Numerous mutualisms have been described from terrestrial and marine communities and many of these mutualisms have significant effects on community structure and function. In contrast, there are far fewer examples of mutualisms from freshwater habitats and there is no evidence that any mutualism has community-wide or ecosystem-level consequences. Northern hemisphere crayfish are host to a variety of ectosymbiotic worms called branchiobdellidans. The association between some of these "crayfish worms" and their hosts is a mutualism. The outcome of the association is context dependent and can be influenced by host size, symbiont number and the environment. Here we document in two experiments that the mutualism between crayfish and these worms alters the effect of crayfish on stream community structure and sediment deposition, an important ecosystem variable. We enclosed crayfish stocked with 0 worms, intermediate (3-6) and high worm densities (12) in cages in streams in Boone, NC and Clemson, SC, USA. At both locations, there was a negative relationship between initial worm density and final macroinvertebrate abundance. There was a significant effect of worm treatment on macroinvertebrate community structure in both the Boone and Clemson experiments. In Boone, there were effects on both overall macroinvertebrate abundance and community composition, while in Clemson, changes to community structure were primarily driven by changes in total abundance. There was a negative relationship between benthic sediment volume and initial worm density in both experiments, primarily later in the experiments, though these effects were influenced by sediment deposition rates. Our results are the first to demonstrate strong effects of a mutualism on freshwater communities. Both members of this mutualism are found throughout the northern hemisphere, so similar impacts may occur in many other waterways. Given that various species in addition to crayfish function as keystone species and ecosystem engineers in freshwater systems throughout the world, mutualisms involving these strongly interacting species may be as important to the structure and functioning of freshwater systems as comparable mutualisms in marine and terrestrial systems.

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互利共生对淡水群落结构的强烈影响

陆地和海洋群落中已经描述了许多互惠互利，其中许多互惠互利对群落结构和功能具有显着影响。相比之下，淡水栖息地的互惠互利的例子要少得多，而且没有证据表明任何互惠互利会在社区范围或生态系统层面产生影响。北半球小龙虾是各种称为 branchiobdellidans 的外共生蠕虫的宿主。其中一些“小龙虾蠕虫”与其宿主之间的关联是一种共生关系。关联的结果取决于上下文，并可能受宿主大小、共生体数量和环境的影响。在这里，我们在两个实验中记录了小龙虾和这些蠕虫之间的共生关系改变了小龙虾对河流群落结构和沉积物沉积的影响，一个重要的生态系统变量。我们在美国北卡罗来纳州布恩和南卡罗来纳州克莱姆森的溪流中的网箱中封闭了 0 只蠕虫、中等密度 (3-6) 和高蠕虫密度 (12) 的小龙虾。在这两个位置，初始蠕虫密度和最终大型无脊椎动物丰度之间存在负相关。在 Boone 和 Clemson 实验中，蠕虫处理对大型无脊椎动物群落结构有显着影响。在布恩，大型无脊椎动物的总体丰度和群落组成都有影响，而在克莱姆森，群落结构的变化主要由总丰度的变化驱动。在两个实验中，底栖沉积物体积和初始蠕虫密度之间存在负相关关系，主要是在实验后期，尽管这些影响受到沉积物沉积率的影响。我们的结果首次证明了互惠互利对淡水社区的强大影响。这种互惠互利的两个成员遍布整个北半球，因此在许多其他水道中可能会发生类似的影响。鉴于除小龙虾之外的各种物种在世界各地的淡水系统中都是关键物种和生态系统工程师，涉及这些强相互作用物种的共生关系对于淡水系统的结构和功能可能与海洋和陆地系统中的可比互利关系一样重要。