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Phoretic relationships often bring large advantages to epibionts. By attaching themselves to mobile hosts, epibionts are able to: expand their ranges without spending energy, reduce their risk of being predated, and increase their probability of finding food. We assessed the phoretic relationship between the siluriform fish Ancistrus multispinis (Regan, 1912) and the chironomid larva Ichthyocladius sp. in three streams of the Atlantic forest in southeastern Brazil. We evaluated changes in epibiont distribution throughout the body regions of the host and among three different aquatic systems. We had predicted that certain body regions are more prone to support epibiont attachment, and that epibiont prevalence increases with increased host size and quality of the aquatic system. Three streams (Santana, São Pedro and D'Ouro), tributaries of the Guandu River, were sampled during 2010 and 2011. A total of 102 specimens of A. multispinis were collected and analyzed. Epibionts were found in fourteen of fifteen body regions of the host. Observation from scanning electron microscopy revealed that Chironomidae larvae fix themselves to the spicules through the anal prolegs, not at the skin, as previously reported. The amount of epibionts (degree of infestation) was significantly correlated with fish size in the Santana Stream (r = 0.6, p < 0.01), and São Pedro Stream (r = 0.56, p < 0.01), but not in the D'Ouro Stream, the most altered of the three. The presence of epibionts on the body of the fish is directly correlated with the availability of spicules on the fish's body, the largest numbers of infestations being found in structures associated with swimming (caudal and pectoral fins), since the swimming movement can create favorable conditions (e.g., suspension of organic particles, increasing oxygenation) for the epibiont. [ABSTRACT FROM AUTHOR]

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