Systematic Parasitology 52: 199–204, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 199 Diplectanum squamatum n. sp. (Monogenea: Diplectanidae) from the gills of Cynoscion guatucupa (Sciaenidae) in Southwest Atlantic waters Cl´ audia P. Santos 1,2 , Juan T. Timi 3 & David I. Gibson 4 1 Universidade Santa ´ Ursula, Instituto de Ciências Biol´ ogicas e Ambientais, R. Fernando Ferrari, 75-P.IV., Rio de Janeiro, RJ 22.231-040, Brazil 2 Present address: Universidade Presbiteriana Mackenzie, Av. Rio Branco 277 grupo 302, Rio de Janeiro, RJ 20040- 009, Brazil 3 Laboratorio de Parasitolog´ ıa, Depto. de Biolog´ ıa, F.C.E. y N., U.N.M.D.P., Funes 3350, (7600) Mar del Plata, Argentina 3 Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Accepted for publication 6th December, 2001 Abstract Diplectanum squamatum n. sp. is described from the gills of the sciaenid Cynoscion guatucupa collected off Mar del Plata, Argentina. The new species is distinguished from other species of the genus by the morphology of the hamuli and the male copulatory organ, and presence of scales on the haptor and body. This is the first species of Diplectanum to be reported from southwest Atlantic waters and, with the possible exception of D. cayennense Euzet & Durette-Desset, 1974, the only species from the Atlantic coast of South America. Introduction Members of the Diplectanidae Bychowsky, 1957 par- asitise the gills of teleosts of the order Perciformes and have a world-wide distribution (Oliver, 1993). Species of Diplectanum Diesing, 1858, which are characterised by the presence of a dorsal and a ven- tral squamodisc, three bars and two pairs of hamuli on the haptor, and a sclerotised tubular copulatory organ, have been recorded from both marine and freshwater fishes (Oliver, 1987, 1993). There have, however, been no reports of their occurrence on marine fish in South American Atlantic waters (Kohn & Paiva, 2000; San- tos & Carbonell, 2000), although species of the related Pseudorhabdosynochus Yamaguti, 1958 (see Santos, Buchmann & Gibson, 2000) are present. During a survey of sciaenids off Mar del Plata, Argentina, a new species of Diplectanum was encountered; this is described below. Materials and methods Hosts, Cynoscion guatucupa (Cuvier in Cuvier & Va- lenciennes), were caught using trawl-nets and landed at the port of Mar del Plata. The worms, collected alive from the gills, were fixed in 70% ethanol. Some worms were stained in Gomori’s trichrome or Mayer’s paracarmine, differentiated in acid-alcohol, dehydrated in an ethanol series, cleared in beech- wood creosote and mounted in Canada balsam. Oth- ers were examined in Hoyer’s medium as temporary mounts. Drawings were made with the aid of a draw- ing tube. Measurements are presented in micrometres. The methodology for the measurements of the hamuli follows Oliver (1968). For scanning electron microscopy (SEM), speci- mens were post-fixed for 1h in a solution of 1% os- mium tetroxide in 0.1 M cacodylate buffer, dehydrated through a graded ethanol series, critical-point dried and sputter-coated to a thickness of 20–40 nm with gold. They were examined using a JSM-8500 scan- ning electron microscope at an accelerating voltage of 15 kV. Type and voucherspecimens are deposited in the helminthological collections of the Insitute Os-