Strong effects of a mutualism on freshwater community structure

Abstract

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 and intermediate (3–6) and high worm densities (12) in cages in streams in Boone, North Carolina and Clemson, South Carolina, United States. 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, whereas 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.

The potential impacts of invasions on native symbionts

“…The invading crayfish Faxonius cristavarius is causing declines in the abundance and diversity of native crayfish worms (Bell, 2018; Creed et al, 2022). Some of these worms can modulate the effect of their native crayfish hosts on community structure and sediment deposition (Creed et al, 2021). Loss of these worms could result in diminished effects of the native hosts on coexisting taxa.…”

Biotische interacties in aquatische ecosystemen

“…Dat mutualisme ook de rest van de gemeenschap kan beïnvloeden is ook bekend van rivierkreeften. De aanwezigheid van de wormen Cambarincola ingens en Xironodrilus appalachius op rivierkreeften Cambarus chasmodactylus en C. chaugaensis resulteerde in een verandering in de abundantie van andere macroinvertebraten (Creed et al, 2021). De wormen zorgen voor verhoogde activiteit en groei van de rivierkreeften.…”

The dilution effect in a freshwater mutualism: Impacts of introduced host species on native symbionts

“…One well‐studied freshwater mutualism occurs between crayfish and some species of ectosymbiotic worms called branchiobdellidans (Ames, Helms, & Stoeckel, 2015; Brown, Creed, & Dobson, 2002; Brown, Creed, Skelton, Rollins, & Farrell, 2012; Creed, Skelton, Farrell, & Brown, 2021; Lee, Kim, & Choe, 2009; Thomas, Creed, Skelton, & Brown, 2016). In these mutualisms, the worms benefit from a relatively safe habitat with resources, and a site for reproduction (Creed, Lomonaco, Thomas, Meeks, & Brown, 2015).…”

“…In these mutualisms, the worms benefit from a relatively safe habitat with resources, and a site for reproduction (Creed, Lomonaco, Thomas, Meeks, & Brown, 2015). The crayfish benefit by having their gills cleaned of epibionts and detritus which results in increased host survival and growth (Brown et al, 2002(Brown et al, , 2012Creed et al, 2021). This host-symbiont system is a useful model for studying symbioses because both the crayfish hosts and the branchiobdellidans are widespread throughout many regions of the world, both taxa can be easily kept in laboratory aquaria or used as subjects in field experiments, and the worms are easy to manipulate and monitor during experiments (Creed et al, 2021;Skelton et al, 2013).…”

“…The crayfish benefit by having their gills cleaned of epibionts and detritus which results in increased host survival and growth (Brown et al, 2002(Brown et al, , 2012Creed et al, 2021). This host-symbiont system is a useful model for studying symbioses because both the crayfish hosts and the branchiobdellidans are widespread throughout many regions of the world, both taxa can be easily kept in laboratory aquaria or used as subjects in field experiments, and the worms are easy to manipulate and monitor during experiments (Creed et al, 2021;Skelton et al, 2013). It is also a good system for studying invasion-related dilution effects as crayfish have been introduced into waterways around the world where they can interact with native crayfish hosts and their symbionts (Gherardi, 2006;Hobbs III, Jass, & Huner, 1989;Twardochleb, Olden, & Larson, 2013).…”