Gymnotiformes

Gymnotiformes is an order of teleost bony fishes found in the freshwater of the Neotropics. These fish are commonly called Neotropical or South American knifefish.

They inhabit rivers and streams from southern Mexico to northern Argentina. Most species are nocturnal, meaning they are active during the night. They are most famous for being electric fish. They can generate electric fields for navigation, communication, and finding prey. In some cases, like the electric eel, they use electricity for attack and defense.

The physical shape of these fish is quite unique. They possess slender, narrow bodies with tapering tails. This shape gives them their common name. They lack both pelvic fins and dorsal fins. Instead, they have an elongated anal fin along their entire underside.

This fin can have approximately 150 fin rays. These rays show active control because they can curve significantly during movement. This is not just passive bending from the water. The fish swims by creating traveling sinusoidal waves along this ribbon-fin. A forward wave moves the fish forward. A reverse wave provides thrust in the opposite direction. This allows them to swim backward as easily as forward.

This movement is a complex mechanical process. As the fin undulates, it produces a system of linked vortex-tubes along its bottom edge. These tubes create a jet of water at an angle to the fin. This jet provides the propulsion needed to move. The speed of the fish depends on the frequency of these waves rather than their amplitude.

The angle of the fin is also vital for efficiency. If the fin were directly underneath the body, it would create upward force. This would require extra downward force to maintain neutral buoyancy. By adjusting wave patterns, they can also hover or move upward. The pectoral fins help them control roll and pitch. They can even roll to generate vertical thrust for ambushing prey.

Most gymnotiforms are considered weakly electric. This means they use active electrolocation to sense their environment. Their electric organs are usually derived from muscle cells. However, adult Apteronotidae are an exception. Their electric organs come from nerve cells called spinal electromotor neurons.

These fish produce tiny discharges of just a few millivolts. These signals are too weak to cause harm. They use these pulses to find bottom-dwelling invertebrates or to communicate with other fish. The electric eel (genus Electrophorus) is different. It is strongly electric and can produce powerful discharges to stun predators or prey. Despite their name, electric eels are not closely related to true eels.

The history of these fish involves millions of years of evolution. Their ancestors likely developed ampullary receptors about 150 million years ago. These receptors allow for passive electroreception. This trait evolved independently from other species like the African Mormyridae. This process is known as convergent evolution.

Their last common ancestor with Mormyridae lived roughly 140 to 208 million years ago. At that time, they did not have electric sensing systems. Today, there are about 250 valid species organized into five families. These include Apteronotidae, Sternopygidae, Gymnotidae, Hypopomidae, and Rhamphichthyidae.

Evolutionary pressures like predation and sexual selection shape their electric signals. Predators like catfish can sense low-frequency electric fields. To stay safe, some species have shifted their signal frequencies to remain invisible.

Sexual selection also plays a major role. Females often prefer males with low-frequency signals. These signals are more dangerous because they attract predators. A male using them demonstrates high fitness by successfully evading capture. Females also prefer longer pulses and larger tail lengths. These traits suggest the male is good at finding resources.

Gymnotiformes are highly diverse across different habitats. Families like Gymnotidae and Hypopomidae are common in small streams and floating meadows. Other families, such as Apteronotidae and Sternopygidae, prefer large rivers.

This diversity is also influenced by genetic drift. Geographical barriers in different streams can lead to very different signal patterns. These fish remain a vital part of the Neotropical freshwater ecosystem. They show how specialized biological tools can help life thrive in dark, complex environments.