Too close for comfort: Elpidium ostracods trigger avoidance and defensive responses in anurans

Commensalism is an interspecific interaction with benefits to one species and null effect on the other. Phoresy has been considered one of these interactions, in which the phoront attaches itself to the dispersal host to achieve new habitat. Though supposedly null for dispersal hosts, studies are increasingly showing costs to them caused by the attachment of phoronts. One phoretic system with few laboratory studies is the one established between anuran amphibians and Elpidium ostracods. On a pilot observation, we registered a treefrog displaying a behavior similarly to a defensive behavior anurans perform to avert parasites. We performed an experimental study to investigate avoidance and defensive behaviors in anurans exposed to Elpidium. We placed 34 treefrogs individually inside Petri dishes with zero, 50, and 100 ostracods, and quantified the occurrence and time until avoidance and the number of defensive behaviors displayed. All anurans in contact with Elpidium performed defensive behaviors, using hind limbs to kick ostracods, and the rate of displays increased with ostracod abundance. Anurans with ostracods avoided the microhabitat more frequently, and higher ostracod abundance avoided faster. The results suggest a behavioral trade-off: anurans perform the defensive behavior until a certain point, and then shift their strategy to habitat avoidance. These changes in behavior might jeopardize the actions anurans perform inside bromeliads, presenting important costs. We provide additional evidence to support the occurrence of costs for dispersal hosts, which distance phoresy from a classic commensalism.