A new species of Potamotrygonocestus Brooks & Thorson, 1976 (Eucestoda: Tetraphyllidea) from Plesiotrygon iwamae Rosa, Castello & Thorson (Mylliobatoidea: Potamotrygonidae) and a redescription of Potamotrygonocestus chaoi Marques, Brooks & Araujo, 2003 N. M. Luchetti Æ F. P. L. Marques Æ P. Charvet-Almeida Received: 9 April 2007 / Accepted: 16 October 2007 Ó Springer Science+Business Media B.V. 2008 Abstract Potamotrygonocestus Brooks & Thorson, 1976 is currently represented by six recognised species of tetraphyllidean cestodes inhabiting Neotropical freshwater stingrays. Potamotrygonid stingrays exam- ined to date have included only a single specimen of Plesiotrygon iwamae. Only one species of tetraphylli- dean, Potamotrygonocestus chaoi Marques, Brooks & Araujo, 2003, has been described from this host, and this description was based on limited material. New efforts to document the diversity in this host species resulted in the collection of eight additional specimens of P. iwamae, one of these from the upper Rio Solimo ˜es, at Sa ˜o Paulo de Olivenc ¸a Amazonas during September, 2003, and seven from the Baı ´a do Marajo ´, Para ´, during November, 2003. The specimen from the upper Solimo ˜es was found to be infected with P. chaoi. Voucher material from this stingray was used for the redescription of this cestode, which is characterized by strobila 8.78–22.83 mm long and a great number of proglottides, 58–93; the new material provided strobilar length and proglottis counts for complete worms. Potamotrygonocestus marajoara n. sp. is the second species of this genus reported from Plesiotrygon iwamae, although it appears to be restricted to the lower Amazon. This new species resembles P. chaoi in possessing filitriches and blade- like spinitriches on the scolex, cephalic peduncle and cirrus, but differs from other species of the genus in the number of testes, which is 44 on average per proglottis, and by having apical sucker measuring 95–175 lm in length. Additional data on the distribu- tion and morphology of the microtriches and a detailed description of the female reproductive system are also provided in this study. Introduction The tetraphyllidean fauna of the Potamotrygonidae is unique in being represented by both typically marine groups (e.g. Acanthobothrium Beneden, 1840, Anin- dobothrium Marques, Brooks & Lasso, 2001 and Rhinebothrium Linton, 1889) and other exclusively freshwater genera (e.g. Rhinebothroides Mayes, Brooks & Thorson, 1981 and Potamotrygonocestus Brooks & Thorson, 1976). This complex parasite fauna makes this system an excellent model for studies of biogeography and historical associations between parasites and hosts. However, knowledge of the parasite diversity of potamotrygonids remains N. M. Luchetti Á F. P. L. Marques (&) Departamento de Zoologia-IB, Laborato ´rio de Helmintologia Evolutiva, Universidade de Sa ˜o Paulo, Rua do Mata ˜o, tv. 14, 101, Cidade Universita ´ria, 05508-090 Sa ˜o Paulo, SP, Brazil e-mail: fernando@ib.usp.br P. Charvet-Almeida Departamento de Sistema ´tica, Laborato ´rio de Ictiologia, Centro de Cie ˆncias Exatas e da Natureza, Universidade Federal da Paraı ´ba, Cidade Universita ´ria, Campus I, 58059-900 Joa ˜o Pessoa, PB, Brazil 123 Syst Parasitol (2008) 70:131–145 DOI 10.1007/s11230-008-9135-9