The alteration of habitat structure by introduced ecosystem engineers imposes direct impacts on native biota but can also exert trait-mediated indirect effects. In this study, we show that the habitat structure provided by invasive Pacific oysters (Crassostrea gigas) can also indirectly affect parasitism in native blue mussels (Mytilus edulis). We conducted a 3-month field experiment, in which uninfected mussels were positioned at the bottom and top of two intertidal oyster reefs in the Wadden Sea. On one reef, we detected a significantly higher prevalence of parasitic copepods (Mytilicola spp.) in mussels positioned on top of oysters than in mussels at the bottom, but no difference in infection intensity. For trematodes (Renicola roscovita), a different pattern was observed, with higher prevalence (one reef) and significantly higher infection intensities (both reefs) in mussels positioned at the bottom of the oyster reef. We suggest that the contrasting pattern results from differences in parasite life cycles. Mytilicola spp. larvae spend 2–3 weeks in the water column before infecting their hosts and, therefore, mussels positioned at the top are exposed to higher numbers of planktonic larvae than mussels at the bottom. In contrast, infective trematode larvae spend less than 12 h in the water column and primarily infect mussels during low tide, which may explain higher prevalence and intensity of R. roscovita in mussels near the bottom of the oyster reef. Our results demonstrate that indirect effects leading to alterations of parasite-host interactions may be a more common but hitherto rarely considered impact of biological invasions.

引进的生态系统工程师对栖息地结构的改变会直接影响原生生物群，但也会产生性状介导的间接影响。在这项研究中，我们表明入侵太平洋牡蛎（Crassostrea gigas）提供的栖息地结构也可以间接影响天然蓝贻贝（Mytilus edulis）的寄生性。我们进行了一个为期3个月的野外实验，其中未感染的贻贝位于沃登海的两个潮间牡蛎礁的底部和顶部。在一块珊瑚礁上，我们发现位于牡蛎顶部的贻贝中的寄生co足类（Mytilicola spp。）的患病率明显高于底部的贻贝，但感染强度没有差异。对于吸虫（Renicola roscovita），观察到了不同的模式，位于牡蛎礁底部的贻贝患病率更高（一个礁石），感染强度（两个礁石）都明显更高。我们建议，相反的模式是由寄生虫生命周期的差异引起的。Mytilicola spp。幼虫在感染宿主之前先在水柱中呆2-3周，因此，位于顶部的贻贝比底部的贻贝面临着更多的浮游幼虫。相比之下，感染性吸虫的幼虫在水柱中的停留时间少于12小时，主要是在退潮时感染贻贝，这可能解释了罗斯科维塔尼亚的流行和强度较高。在牡蛎礁底部附近的贻贝中。我们的研究结果表明，导致寄生虫-宿主相互作用改变的间接影响可能是更普遍但迄今很少考虑的生物入侵影响。