Ectosymbionts improve host gill function in a freshwater cleaning symbiosis

Abstract

Mutualisms are common interactions that involve a diverse array of taxa. An important aspect in the study of mutualisms is determining the mechanisms that underlie benefits in survival, growth and reproduction of the partner species involved. Crayfish are engaged in mutualisms with ectosymbiotic worms called branchiobdellidans. These worms benefit by using their crayfish hosts as safe habitat and sites for feeding and reproduction. Some of these worm species can improve crayfish survival and growth. We hypothesized that the worms influenced crayfish survival and growth by cleaning their gills of debris and fouling biota and that this cleaning would increase rates of host gas exchange and ammonia excretion. Using surveys and experiments, we examined the effect of the worms on crayfish ammonia excretion and oxygen consumption. There were positive effects of increasing worm density on ammonia excretion by crayfish hosts in both our survey and experiment. No effect of worm density was observed on oxygen consumption in either our survey or experiment. However, molt status of the surveyed crayfish (intermolt/premolt vs. recently molted) did affect oxygen uptake. Recently molted crayfish, which should have gill epithelia largely free of fouling organisms and debris, had higher levels of dissolved oxygen uptake than intermolt/premolt crayfish which should have had higher levels of gill fouling. Increased ammonia excretion at moderate worm densities likely underlies the increased survival and growth responses observed at these densities in previous experiments. Increased growth of crayfish hosting worms probably influences the impacts of these crustaceans on the benthic communities they occupy.