Ontogenetic Shifts and Symbiont Succession in a Freshwater Cleaning Symbiosis Mutualism

Host-symbiont associations and the outcome of symbioses may vary over host ontogeny. I examined how changes in crayfish size can influence their associations with ectosymbiotic branchiobdellidan worms and the consequences of these associations for both the host and the symbiont. In the New River, the dominant worm species shifts from Cambarincola philadelphicus to Cambarincola ingens as crayfish (Cambarus bartonii) increase in size. I evaluated whether this shift had effects on different size classes of crayfish, how the worms responded to different sized crayfish and what mechanisms promoted the shift. In a lab experiment, small and large crayfish were stocked with either C. philadelphicus or C. ingens. I monitored crayfish growth, worm numbers and worm reproduction. Neither worm species had an effect on small crayfish but both increased the growth of large crayfish. Cambarincola philadelphicus exhibited greater fitness on small crayfish while C. ingens fitness was greater on large crayfish. A subsequent lab experiment demonstrated that intraguild predation by C. ingens was responsible for the decline in C. philadelphicus on large crayfish. My results demonstrate that the outcome of the crayfish-branchiobdellidan association changes over crayfish ontogeny. Further, host regulation and intraguild predation by C. ingens determine ectosymbiont succession in this mutualism.