The Shocking Electric Eel

credit: Flickr

For a species that has a prominent place in the lore of science--Alexander Volta designed the first synthetic battery inspired by the eel's body design, and Michael Faraday placed his bare hands on the animal to understand the phenomenon of electricity--the electric eel or Electrophorus electricus, has some shocks in store for modern researchers. First, the eel is not an eel at all, but a type of knifefish, (Gymnotiformes) which have a flattened body, use gills to breath, and employs electric impulses for navigation and communication. The eel is unique in the order because it breaths air, has a thick cylindrical body shape, and has a organ that can produce shocks of more than 600 volts, strong enough to incapacitate a horse or human being. Its electric organ stretches 80% of its two meter body length with a positive pole in the head and a negative pole in the tail.  The organ contains thousands of specialized muscle cells called electrocytes that can generate electric potential of about 150mV each. [see chart]

credit: Schroeder et al / Nature

But the oddities do not stop there. Thought to comprise a unique species in its own genus for two hundred and fifty years since the famous taxonomist Linnaeus named it, the electric 'fish' is actually three distinct species according to Brazilian researchers. Carlos David de Santana of the Smithsonian Natural History Museum says his research based on museum specimens and live captures shows the three species have distinct genetics, physiques, habitats and electric power. Their DNA suggests they diverged from a common ancestor seven million years ago, before lions and tigers, brown bears and polar bears, humans and chimpanzees.

One of the species retains the original name of E. electricus. (Linnaeus' eel) The others are now known as E. voltai after the Italian battery builder, and E. varii after Peter Vari, a famous ichthyologist who was a member of Stantana's research team until be died in 2016. Santana can now identify the species by sight, but to make scientifically significant distinctions, his team focused on ten important genes. They found little variation in the genes within a group, but much variation between the three. These genetic differences express themselves in subtle physical characteristics such as head shape and the number of touch sensitive pores along the flanks.

In the wild, the eels live in distinctly different habitats. E. vari lives in muddy, oxygen-deprived lowland streams. The other two live in highland rivers with fast flowing, clear water that is well oxygenated. This difference in habitats probably occurred when the Amazon River reversed its westerly flow about 2.5 million years ago, the same time frame when Volta's and Linnaeus' eels variegated into two separate species. Murky water quality allows E. Vari to hunt stealthily alone, using weaker shocks to paralyze its prey. The clear water species hunt in groups and use much stronger shocks because clear water is less conductive. Volta's eel can discharge up to 860 volts, the strongest shock emitted by any electric fish, and strong enough to kill a man. Eels have been known to leap out of the water in order to stun potential prey with maximum effect by creating a short circuit through their target. Alexander von Humboldt, the German naturalist, described this behavior in 1807.

Recently Ken Catania from Vanderbilt University discovered that eels use their electric battery power for more than just hunting. They release 400 volt pulses to cause their hiding targets to twitch with muscular contractions. Eels can then detect distortions in the weak electrical field that surrounds them with electrosensors in their head, and use them to track their victims. They share this ability with other electric fish, which use weak fields to navigate and communicate. But eels take it up a level by immobilizing their prey using their electric superpowers  to transmit high voltage pulses. Helpless prey is then sucked up by the Tazer-equipped monster. What other stunning discoveries await researchers in Amazonia? We may never know since the eels' habitat is rapidly being destroyed by humans..