Some parasites manipulate their host’s phenotype to enhance predation rates by the next host in the parasite’s life cycle. Our understanding of this parasite-increased trophic transmission is often stymied by study-design challenges. A recurring difficulty has been obtaining uninfected hosts with a coevolutionary history with the parasites, and conducting experimental infections that mimic natural processes. In 1996, Lafferty and Morris provided what has become a classic example of parasite-increased trophic transmission; they reported a positive association between the intensity of a brain-infecting trematode (Euhaplorchis californiensis) in naturally infected California killifish (Fundulus parvipinnis) and the frequency of conspicuous behaviors, which was thought to explain the documented 10–30× increase in predation by the final host birds. Here, we address the primary gap in that study by using experimental infections to assess the causality of E. californiensis infection for increased conspicuous behaviors in F. parvipinnis. We hatched and reared uninfected F. parvipinnis from a population co-occurring with E. californiensis, and infected them 1–2 times/week over half their life span with E. californiensis and a small cyathocotylid trematode (SMCY) that targets the host’s muscle tissue. At 3 time points throughout the hosts’ lives, we quantified several conspicuous behaviors: contorting, darting, scratching, surfacing, and vertical positioning relative to the water’s surface. Euhaplorchis californiensis and SMCY infection caused 1.8- and 2.5-fold overall increases in conspicuous behaviors, respectively. Each parasite was also associated with increases in specific conspicuous behaviors, particularly 1.9- and 1.4-fold more darting. These experimental findings help solidify E. californiensis–F. parvipinnis as a classic example of behavioral manipulation. Yet our findings for E. californiensis infection–induced behavioral change were less consistent and strong than those previously documented. We discuss potential explanations for this discrepancy, particularly the idea that behavioral manipulation may be most apparent when fish are actively attacked by predators. Our findings concerning the other studied trematode species, SMCY, highlight that trophically transmitted parasites infecting various host tissues are known to be associated with conspicuous behaviors, reinforcing calls for research examining how communities of trophically transmitted parasites influence host behavior.

一些寄生虫操纵宿主的表型，以提高寄生虫生命周期中下一个宿主的捕食率。我们对这种寄生虫增加的营养传播的理解常常受到研究设计挑战的阻碍。一个反复出现的困难是获得与寄生虫有共同进化历史的未感染宿主，并进行模仿自然过程的实验感染。1996 年，Lafferty 和 Morris 提供了寄生虫增强营养传播的经典例子；他们报告说，自然感染的加州鳉鱼（Fundulus parvipinnis ）中脑部感染吸虫（ Euhaplorchis californiensis ）的强度与显眼行为的频率之间存在正相关关系，这被认为解释了有记录的吸虫捕食量增加 10-30 倍的原因。最终宿主鸟。在这里，我们通过使用实验感染来评估加州大肠杆菌感染与F. parvipinnis明显行为增加的因果关系，解决了该研究中的主要空白。我们从与E. californiensis共存的群体中孵化和饲养未感染的F. parvipinnis，并在其一半寿命中每周感染 1-2 次E. californiensis和一种针对宿主肌肉的小细胞轴吸虫 (SMCY)组织。在宿主一生中的 3 个时间点，我们量化了几种明显的行为：扭曲、猛冲、抓挠、浮出水面以及相对于水面的垂直定位。加州真鞭毛虫和 SMCY 感染分别导致显眼行为总体增加 1.8 倍和 2.5 倍。每种寄生虫还与特定显眼行为的增加有关，特别是飞镖行为增加 1.9 倍和 1.4 倍。这些实验结果有助于巩固E. californiensis–F. parvipinnis是行为操纵的典型例子。然而，我们对加州大肠杆菌感染引起的行为变化的发现不如以前记录的一致和强烈。我们讨论了这种差异的可能解释，特别是当鱼类受到捕食者的主动攻击时，行为操纵可能最为明显。我们对其他研究的吸虫物种 SMCY 的研究结果强调，感染各种宿主组织的营养传播寄生虫已知与明显的行为相关，这加强了对研究营养传播寄生虫群落如何影响宿主行为的呼声。