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Temporal (May 2005 to February 2006) and habitat distribution (pools and riffles) of Hirudinea species was analyzed at a post urban reach from Esquel stream (Chubut province, Patagonia, Argentina). Site was located 5.7 km downstream a Waste Treatment Plant. Mean values of nutrients: ammonia, nitrates and soluble reactive phosphate, as well water conductivity, turbidity and total suspended solids indicated physical and organic pollution. Leeches assemblage was composed by the glossiphonids: Helobdella scutifera Blanchard, 1900, H. michaelseni (Blanchard, 1900), H. simplex (Moore, 1911), Helobdella sp., H. hyalina Ringuelet, 1942, H. obscura Ringuelet, 1942 and the semiscolecid Patagoniobdella variabilis (Blanchard, 1900). From these H. hyalina and H. obscura are new records for Chubut province. Helobdella hyalina (810 ind.m-2) and H. simplex (465 ind. m-2) clearly dominated the assemblage at the reach. Only H. simplex displayed a spatial preference being significantly more abundant in pools than in riffle habitats (p

A new freshwater leech species Helobdella fantasmae n. sp. is described. This description is based on the examination of 12 specimens collected in Laguna Fantasma, Bariloche (41° 05'S 71° 28'W), during December 2002. Leeches were found attached to submerged plants. They were relaxed with gradual addition of 70% ethanol, fixed in 10% formalin, stored in 70% ethanol and stained with borax carmine. Examination of external morphology, dissections and microphotographs were accomplished with a Leica Wild M3Z stereo microscope aided with an Olympus C-4000 digital camera. H. fantasmae n. sp. differs from other species of the genus by the presence of 1 pair of eyes on somite III, crop without gastric chambers, digitiform caeca and postcaeca., short sperm ducts reaching the back of somite XV, atrium pyriform, short ovisacs reaching somite XIII. This is the first record of leeches from an ephemeral wetland in North Patagonia. This finding expands current knowledge of the biodiversity of Hirudinea in South America, increasing the number of known Helobdella spp. from the Río Negro province (North Patagonia) to 10 species.Se describe una especie de sanguijuela dulceacuícola Helobdella fantasmae n. sp. Esta descripción se basa en el examen de 12 ejemplares recolectados en la Laguna Fantasma, Bariloche (41° 05'S 71° 28'O) durante diciembre de 2002. Los individuos se hallaron asociados a la vegetación sumergida. Fueron relajados con la adición gradual de etanol 70%, fijados en formalina al 10%, preservados en etanol 70% y teñidos con carmín borácico. El examen de la morfología externa, disecciones y microfotografías fueron realizados con la ayuda de un estereomicroscopio Leica Wild M3Z con cámara digital Olympus C-4000. H. fantasmae n. sp. difiere de otras especies del género por los siguientes caracteres: un par de ojos en el somito III, estómago recto sin cámaras, ciegos laterales ni postciegos, espermiductos cortos que descienden hasta el somito XV, atrio piriforme, ovisacos cortos que se extienden hasta el somito XIII. Este es el primer registro de hirudíneos en un humedal efímero del norte patagónico. El hallazgo incrementa el conocimiento actual de la diversidad biológica de hirudíneos sudamericana alcanzando las 10 especies de Helobdella spp. descritas hasta el momento para la provincia de Río Negro (Norte de la Patagonia).