What do James Brown and ghost knifefish (Apteronotus) have in common? They've both harnessed the power of electric organs in order to deliver declarations of adoration. As close relatives of the electric eel (Electrophorus electricus), it's probably not too shocking that ghost knifefish also possess an electric organ that generates an electrical discharge. As well as emitting electricity, ghost knifefish are covered in electro-sensitive receptors that allow them to sense their surroundings, detect the presence of other individuals and communicate with other fish through electric organ discharge. As ghost knifefish are nocturnal and frequently hide amongst rocks and roots, their natural behaviours are rarely ever observed and the characteristics of their electrical discharges have previously been examined almost exclusively through laboratory-based tank studies. Until now, the natural electric conversations of ghost knifefish and their role in wild behaviours have largely remained a mystery.
A recent study reveals the importance of subtle bio-electric communication during the courting and aggression behaviours of wild weakly electric fish. Lead authors of the study, Jörg Henninger and Jan Benda, from the Eberhard Karls University of Tübingen, Germany, set out deep into the rainforests of Panama to record the electro-communications of wild brown ghost knifefish (Apteronotus rostratus). Using a grid of electrodes placed in a knifefish-inhabited creek, the team were able to record electric organ discharges with high precision. They found that each fish emitted electric discharges at their own unique frequency, producing ‘electrical fingerprints’ that they used to identify and track individuals, as well as distinguish between the sexes.
The team discovered that the electric discharges were closely associated with two types of behaviour: male–female courting and male–male aggression. During courting, males would hound females with short electric ‘chirps’ that lasted less than 20 ms, until the female responded with a long chirp that lasted over 150 ms, signalling her intention to release eggs. These long chirps were responded to with precisely timed response ‘doublet’ chirps, which indicated the intention of the male to release sperm, resulting in a tightly synchronised fertilisation event. As well as courtship, the team found interesting patterns of electro-communication between males competing for access to females. These events occurred when the electric discharges of a rival male interfered with the signals emitting from a male courting a female and tended to end with the rival male submissively retreating from the area.
The team was also able to demonstrate how these short and long chirps were closely linked to spawning behaviours in another species of brown ghost knifefish (Apteronotus leptorhynchus) in a more closely monitored laboratory environment. By simultaneously filming and recording the fish's electric discharges for over 5 months, collecting a staggering 1.3 million individual chirps in the process, the team demonstrated that the same short and long chirps recorded in the wild were closely correlated with critical mating events such as the synchronised release of eggs and sperm.
However, in contrast to previous laboratory-based tank studies, many of the EOD signals detected in the wild were much weaker than expected and barely activated the fish’s own electro-receptors. These subtle electric whispers had never previously been recorded in laboratory experiments as the artificial conditions may interfere with natural courtship. This study highlights the importance of investigating physiological phenomena in natural habitats. The team also note that the insights into how weak electrical signals are interpreted by the fish's brains may help us to improve the design of bionic devices, such as retinal and cochlear implants for people with impaired vision and hearing.
