ABSTRACT
This chapter presents a survey of the chromosome studies carried out in the five families of Gymnotiformes and discusses the chromosome variability in this group. The fish order Gymnotiformes is a monophyletic group of electrogenic freshwater fish with a wide Neotropical distribution. According to Mago-Leccia, the order Gymnotiformes encompasses 6 fish families: Sternopygidae, Apteronotidae, Rhamphichthyidae, Hypopomidae, Gymnotidae and Electrophoridae. Species diversity of Gymnotiformes has sometimes been overlooked, because the fishes are often difficult to identify, and a number of cryptic species and species complexes may exist that are not evident by their morphological characteristics. While populations of Electrophorus are found throughout northern South America, especially in the Amazon and Orinoco River basins, the genus Gymnotus stands as the most widespread genus in the order Gymnotiformes, being reported in waters from Argentina in South America, to Mexico in North America.
