Fickle food on a shifting plate: Variable benefits and partner control mechanisms in a crayfish-annelid cleaning symbiosis

Background/Question/Methods

The outcomes of symbioses are more flexible and far-reaching than previously assumed. While past portrayals of symbiotic interactions—especially mutualisms—have emphasized mutual benefit, many symbioses are probably best characterized as reciprocal exploitations in which each partner exploits, and is in turn exploited.  For such interactions to be evolutionarily stable, it is often necessary for organisms to regulate exploitation by their symbiotic partners.  This regulation, known as partner control, is well described in some symbiotic systems, but in other systems, the mechanisms are poorly understood.  Our previous research has described a cleaning symbiosis between a keystone species/ecosystem engineer (crayfish) and ectosymbiotic annelid worms (branchiobdellids).  Branchiobdellids benefit from their relationship with the crayfish through food acquisition, protection, and reproductive dependence.  However, the reciprocal effects of worms on crayfish are variable and more complex.  At lower densities, worms appear to clean the gills of their host, likely improving gas exchange and ammonia excretion.  However, at higher densities, branchiobdellids will actually begin to consume the gills of their hosts.  Using observations, field surveys, and experiments, we sought to identify mechanisms through which crayfish may regulate their exploitation at the hands of their branchiobdellid symbionts.    

Results/Conclusions

We demonstrate that crayfish can regulate their interaction with branchiobdellids through grooming behavior in which crayfish systematically remove the worms from their exoskeleton.  However, this crayfish behavior was not a constant and we identified variability in the behavior among crayfish species, across crayfish age classes, and across differing environmental conditions.  Crayfish reproductive status can also influence branchiobdellid abundance.  From the branchiobdellid’s perspective, we found differences in the abundance of branchiobdellids on crayfish of different age/size classes.  Branchiobdellid reproduction can also be influenced by environmental conditions, specifically the degree of environmental fouling.  These results suggest that both partners in the crayfish-branchiobdellid symbiosis can assert some measure of control over the mutually-exploitative relationship.