Retzer's elephant-nose knifefish (Sternarchorhynchus retzeri). X-ray of a section through two Retzer's elephant-nose knifefish, showing their skeletons. This species of ghost knifefish (family Apteronotidae) lives in deep waters of the Amazon Basin, South America, where it feeds on small invertebrates. Elephant-nose knifefish have extremely small eyes (black dot on head) as they don't rely on sight. They find prey by electrolocation, producing and detecting weak electrical impulses. This specimen measures 28.5 centimetres in length.