The diversity and evolution of electric organs in Neotropical knifefishes

The Gymnotiformes, also known as the South American or Neotropical knifefishes, include the strongly electric Electrophorus electricus and many other weakly electric species. These fish possess specialised electric organs that are able to release electric discharges into the water, for electrolocation and communication, and sometimes for predation and defence. All Gymnotiform species possess a myogenic electric organ (mEO) derived from the muscle tissue, and members of the Apteronotidae family uniquely possess a neurogenic electric organ (nEOs) derived from the nervous tissue. A mEO may consist of ‘Type A’ electrocytes that develop within the tail muscle (for example, in Apteronotus leptorhynchus), or ‘Type B’ electrocytes that develop below the tail muscle (for example, in Brachyhypopomus gauderio). In this review, we discuss the diversity in the anatomy, electric discharge and development of electric organs found in different Gymnotiform species, as well as the ecological and environmental factors that have likely contributed to this diversity. We then describe various hypotheses regarding the evolution of electric organs, and discuss the potential evolutionary origin of the nEO: a pair of nerve cords that are located on either side of the aorta in B. gauderio, and which may have expanded and developed into a nEO in the Apteronotidae family during its evolution from a common ancestral species. Finally, we compare potential Gymnotiform phylogenies and their supporting evidence.