prey to detect them. The electroreceptive African sharptooth catfish (Clarias gariepinus) may hunt the weakly electric mormyrid, Marcusenius macrolepidotus in this way. This has driven the prey, in an evolutionary arms race, to develop more complex or higher frequency signals that are harder to detect. Some shark embryos and pups "freeze" when they detect the characteristic electric signal of their predators. Evolution and taxonomic distribution In vertebrates, passive electroreception is an ancestral trait, meaning that it was present in their last common ancestor. The ancestral mechanism is called ampullary electroreception, from the name of the receptive organs involved, ampullae of Lorenzini. These evolved from the mechanical sensors of the lateral line, and exist in cartilaginous fishes (sharks, rays, and chimaeras), lungfishes, bichirs, coelacanths, sturgeons, paddlefishes, aquatic salamanders, and caecilians. Ampullae of Lorenzini appear to have been lost early in the evolution of bony fishes and tetrapods, though the evidence for absence in many groups is incomplete and unsatisfactory. Where electroreception does occur in these groups, it has secondarily been acquired in evolution, using organs other than and not homologous with ampullae of Lorenzini. Electric organs have evolved at least eight separate times, each one forming a clade: twice during the evolution of cartilaginous fishes, creating the electric skates and rays, and six times during the evolution of the bony fishes. Passively-electrolocating groups, including those that move their heads to direct their electroreceptors, are shown without symbols. Non-electrolocating species are not shown. Actively electrolocating fish are marked with a small yellow lightning flash and their characteristic discharge waveforms. Fish able to deliver electric shocks are marked with a red lightning flash . {{clade |label1=Vertebrates |1={{clade |label1=Lampreys |1= |sublabel1=End-bud recep. |label2=Jawed fishes |sublabel2=Amp. of Lorenz. |2= }} }} Cartilaginous fish Sharks and rays (Elasmobranchii) rely on electrolocation using their ampullae of Lorenzini in the final stages of their attacks, as can be demonstrated by the robust feeding response elicited by electric fields similar to those of their prey. Sharks are the most electrically sensitive animals known, responding to direct current fields as low as 5 nV/cm. Bony fish Two groups of teleost fishes are weakly electric and actively electroreceptive: the Neotropical knifefishes (Gymnotiformes) and the African elephantfishes (Notopteroidei), enabling them to navigate and find food in turbid water. The Gymnotiformes include the electric eel, which besides the group's use of low-voltage electrolocation, is able to generate high voltage electric shocks to stun