are some 350 species of electric fish. Electric organs have evolved eight times, four of these being organs powerful enough to deliver an electric shock. Each such group is a clade. Most electric organs evolved from myogenic tissue (which forms muscle), however one group of Gymnotiformes, the Apteronotidae, derived their electric organ from neurogenic tissue (which forms nerves). In Gymnarchus niloticus (the African knifefish), the tail, trunk, hypobranchial, and eye muscles are incorporated into the organ, most likely to provide rigid fixation for the electrodes while swimming. In some other species, the tail fin is lost or reduced. This may reduce lateral bending while swimming, allowing the electric field to remain stable for electrolocation. There has been convergent evolution in these features among the mormyrids and gymnotids. Electric fish species that live in habitats with few obstructions, such as some bottom-living fish, display these features less prominently. This implies that convergence for electrolocation is indeed what has driven the evolution of the electric organs in the two groups. Actively electrolocating fish are marked on the phylogenetic tree with a small yellow lightning flash . Fish able to deliver electric shocks are marked with a red lightning flash . Non-electric and purely passively electrolocating species are not shown. Weakly electric fish Weakly electric fish generate a discharge that is typically less than one volt. These are too weak to stun prey and instead are used for navigation, electrolocation in conjunction with electroreceptors in their skin, and electrocommunication with other electric fish. The major groups of weakly electric fish are the Osteoglossiformes, which include the Mormyridae (elephantfishes) and the African knifefish Gymnarchus, and the Gymnotiformes (South American knifefishes). These two groups have evolved convergently, with similar behaviour and abilities but different types of electroreceptors and differently-sited electric organs. Strongly electric fish Strongly electric fish, namely the electric eels, the electric catfishes, the electric rays, and the stargazers, have an electric organ discharge powerful enough to stun prey or be used for defence, and navigation. The electric eel, even when very small in size, can deliver substantial electric power, and enough current to exceed many species' pain threshold. Electric eels sometimes leap out of the water to electrify possible predators directly, as has been tested with a human arm. The amplitude of the electrical output from these fish can range from 10 to 860 volts with a current of up to 1 ampere,