Gyrodactylus infections in intensively-reared populations of Nile tilapia, Oreochromis niloticus niloticus, have been associated world-wide with high mortalities of juvenile fish. In this study, 26 populations of Gyrodactylus parasitising either O. n. niloticus or Mozambique tilapia, Oreochromis mossambicus, were sampled from fourteen countries and compared with type material of Gyrodactylus cichlidarum Paperna, 1968, Gyrodactylus niloticus (syn. of G. cichlidarum) and Gyrodactylus shariffi Cone, Arthur et Bondad-Reantaso, 1995. Representative specimens from each population were bisected, each half being used for morphological and molecular analyses. Principal component analyses (PCA) identified five distinct clusters: (1) a cluster representing G. cichlidarum collected from O. n. niloticus from 13 countries; (2) the G. shariffi paratype; (3) three specimens with pronounced ventral bar processes collected from two populations of Mexican O. n. niloticus (Gyrodactylus sp. 1); (4) four specimens collected from an Ethiopian population nominally identified as O. n. niloticus (Gyrodactylus sp. 2); (5) nine gyrodactylids from South African O. mossambicus (Gyrodactylus sp. 3). Molecular analyses comparing the sequence of the ribosomal transcribed spacer regions (ITS 1 and 2) and the 5.8S gene from the non-hook bearing half of worms representative for each population and for each cluster of parasites, confirmed the presence of G. cichlidarum in most samples analysed. Molecular data also confirmed that the DNA sequence of Gyrodactylus sp. 2 and Gyrodactylus sp. 3 (the morphologically-cryptic group of South African specimens from O. mossambicus) differed from that of G. cichlidarum and therefore represent new species; no sequences were obtained from Gyrodactylus sp. 1. The current study demonstrates that G. cichlidarum is the dominant species infecting O. n. niloticus, being found in 13 of the 15 countries sampled.
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