The rules for symbiont community assembly change along a mutualism-parasitism continuum

Symbiont community assembly is driven by host-symbiont and symbiont-symbiont interactions.The effects that symbionts exert on their hosts are often context-dependent and existing theoretical frameworks of symbiont community assembly do not consider the implications of variable outcomes to assembly processes. We hypothesized that symbiont-symbiont interactions become increasingly important along a parasitism/mutualism continuum because; a) negative outcomes favor host resistance which in turn reduces symbiont colonization and subsequently reduce symbiont-symbiont interactions, whereas b) positive host outcomes favor tolerance and consequently higher symbiont colonization rates, leading to stronger interactions among symbionts. We found support for this hypothesis in the cleaning symbiosis between crayfish and ectosymbiotic branchiobdellidan worms. The symbiosis between crayfish and their worms can shift from parasitism/commensalism to mutualism as crayfish age. Here, field surveys identi...