Max Bernt: There is a key in Tiago Carvalho�s dissertation, but I don�t have it handy. However, the working concept I have of the 5 Gymnorhamphichtys spp is as follows:

G. hypostomus � Long snout and dark, thick bands along the back/sides.
G. rosamariae � Moderate snout length, usually no pigment on the body, pale yellow to transparent. Amazon Basin.
G. bogardusi � Very similar to G. rosamariae with no pigment on the back or sides, but with a dark tail. Limited to the Orinoco Basin.
G. britskii � Shorter snout with dark, thick bands along the back/sides. Limited to the La Plata Basin.
G. rondoni � Shorter snout. Color variable, sometimes transparent, bands normally not extending onto the fishes� sides. Widespread.