, our internal marker) and parasite abundance and biomass, thus showing that the use of classical epidemiological parameters was not relevant here. Surprisingly, we found that the most severely affected eels (with damaged swimbladder) had greater body length and mass (+11% and +41%, respectively) than unaffected eels of same age. We discuss mechanisms that could explain this finding and other counter-intuitive results in this host–parasite system, and highlight the likely importance of host panmixia in generating great inter-individual variability in growth potential and infection risk. Under that scenario, the most active foragers would not only have the greatest size increase, but also the highest probability of becoming repeatedly infected –via trophic parasite transmission– during their continental life.量化寄生虫对野生宿主造成的适合度代价（fitness cost）是一项极具挑战性的工作，因野外采样宿主的流行病学史通常难以获知。本研究以宿主个体所受寄生虫压力的内部标志物（internal marker）为分析工具，针对野外采集且已明确个体年龄的受试动物，从宿主身体状况、体长增长与繁殖潜力三个维度评估寄生代价。本次研究的宿主-寄生虫系统为欧洲鳗鲡（*Anguilla anguilla*）与寄生入侵物种粗厚鳗居线虫（*Anguillicoloides crassus*），由于宿主缺乏适应性免疫应答，预期该系统将表现出高毒力与严重危害。我们发现宿主受损伤官的损伤程度与寄生虫丰度（parasite abundance）、生物量（biomass）之间存在非线性关联，这表明经典流行病学参数（epidemiological parameters）在此研究系统中并不适用。令人意外的是，我们发现与同龄未感染鳗鲡相比，受作为内部标志物的鳔（swimbladder）损伤最严重的鳗鲡，其体长与体质量分别高出11%与41%。我们针对该宿主-寄生虫系统中的这一发现与其他反直觉结果探讨了可能的解释机制，并强调宿主泛交（panmixia）在塑造生长潜力与感染风险的个体间巨大差异中可能发挥的重要作用。在此场景下，觅食活动最为活跃的个体不仅体长增长幅度最大，同时在其淡水生活阶段通过营养级寄生虫传播（trophic parasite transmission）途径反复感染的概率也最高。