A new species of monorchiid digenean from marine fishes in the Southwestern Atlantic Ocean off Patagonia.

Proctotrema bartolii n. sp. (Digenea: Monorchiidae) is described based on naturally and experimental obtained adults from the marine fishes Odontesthes smitti (Lahille), O. nigricans (Richardson) (Atherinopsidae) and Eleginops maclovinus (Cuvier) (Eleginopidae) off Patagonia, Argentina, in the Southwestern Atlantic Ocean. Its generic identification is based on the presence of a unipartite terminal organ with the metraterm uniting with its distal region, an unarmed genital atrium, a single testis, a vitellarium follicular lateral to the ovary and ventral sucker, and uterine coils occupying most of hindbody. The new species differs from P. bacilliovatum Odhner, 1911, P. amphitruncatum Fischthal & Thomas, 1969 and P. guptai Ahmad & Dhar, 1987 in having a smaller body (305-650 vs 1,600-3,080, 1,500-1,800 and 2,150-2,670 μm, respectively), a round vs funnel-shaped oral sucker, a smooth vs lobed ovary, a saccular rather than tubular excretory vesicle, the number of vitelline follicles (12-16 vs 8-9, 9 and 6-8, respectively), and wider eggs (25-31 × 15-20 vs 28-37 × 9-12, 24-28 × 7-10, and 24-30 × 8-10 μm, respectively). Moreover, the new species differs from P. bacilliovatum and P. amphitruncatum in having a saccular rather than a coiled seminal vesicle, and from P. bacilliovatum and P. guptai in having its tegument completely vs partly spined. Proctotrema Odhner, 1911 is considered to be restricted to these four species. This is the first report of a species of this genus from South American waters.