A new species of Gieysztoria Ruebush & Hayes, 1939 (Platyhelminthes, Rhabdocoela, Dalyelliidae) from Argentina with comments on geographical distribution of the genus in the Neotropical region

A new Gieysztoria Ruebush & Hayes, 1939 species from limnic environments in Argentina is described. The arrangement of its genital muscle system is described using rhodamine-labeled phalloidin and confocal laser scanning microscopy (CLSM), a novel methodological approach for this genus. Gieysztoria reta n. sp. differs from its congeners in terms of stylet shape and length. The complex stylet is 50-75 μm long, with a fenestrate fibrous girdle. Distally it has four sets of spines: the largest, robust, ventral fang-shaped spine slightly curved toward the center and proximately grooved; a shorter ventral fang-shaped spine; a dorsal group of seven to eight fine spines arranged like bristles; and a group of 12 to 13 dorsal hollow, broad-based spines of different lengths. These are arranged in three rows, forming a fan shape. The major advantage of using rhodamine-labeled phalloidin is that we have been able to visualize the 3D structure of the genital system and the relationship between different sets of muscle fibers, both in the female and male genital organs. Finally, the Neotropical biogeographic distribution of the genus is provided and its relation to stylet morphology is discussed.