Shocking Adaptations Create 'Spooky Interactions' in Amazon's Electric Fish

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A study of electric fish from a remote area of the Brazilian Amazon Basin has revealed for the first time that electric fish are able to interact with each other over long distances, in a way very similar to AM radio.

In findings published in the journal Frontiers, researchers found that a cave-adapted species of glass knifefish (Eigenmannia vicentespelea) has evolved from its surface-dwelling relatives (Eigenmannia trilineata) by sacrificing their eyes and pigmentation but gaining slightly more powerful electric organs that enhance the way they sense prey and communicate in absolute darkness.

The study analyzed the fishes’ electric-based communication and behaviour and found that the electric fish tap into a special channel for long-distance messaging via changes in the amplitude of electrical signals they generate. The researchers adopted Einstein’s famous quote on the theory of quantum entanglement, 'spooky interaction at a distance' – in which particles appear to instantly interact even when separated by vast distances – to describe how the weakly electric fishes perceive these social messages and affect each other’s behaviour, even when several metres apart.

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Researchers at the mouth of Brazil’s São Vicente Cave system (Photo: Adriano Gambarini)

There are currently 80 species of cavefish known to have evolved from surface-dwelling relatives, which have, over millions of years, developed some form of sensory enhancements to adapt to life in caves, often losing sensory organs they no longer need in the process. These adaptations caused biologists to question how weakly electric fish, which use their electric senses for navigating the dark and murky conditions of the Amazon River, might also adapt – either by evolving heightened electric senses to see and communicate in absolute darkness or by powering down their electric fields to save energy in caves where resources are scarce.

'One of the big questions about fish that successfully adapt to living in caves is how they adapt to life without light,' said Eric Fortune, lead author of the study and biologist at New Jersey Institute of Technology (NJIT). 'My colleagues were split between two groups … one group that predicted that the electric fields of the cavefish would be weaker due to limited food supplies, and another that bet that the electric fields would be stronger, allowing the fish to use their electric signals to see and talk more clearly in the complete darkness of the cave.

'It seems that using their electric sense to detect prey and communicate with each other is quite valuable to these animals; they have greater electric field strengths,' said Fortune. 'Interestingly, our analysis of their electric fields and movement shows that they can communicate at distances of metres, which is quite a long way for fish that are around 10cm in length.'

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Researchers deploying their apparatus inside the São Vicente Cave system (Photo: Dayana Ferreira Torres)

'Nearly all research of cavefish species until now has been limited to behavioural experiments in labs, and that is why this study is special,' said Daphne Soares, NJIT associate professor of biology and co-author on the study. 'This is the first time we’ve been able to continuously monitor the behaviour of any cavefish in their natural setting over days. We’ve gained great insight into their nervous system and specialized adaptations for cave life, but it’s just as exciting to learn how sociable and chatty they are with each other … it’s like middle school.'

For the investigation, NJIT and Johns Hopkins researchers teamed with biologist Maria Elina Bichuette from the Federal University of São Carlos, who began studying the two groups of fish nearly two decades ago in the remote São Vicente II Cave system of Central Brazil’s Upper Tocantins river basin.

Over several days, the team applied a customized electric fish-tracking technique involving placing electrode grids throughout the fishes’ water habitats to record and measure the electric fields generated by each animal, allowing the team to analyze the fishes’ movements and electricity-based social interactions. The researchers tracked more than 1,000 electrical-based social interactions over 20-minute-long recordings taken from both surface and cavefish populations, discovering hundreds of specialized long-distance exchanges in the process.

'Basically, our evidence shows that the fish are talking to each other at distance through electricity using a secret hidden channel,' said Fortune. 'It is not unlike how an AM radio works, which relies on amplitude modulations of a radio signal.' 

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Contrast between two distantly-related species of transparent knifefish neighbouring each other in Central Brazil’s Upper Tocantins river basin, state of Goiás. The surface-dwelling fish Eigenmannia trilineata (left) inhabits the Rio da Lapa at the mouth of the São Vicente II Cave where the blind cavefish Eigenmannia vicentespelea (right) has adapted to life without light over tens of thousands of years (Photo: NJIT)

The recordings showed that strengths of electric discharges in the cavefish were about 1.5 times greater than those of surface fish, despite coming at a cost of up to a quarter of their overall energy budget. CT scans of both species found that the cavefish possess larger electric organs than their stream-mates, which could explain the source of the cavefishes’ extra electrical power.

The discovery of the fishes’ AM radio-style communication is claimed to be the first of its kind reported among electric cavefish, although similar phenomena are being reported in some other species as well. Researchers in Germany have recently reported observing a form of long-distance electrical communication among groups of knifefish from the related Apteronotus genus.

Fortune says the finding could have implications for the field of neurobiology, where weakly electric fish present a unique and powerful model for exploring the nature of the brain-body connection in other animals – including humans. 'Electric fish are great systems for understanding the neural basis of behaviour, so we have been studying their brains for decades,' said Fortune. 'These new data are forcing a reexamination of the neural circuits used for the control of behaviour of these fishes.'

 

The complete study: 'Spooky Interaction at a Distance in Cave and Surface Dwelling Electric Fishes' by Eric S. Fortune et al can be found at www.frontiersin.org/articles/10.3389/fnint.2020.561524/full

 

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