Catania, K. The shocking predatory strike of the electric eel. Science. 346, 1231 (2014). doi: 10.1126/science.1260807
It’s Friday night and you’re at home with nothing in particular to do, so, naturally, you throw yourself on the couch and turn on the TV for an evening of vegetative entertainment. At some point in the middle of watching re-runs of Friends, you realize you’re hungry. Fortunately, in anticipation of the considerable energy it takes to prepare a home cooked meal, you’ve stocked up on TV dinners, but even that will require you to get up off your … couch. Just then an ad for Lunchables airs and you’re practically drooling, the pudding looks so good! And you think to yourself, wouldn’t it be a wonderful world where you could use your remote control to extract sustenance from the TV itself?
As it turns out, electric eels are eons beyond the human couch potato, and can do just that—use electric pulses to remotely control their prey. While electric eels have a storied history in scientific research from early investigations into the nature of electricity to modern pharmacology, it was only recently in a study conducted by Professor Kenneth Catania of Vanderbilt University published in Science that the fascinating mechanism by which the electric eel Electrophorus electricus detects and immobilizes its prey was revealed.
Catania initially set out to mimic a “naturalistic” encounter of an eel with its swimming prey. To this end, a fish was placed in a tank with an eel and time between a “high voltage volley” emitted by the eel and fish immobilization (or “tetanus”) were recorded. Once a baseline was established for prey response, Catania moved on to demonstrate that tetanus in fish was indeed an involuntary response to the eel’s electric pulse. This time, instead of placing the fish in the same compartment with the eel, he separated the two with a conductive agar barrier, so that electrical signals could pass through without the eel ever seeing the fish. The unfortunate fish on the other side of the barrier had its brain surgically detached from its spine (“pithed”) in order to ensure that strictly involuntary responses were measured. To measure fish response, the fish were immobilized at one end and attached at the other end to a “tension transducer,” which measures tension induced by muscle contractions. In order to provoke electric volley responses from the eel, worms were added into the eel’s compartment. Sure enough, upon encounter with worms, eels emitted a high frequency volley, triggering a tetanus response in fish with a time delay similar to that observed in the naturalistic experiment. Hence, the prey tetanus response is an involuntary response triggered by the eel’s high frequency volley.
Catania went on to localize the point of initiation of the prey response in the fish’s nervous system. He first asked whether the remote control signal from the eel initiates at motor neurons (those neurons that control muscle movement) or occurs through direct control of prey muscles. To test this, Catania treated fish with with “curare,” a plant toxin that blocks the action of motor neurons, or with a “sham” injection, which should have no effect on the fish’s motor neurons. He observed a drop in tension to zero with curare treated fish as compared to the negative control, indicating the eels were triggering a response in the motor neurons. Catania further asked whether tetanus originated via stimulus of the central nervous system (spine) or further downstream in “efferent” branches of motor neurons. He compared fish whose brains and spinal cords (double pithed) were destroyed to those of fish with only brains destroyed, and observed no reduction in the response, indicating that the eel triggered a response in motor neurons efferents. A more precise dissection of the “form” or rhythm of the eel’s “pulse train” lead to the observation that each pulse begins in “doublet,” a phenomenon also observed at the onset of motor neuron trains associated with high rates of muscle tension. Hence, Catania concluded that the eel’s volleys might be exquisitely optimized to induce rapid muscle tension in prey by mimicking motor neuron signaling.
In the course of his experiments, Catania chanced upon an observation that led to perhaps the most fascinating finding of his study. While the eels passed the time between worm feedings, they would occasionally emit high frequency doublets or triplets (series of two or three electrical pulses, respectively). Subsequently, they would attempt to break through the barrier, presumably to reach to the fish on the other side, suggesting the eels were able to detect movements elicited in fish as result of exploratory pulses. To test this hypothesis, Catania placed live prey opposite the agar barrier from the eels. In some cases the eels detected prey through the barrier and proceeded with a high voltage volley. In the more interesting case, eels explored the barrier with low frequency pulses, followed by high voltage doublet, which stimulated prey movement, and resulted in a full on high voltage volley. To further interrogate the eel’s response to induced prey movements, Catania placed fish in insulating body bags that shielded them from electrical pulses. Although eels continued to emit sensory pulses, they never proceeded to attack since the electrically insulated fish never felt the stimuli (i.e., never twitched). However, when Catania induced twitches in body-bagged fish via an external voltage source, the eels responded in all cases with a high voltage volley, demonstrating “mechanosensory” feedback between the hunter and the hunted.
Catania’s study shows that electrical discharge is not simply a brute force feature of the electric eel, but rather the patterns and intensities of electrical pulses make up a deliberate hunting “vocabulary”. Indeed, Catania’s findings show that the language of electric eel is composed of three types of signals, low voltage pulses for sensing the eel’s environment, pairs and triplets of high voltage pulses for exploring more complex environments, and high voltage volleys are used to inflict immobilizing tetanus, massive whole-body contractions, prior to the final attack.
In a few words, the electric eel’s hunt is something more than shocking.
Abrahim is a PhD student at Scripps Institution of Oceanography in San Diego where he studies marine chemical biology.