The Glass Knifefish Eigenmannia virescens are unusual, even for knife fish. The fins and much of the body are transparent and they have a most interesting swimming motion. Their bodies are relatively stiff and they don't have a dorsal fin, so Its hard to imagine how they can move around so adeptly. When they move they appear to be jet propelled. Its that hard to see anal fin using a rippling motion that provides their means of locomotion. If they are going forward and want to reverse direction, they simply reverse the rippling motion and are moving backward.
This transparent knife fish is a member of the Sternopygidae Family found in tropical South America. As a group they are referred to as Glass knifefish or Rattail Knifefish. There are currently 30 species of glass knifefish grouped into six genera. The Eigenmannia genus has eight described species including E. virescens.