Gymnotiformes - Evolution

Evolution

Gymnotiformes are among the more derived members of Ostariophysi, a lineage of primary freshwater fishes. The only known fossils are from the Miocene about 7 million years ago (Mya) of Bolivia.

Gymnotiformes has no extant species in Africa. This may be because they did not spread into Africa before South America and Africa split, or it may be that they were outcompeted by Mormyriformes, which are similar in that they also use electrolocation.

Interestingly, Gymnotiformes and Mormyriformes have developed their electric organs and electrosensory systems (ESSs) through convergent evolution. As Arnegard et al. (2005) and Albert and Crampton (2005) show, their last common ancestor was roughly 140 to 208 Mya, and at this time they did not possess ESSs. Each species of Mormyrus (family: Mormyridae) and Gymnotus (family: Gymnotidae) have evolved a completely unique waveform that allows the individual fish to identify between species, genders, individuals and even between mates with better fitness levels. The differences include the direction of the initial phase of the wave (positive or negative, which correlates to the direction of flow of current through the electrocytes in the electric organ), the amplitude of the wave, the frequency of the wave, and the number of phases of the wave. Thus, the parallels between these distantly related species is remarkable.

One significant force driving this evolution is predation. The most common predators of Gymnotiformes include the closely related Siluriformes (catfish), as well as predation within families (E. electricus is one of the largest predators of Gymnotus). These predators sense electric fields, but only at low frequencies, thus certain species of Gymnotiformes, such as those in Gymnotus, have shifted the frequency of their signals so they can be effectively invisible.

As sexual selection is another driving force with an unusual influence, in that females exhibit preference for males with low-frequency signals (which are energetically expensive and easily detected by predators), but most males exhibit this frequency only intermittently. They also prefer males with longer pulses, also energetically expensive, and large tail lengths. These signs indicate some ability to exploit resources, thus indicating better lifetime reproductive success.