Multiple mechanisms of partner regulation may maintain symbiont densities at beneficial levels in a cleaning symbiosis

Background/Question/Methods The current conceptualization of mutualisms is that these interactions are reciprocal exploitations between two partner species. However, there appears to be an asymmetry in the degree to which each partner can exploit the other. In most mutualisms, the smaller partners (e.g., pollinators in plant-pollinator systems, cleaner fish in cleaning symbioses) have greater potential to overexploit their larger partners. Partner regulation, i.e., reducing interaction duration or partner density, therefore, is generally performed by the larger partner. Other mechanisms that may regulate small partner abundance have received little attention. Mutualistic ectosymbioses in which the smaller partner spends all of its life on the exterior of the larger partner (e.g., the crayfish-branchiobdellidan cleaning symbiosis) are systems in which other potential regulatory mechanisms that limit growth and size of the small partners’ population may occur. We evaluated whether multiple types of partner regulation  occur in the crayfish-branchiobdellidan mutualism. We stocked crayfish with 2, 5 or 10 large, mature worms in a lab experiment. We monitored branchiobdellidan abundance and reproduction over a 70 day period. We also evaluated the effect of crayfish molting on worm populations as this may also allow for some control of branchiobdellidan abundance.

Results/Conclusions Similar to previous experiments, we saw clear evidence that crayfish removed excess large branchiobdellidans early in the experiment. Worms in all treatments laid their cocoons in a synchronized pulse during the first 3 weeks. During that period, we saw evidence of an Allee effect, i.e., highest total and per capita cocoon production at intermediate worm densities. This resulted in more juvenile worms on the 5 worm crayfish later in the experiment. However, treatments did not differ in total branchiobdellidan number by the end of the experiment. There was no significant relationship between worm treatment and the proportion of worms lost during the crayfish molt. Instead, the best predictor of the number of worms on a crayfish after the molt was the number of worms it had prior to molting. Our results suggest that worm removal by the crayfish in conjunction with an Allee effect in branchiobdellidan reproduction maintain branchiobdellidan densities at those which benefit the crayfish in this cleaning symbiosis.