Zootaxa 3893 (3): 382–396 www.mapress.com /zootaxa / ISSN 1175-5326 (print edition) Article Copyright © 2014 Magnolia Press ZOOTAXA ISSN 1175-5334 (online edition) http://dx.doi.org/10.11646/zootaxa.3893.3.4 http://zoobank.org/urn:lsid:zoobank.org:pub:C8D52425-9F98-42CC-95BB-6553D943AA50 Characithecium (Monogenoidea: Dactylogyridae) parasitic on the Neotropical fish Oligosarcus jenynsii (Teleostei: Characidae) from the Pampasic region, Argentina, with the emendation of the genus MARÍA ALEJANDRA ROSSIN & JUAN TOMÁS TIMI Laboratorio de Parasitología, Instituto de Investigaciones Marinas y Costeras (IIMIyC), Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata-Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Funes 3350, (7600) Mar del Plata, Argentina. E-mail: mrossin@mdp.edu.ar; jtimi@mdp.edu.ar Abstract Presently, only 2 species of dactylogyrid monogeneans have been reported Characidae inhabiting lakes and streams from the Pampasic region (central Argentina). During a parasitological survey on the characid Oligosarcus jenynsii, from Nahuel Rucá Lake (Buenos Aires province, Argentina), dactylogyrids were found on the gills. Specimens were identified as members of Characithecium: C. chascomusensis n. comb, C. longianchoratum n. sp., C. robustum n. sp., C. quadratum n. sp. and C. chelatum n. sp. These species can be distinguished from each other mainly by differences in the shape of the accessory piece of the male copulatory organ, morphology of anchors and ventral bars as well as position of vaginal aperture. The observation of some additional features present in all species studied, such as the vaginal aperture position (as midventral, lateroventral or lateromarginal) and the variability in the morphology of ventral bar (posteromedial projection present or absent) justified an emended diagnosis of Characithecium. Key words: South America parasites, Neotropical monogenean, dactilogyrids, Characithecium chascomusensis n. comb, Characithecium longianchoratum n. sp., Characithecium robustum n. sp., Characithecium quadratum n. sp., Characithecium chelatum n. sp., characid fish, Oligosarcus jenynsii Introduction Characidae is the fourth most diverse family of fishes and the most diverse among Neotropical fishes, with more than 1,000 species distributed from the southern USA to northern Patagonia (Argentina) in South America (Ornelas-García et al. 2008; Mirande 2010). The dactylogyrid fauna of characids has been extensively studied in northern South America, especially in Brazil (Thatcher 2006; Cohen & Kohn 2009; Cohen et al. 2013) and, in lesser extent, in Central America (Mendoza-Franco et al. 2009). At present, the knowledge of monogenean parasitizing characids in the southern boundaries of the family distribution, represented by the Pampasic region in central Argentina, is scarce and fragmentary, in comparison with their northern counterparts. This fauna comprises, to date, the records of only 2 species of Palombitrema Price & Bussing, 1968 that have been reported from Chascomus Lake (Buenos Aires province). These species include P. heteroancistrium (Price & Bussing, 1968) Suriano, 1997 a parasite of Astyanax fasciatus fasciatus Cuvier, and P. chascomusense (Suriano, 1981) Suriano, 1997, a parasite of the curimatid Cyphocharax voga (Hensel) as well as of Oligosarcus jenynsii (Günther) (Suriano 1981; 1997). During a parasitological examination of O. jenynsii captured in Nahuel Rucá, a shallow lake located in Buenos Aires province, Argentina, dactylogyrid monogeneans were found in the gills. Oligosarcus jenynsii is one of the most widely distributed and dominant species in the ecosystems at regional scale (Rosso 2006). However, little is known about its parasite fauna. Herein, 5 species were identified as members of Characithecium Mendoza-Franco, Reina & Torchin, 2009 a monospecific genus containing C. costaricensis (Price & Bussing, 1967) MendozaFranco, Reina & Torchin, 2009, reported from 3 species of Astyanax across Central America (Mendoza-Franco et al. 2009). Four of the species found are described for first time, whereas the fifth species was identified as P. 382 Accepted by N. Dronen: 31 Oct. 2014; published: 9 Dec. 2014 Acknowledgments The authors wish to thank Dr. Matías Merlo for their assistance in the collection of fish samples; to Mr. Pedro Urrutia for allowing us to work in Nahuel Rucá Lake and to Dr. Darío Colautti and his research team (Instituto de Limnología Raúl Ringuelet-CONICET) for providing fish samples from Chascomús Lake. We also thank Dr. Walter Boeger for their valuable comments and suggestions on the manuscript and drawings. Financial support was provided by grants from Agencia Nacional de Promoción Científica y Tecnológica, Argentina (ANPCYT; PICT # 2131) and Universidad Nacional de Mar del Plata (EXA 713/14). References Agarwal, N. & Kritsky, D.C. (1998) Neotropical Monogenoidea. 33. Three new species of Ancistrohaptor n. g. (Dactylogyridae, Ancyrocephalinae) on Triportheus spp. (Teleostei, Characidae) from Brazil, with checklist of ancyrocephalines recorded from Neotropical characiform fishes. 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