Monday Article #73: The Electrifying Science of Electric Eels!

A magnificent creature known as the electric eel resides among the luscious Amazon rainforest. This slimy, long fish has a special power unlike any other; it possesses the ability to deliver painful shocks and kill its prey from a distance. With an electrifying power, rivaling that of Zeus, this animal has long been fascinating to scientists and is even the inspiration for the invention of batteries (Jorgensen, 2022). In this article, we’ll be diving into the anatomy, physiology, and behavior that make these creatures the masters of bioelectrogenesis.

Basic overview

First of all, let’s clear up a misconception. Although the electric eel has “eel” in its name, they are actually part of the knifefish order (Gymnotiformes) rather than true eels (Anguilliformes), and more closely related to catfish than to eels. With that out of the way, electric eels (electrophorus genus) are freshwater fish that originate from South America. Electric eels are large fish that can grow up to 2m in length and 20kg in weight. Aside from their electrical prowess which will be discussed later on, electric eels also have the ability to survive on land for some hours if their skin is wet enough (Berra and Netlibrary, 2007). This is because they are obligatory air breathers and rely on buccal pumping to get their oxygen directly from the air instead of through the gills in water (Kramer et al., 1978). The majority of carbon dioxide is expelled through the skin hence the need for a moist skin surface (Finger and Piccolino, 2011).

Electrophysiology

Electric eels are best known for their special ability to produce and discharge electricity to stun their prey. But how? How does an animal produce electricity at such high voltage in the first place, and doesn’t kill itself?

The electric eel possesses three pair of electric organs arranged longitudinally that makes up 80% of its body: the Sachs’ organ, Hunter’s organ, and Main organ. These organs give the eels the ability to produce a discharge of either high voltage or low voltage (de Santana et al., 2019).

Figure 1: Anatomy of an electric eel’s electric organ. Image taken from Creation Ministries International.

These organs are made from electrocytes, modified muscle cells with a high concentration sodium ions channel, as well as acetylcholine receptors (Gotter, Kaetzel and Dedman, 2012). The mechanisms whereby these electrocytes work is remarkably similar to that in excitable cells in our body. At a resting state, the sodium-potassium pump on the surface of the electrocytes actively transports 3 sodium ions out of the cell, and 2 potassium ions into the cell. This causes the resting membrane potential of the electrocyte to be around -85mV (Cao et al., 2020). When presented with a stimulus, the nerve cells innervating the electrocytes release acetylcholine. The binding of acetylcholine to its receptors opens ion channels that cause an inflow of positive ions into the electrocytes on the innervated side, making its potential around +65mV. The other side of the electrocyte continues to maintain a negative charge, thus creating a potential difference (voltage) whereby electricity can flow. Similar to a muscle cell, this potential difference is terminated via the outflow of potassium ions through channels on the cell membrane (Traeger et al., 2017).

In order to produce a high voltage, the electric eels have over 6000 electrocytes lined up in series in the main organ, with 35 such stacks in parallel on each side of the body. Skipping over the physics of electricity, the electrocytes in series amplify the output voltage, allowing the electric eels to generate high-voltage shocks (Traeger et al., 2015). Each of these electrocytes is innervated by one electromotor neuron, and is stimulated to open their voltage-gated sodium channels simultaneously upon presentation of a stimulus (Bennett and Sandri, 1989). This means that action potentials to the proximal (closer to the head) electrocytes should be delayed to varying degrees compared to the distal (farther from the head) electrocytes. As a result, neurons innervating proximal electrocytes are smaller in diameter and conduct action potentials more slowly, due to higher resistance (Bennett, 1971).

Figure 2: Diagrammatic representation of electrocytes. Image taken from Electrocytes of Electric Fish (Gotter, Kaetzel and Dedman, 2012).

Behavior

After all this rambling about physiology, how do electric eels actually use these electrocytes to their advantage?

Using these high-voltage shocks that are also high in frequency (500 times a second), electric eels can induce involuntary muscle contractions in their prey. This causes immobilization of their prey which renders the prey vulnerable (Catania, 2015a). An analogy to this would be that of getting stunned by a TASER. In pursuit of their prey, electric eels are also able to produce high-voltage doublets with an interval of 2ms first. The purpose of this doublet is proposed to be used to detect the location of their prey. The high-voltage doublet induces intense muscle contraction in its prey that can be detected in the water movement by the electric eel’s mechanoreceptor (Catania, 2014). Furthermore, these high-voltage shocks can also be used to track fast-moving prey as the prey’s momentum will usually keep it moving even after muscle paralysis (Catania, 2015b).

Figure 3: Utilization of high-voltage doublet to detect prey location. Image taken from An Optimized Biological Taser: Electric Eels Remotely Induce or Arrest Movement in Nearby Prey (Catania, 2015a).

Besides, these eels are also able to activate only a subset of their electrocytes to produce low-voltage discharge for the use of communication and electrolocation (Hagiwara, Szabo and Enger, 1965). In fact, electric eels have poor eyesight and rely on the disturbance of the electrical discharge wave to identify their environment.

In order to deal with larger or more resistant prey, electric eels employ a strategy of physics. Under normal circumstances, the head of the eel represents the positive pole, while the tail represents the negative pole. A prey near the head only experiences the electric field of the positive pole. Hence, when facing off against strong prey, the electric eel will curl up and surround the prey between its head and tail. This concentrates the electric field line acting on the prey which allows for higher intensity shocks. By employing these high intensities, high frequency, and high voltage discharges, the electric eel can induce constant muscle contractions which leads to muscle fatigue (Catania, 2015c). This is especially useful when the electric eel is feeding on prey like crayfish as it stops the crayfish from retaliating with its claw.

Figure 4: Diagrammatic representation of electric field lines concentration when an electric eel curls its body. Image taken from Electric Eels Concentrate Their Electric Field to Induce Involuntary Fatigue in Struggling Prey (Catania, 2015c).

Contrary to popular belief, electric eels do not always play solo and have been observed to work in groups. Groups of electric eels can round up their prey into a circle and can simultaneously shock their prey to increase the discharge voltage and efficacy (Bastos et al., 2021).

Figure 5: Electric eels hunting in a group. Image taken from Daily Mail.

Why don’t eels shock themselves?

Ending this article, let’s discuss why don’t such powerful discharge affect the electric eels themselves. The consensus behind this is that electric eels possess a high amount of adipose tissue and connective tissue that insulates vital organs and prevents them from getting harmed (Nelson, 2011). Their large size also allows them to resist higher electric currents than their smaller prey counterparts.

Concluding remarks

In a nutshell, electric eels are powerful creatures in the Amazon rainforest and should not be viewed lightly. Let us all appreciate the biodiversity present in the Amazon rainforest and work together to prevent further destruction of this wonderful place.

References

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Article prepared by: Jared Ong Kang Jie, R&D Director of MBIOS 2023/2024