That's because they are 
(AKA freshwater stingray),
Paratrygon, Plesiotrygon, Potamotrygon.
Length: Most river rays are about 12 to 18 inches across (30 to 45
centimeters), but Paratrygon can be up to 60 inches across (152
centimeters).
Color: Wide variety of camouflage patterns.
Range: Freshwater river systems of northern South America.
Remarks: There are about 20 species of South American freshwater
river rays, or stingrays, with more than 50 varieties of color
patterns. Most are found in the Amazon River basin and belong to
the 3 genera. Feared more by locals than piranha, people can become
sick for days from the river ray's sting. The enzyme on the
poisonous barb can cause muscle tissue to dissolve, which is very
painful. If the wound is not treated properly, death is a
possibilty. River rays are the only chondrichthyans to live their
entire lives in freshwater. They have evolved from stingrays that
once inhabited the oceans but were eventually cut off from
saltwater a long time ago.
Status: Common throughout northern South America, some of the more
exotically colored species are popular in the aquarium trade and
wild populations should be monitored.
http://www.anchoragemuseum.org/arch_Ray_Troll/changing_exb1_8.asp
