Habitat mediates the outcome of a cleaning symbiosis for a facultatively burrowing crayfish

Summary Branchiobdellidans are small, symbiotic annelids (Clitellata) that can have positive effects on the survival and growth of the crayfish host. We hypothesised that branchiobdellidans would be particularly beneficial to crayfish that move to underground burrows (characterised by poor water quality) as surface waters recede. In surface water mesocosms, the symbioses were mutualistic, with worm (Cambarincola barbarae.) density increasing over time and crayfish (Procambarus clarkii) growing faster when worms were present. Contrary to previous studies, the outcome did not shift from mutualism to parasitism at high worm densities. In burrowing chambers in the laboratory, the symbiotic relationship was lost, as worm density decreased by 92% and crayfish growth ceased. Similarly, in burrows excavated in field experiments, worm density decreased by 84%. Rather than the symbiotic relationship increasing survival when crayfish moved underground, this crayfish species may be faced with a trade-off between the benefits of a mutualism with worms while on the surface and protection from heat stress and desiccation by burrowing when water levels are low.