Abstract
This study aimed to verify preferences for microhabitat by the monogeneans on the gills of the south american catfish, Rhamdia quelen (Quoy & Gaimard, 1824), at different stocking densities under controlled laboratory conditions. Three stocking densities were used: 14, 28 and 42 fish per tank (50 L) and the fish were sampled initially, at day 5 and 10 of the experiment. Aphanoblastella mastigatus (Suriano, 1986) was noted as the most abundant species at all stocking densities, except for the initial collection day. The gill arches I and II were the most parasitized, showing the highest mean abundance in the dorsal region, at the highest density. The pattern of microhabitat preference of A. mastigatus for outer regions of the gills of the host was independent of the stocking density and collection day. Higher prevalence and abundance of Scleroductus sp. at the initial collection reflected the parasitic infection of the fish at the farm from where the fish were taken. The abundance of Scleroductus sp. decreased along the experiment, and no preference for gill arches was recorded for the species, probably due to the low abundance of this parasite on the gills.