Monogeneans of the catfish Pimelodus blochii Valenciennes (Siluriformes: Pimelodidae) from the Brazilian Amazon, with a description of a new species of Ameloblastella Kritsky, Mendoza-Franco & Scholz, 2000 (Monogenea: Dactylogyridae).

Ameloblastella amazonica n. sp. (Monogenea: Dactylogyridae) collected on the gills of Pimelodus blochii Valenciennes (Siluriformes: Pimelodidae) from Rivers Acre and Iaco, State of Acre (Brazil) is described. The new species differs from the congeners mainly by the morphology of the dorsal bar, i.e. slightly U-shaped bearing a posteriorly directed flap, arising from the anterior edge of its middle third. The most morphologically similar congeners, A. paranaensis (França, Isaac, Pavanelli & Takemoto, 2003) and A. satoi Monteiro, Kritsky & Brasil-Sato, 2010, resemble A. amazonica n. sp. regarding the general structure of the copulatory complex; however, besides other features, they also differ from the new species because their hooks are similar in size (vs hooks with three different sizes). In addition to the dorsal bar lacking a flap, A. amazonica n. sp. can be differentiated from all congeners based on the combination of the following features: size of hooks (one or two different sizes), shape of the dorsal bar (other than slightly U-shaped), anchor points largely passing the level of superficial root (vs not or slightly passing it), number of MCO rings (other than 3-4) and structure of the articulation between the MCO base and the accessory piece (other than thick, medium-sized, smooth and tubular). Demidospermus leptosynophallus Kritsky & Gutierrez, 1998 and Scleroductus yuncensi Jara & Cone, 1989 were also found infesting P. blochii, both representing new host and locality records.