Gyrodactylus pictae n. sp. (Monogenea: Gyrodactylidae) from the Trinidadian swamp guppy Poecilia picta Regan, with a discussion on species of Gyrodactylus von Nordmann, 1832 and their poeciliid hosts J. Cable 1 , C. van Oosterhout 2 , N. Barson 1 & P.D. Harris 3 1 School of Biosciences, Cardiff University, Cardiff CF10 3TL, UK 2 Molecular Ecology and Fisheries Genetics Laboratory, University of Hull, Hull HU6 7RX, UK 3 School of Education, University of Nottingham, Nottingham NG8 1UG, UK Accepted for publication 19th August, 2004 Abstract Gyrodactylus pictae n. sp. is recorded from Poecilia picta in heterospecific shoals with the guppy P. reticulata in Northern Trinidad. G. pictae is morphologically similar to G. turnbulli Harris, 1986, but the hamuli and marginal hooks are slightly smaller and more gracile. The toe and the point of the marginal hook have a distinctly different shape, providing the best morphological characters for dis- tinguishing the two species. The rDNA ITS1 and ITS2 sequences differ from those of G. turnbulli (the closest relative) by >5%, suggesting that these two taxa are not sibling species. The origin of the two species on poeciliids of the subgenus Micropoecilia is discussed, and it is suggested that this may represent a case of host–parasite co-evolution. Introduction A range of gyrodactylid monogeneans have been collected from poeciliid fishes from tropical Central America. The best known are G. bullatarudis Turnbull, 1956 and G. turnbulli Harris, 1986 from the guppy Poecilia reticulata Peters, but other species have been recorded from Xiphophorus spp., P. sphenops Valenciennes, P. caucana (Steindach- ner) and Gambusia affinis (Baird & Girard) (see Rogers & Wellborn, 1965; Kritsky & Fritts, 1970; Lucky, 1973; Harris, 1986; Harris & Cable, 2000; Richards et al., 2000). These studies indicate that poeciliid fishes have a complex gyrodactylid fauna dominated by G. bullatarudis-like species, including G. bullatarudis, G. gambusiae Rogers & Wellborn, 1965, G. poeciliae Harris & Cable, 2002, G. rasini Lucky, 1973 and G. costaricensis Kritsky & Fritts, 1970, and by G. turnbulli-like species, including G. turnbulli and possibly G. milleri Harris & Cable, 2000. In the current work, samples of gyrodactylids were collected from the swamp guppy Poecilia picta Reagan. This poeciliid occurs in sympatry with P. reticulata in some rivers in Trinidad and Tobago, forming mixed-species shoals. Specimens of G. pictae were first recognised as different because of their reluctance to infect P. reticulata. Further studies revealed morphological and molecular differences. On the basis of this material, G. pictae n. sp. is described. Materials and methods Shoals of poeciliids were collected using a siene net from streams in the Northern Mountain Range of Trinidad during June and July, 2003. Shoals were sorted according to species, either P. picta or P. reticulata, and fish of the two species were either individually fixed in 95% ethanol or examined for gyrodactylids using a stereo-micro- scope. Specimens of Gyrodactylus von Nordmann, 1832 found on P. picta were removed individually and transferred to fresh 95% ethanol for storage Systematic Parasitology (2005) 60: 159–164 Ó Springer 2005