Crayfish and Their Hidden Tenants: Direct and Indirect Effects of Ectosymbionts on Crayfish Behaviors and Health.

Symbiotic relationships are a vital component of the stability and function of ecosystems. Despite their importance, our understanding of many key symbiotic relationships is rudimentary. Within freshwater ecosystems, crayfish are considered keystone species and ecosystem engineers, in addition to serving as a host to multiple ectosymbionts. Recent studies on crayfish symbioses have primarily focused on branchiobdellidan worms in isolation, showcasing context-dependent mutualistic and parasitic fitness outcomes for the crayfish hosts. However, in natural environments, branchiobdellidan worms often share their crayfish host with other symbiont taxa, such as freshwater ostracods, potentially leading to interactions that may have unique effects on their host's biology. In this study, we used a correlational approach to assess the relationships among ostracod and branchiobdellidan abundances, the direct sublethal effects of these symbionts, crayfish health, and crayfish behavioral measures, using regression analyses. In addition, to examine potential interactive effects of our symbiont variables, we used path analyses determining the direct, indirect, and total effects of both symbionts on crayfish health and behavior. We found that ostracod and branchiobdellidan abundance was significantly related to gill scarring on crayfish. Similarly, we found that gill scarring was significantly negatively related to hepatosomatic index, suggesting the possibility of parasitism occurring in these symbiotic relationships under certain contexts. Additionally, our regression analyses indicated symbiont abundance and gill scars were related to both locomotion and grooming behavior. However, these relationships were in opposite directions, suggesting these symbiotic relationships may result in behavioral tradeoffs. While we found no significant indirect effects, the total effects of our path analyses indicated that positive and negative reciprocal feedback loops may occur between symbiont abundance and direct sublethal symbiont effects on host crayfish. The results of this study provide further support for context-dependent outcomes in crayfish symbiotic relationships, with symbiont abundance and the direct sublethal effects of symbionts as potential drivers of fitness outcomes. However, the correlational nature of the study cannot provide direct evidence of either parasitism or mutualism, indicating that further work is necessary to determine the mechanistic links between both symbionts and their host crayfish. The results of this study highlight the importance of examining direct sublethal symbiont effects on crayfish hosts as well as multisymbiont interactions to understand the outcomes of crayfish symbiotic relationships in natural environments. Additionally, while the study is based on correlational analyses, the findings suggest novel directions for future studies to help further understand the symbiotic relationships of crayfish.