Four new species of Characithecium (Monopisthocotyla: Dactylogyridae) from Acestrorhynchus falcirostris (Osteichthyes, Acestrorhynchidae) from the Itaya River, Peru.

Studies concentrating on description, identification, and analysis of monopisthocotyls are required to further our understanding of this group of parasites connected to Amazonian fish. Based on the above, the main objective of this research was to identify new species of monopisthocotyls parasitizing the gills of Acestrorhynchus falcirostris (Cuvier) collected in the Itaya River, Loreto-Peru. Twenty (20) A. falcitrostris were collected in the Itaya River (3°47'20.70″S 73°15'40.90″W), in Loreto-Peru in October 2025. The collected fish were transported in boxes with ice to the Laboratory of Parasitology of the "Instituto de Investigaciones de la Amazonía Peruana" (IIAP), in Iquitos, Loreto-Peru for parasitological analyses. Four new species were discovered and described: Characithecium peruense n. sp., C. cachorroi n. sp., C. iquitense n. sp., and C. loretoense n. sp. The new species are differentiated from their congeners, mainly by the morphology of the copulatory complex. In C. peruense n. sp. the male copulatory organ (MCO) is a J-shaped tube, the accessory piece is articulated to the base of the MCO and comprises two subunits, being the second unit hook-shaped. In C. cachorroi n. sp. the accessory piece consists of an elongated proximal unit, with a gentle curvature in its middle portion, whose upper end forms a loop that passes behind the distal end of the cirrus; and an equally elongated distal unit, which forms a loop at its anterior end and has an expansion in its middle region that projects posteriorly. In C. iquitense n. sp. the MCO consists of an elongated sclerotized tube forming two and a half counterclockwise rings before reaching the base, which is articulated to the accessory piece, presenting two units, the first unit articulated to the base of the MCO, "L" shaped, with a rounded termination, the second unit is complex, featuring a stippled (dotted) subunit, formed by two asymmetrical arms. In C. loretoense n. sp. the MCO is tubular, elongated, whose sinuous path describes four and a half clockwise rings; the base of the cirrus articulates with the accessory piece, that is presented as robust plate-like structure with a broad bulbous basal region that tapers to an acutely pointed, down-curved distal tip. The results of the present investigation represent the first species of the Characithecium described from a host collected in the Peruvian Amazon, increasing the knowledge of the biodiversity of monopisthocotyls reported for Peru and the Neotropical region.