Knife fishes — they're electric!

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Dr Will Crampton, of the University of Central Florida, has been studying electric organ discharges (EODs) in knifefishes. Here he explains how and why they they use it.

The South and Central American electric fish order Gymnotiformes currently comprises 181 species.

One species, the strongly electric eel (Electrophorus electricus) can generate electric organ discharges (EODs) exceeding 600 volts.

The remaining "knife fishes" generate weak EODs typically not exceeding 1 volt. These weak discharges cannot be felt, but are easily registered with electronic instruments.

Gymnotiformes are distributed from southern Mexico to northern Argentina, and reach by far their highest diversity in the giant tropical lowland basins of the Amazon, Orinoco and Guianas.

Just 15 years ago, when I began working on gymnotiforms, only around 100 species were known. Many new species still remain to be described, and the total diversity in the order is likely to reach over 250 species in the next decade.

For example, I am currently working on a revision of the family Hypopomidae with my colleague Dr. Carlos David de Santana. This will describe no less than 23 new species in the genera Brachyhypopomus and Hypopygus. The most recently described new species of gymnotiform is Porotergus duende – a miniaturised electric fish known only from the deep channel of the Amazon River in Brazil and Peru. As a reference to its small size, Dr. Santana and I named this species "duende" from the Portuguese for elf or gnome.

Many other new species of the genus Gymnotus are currently being described by me and colleagues with generous funding from the National Science Foundation of the USA. 

By searching on Google Scholar, PFK readers can see gymnotiform species appear almost every month.  The richest source of material for species descriptions are natural history collections of fishes in South America, North America, and Europe. Expeditions to remote areas donate collections to these museums, and these become available to scientists. Bona fide researchers can even request material on loan, like books from a library. Indeed natural history museums are 'libraries' of biodiversity and centers of active research.

I became interested in electric fishes during my doctoral field work in the early 90s, when I was based at the University of Oxford. At that time electric fishes were becoming very popular for neurobiological studies, but little was known about their natural history.

I spent three years studying the ecology and electric signal diversity of gymnotiforms near the town of Tefé in the Brazilian state of Amazonas, and then worked as a researcher in a Brazilian conservation organization in the Amazon for another four years.

In 2006 I began a faculty position at the University of Central Florida. Here my research continues to focus on electric fish. I am especially interested in how they use their electric organ discharges (EODs) to recognise and discriminate species in electrically crowded ecological communities. This research takes me to many interesting and exotic places around South America - mainly Brazil and Peru, but also other countries such as Uruguay, Suriname, and Colombia.

So what are EODs used for?

The EOD of gymnotiforms serves a dual purpose; first, to locate objects in the dark, much like bats use ultrasound echolocation to navigate in darkness, and, second; to communicate. 

The EOD is itself the product of synchronous action potentials generated by a large array of excitable cells in the electric organ. Electrocytes arranged in the electric organ both in series (like batteries in a flashlight) to produce a larger voltage, and also in parallel, to increase the current. The action potential in a single electrocyte is a short-lasting event (usually less than one millisecond) in which the electrical membrane potential of the cell rises and falls.

The action potential is generated by special types of voltage-gated ion channels in a cell’s plasma membrane through which charged ions (mainly sodium ions) flow. When the channels open they allow an inward flow of current. The sum of all the electrocytes in the electric organ generates an electrostatic field in the water around the fish.

Objects in this field that differ in electrical resistance and impedance from the water are then detected by minute electroreceptors embedded in the skin of electric fishes, and information passed to the brain for processing.

Gymnotiforms have very large brain weights (as a proportion of body weights) in comparison to most other fishes, and most of this extra weight is taken up by the computationally intensive electric sense.

The electroreceptors used to detect objects are also used to detect the signals of neighbouring electric fishes – permitting communication.

Other kinds of excitable cells in animals include neurons (nerve cells) and muscle cells. The electrocytes of most gymnotiforms are derived from muscle cells. However, gymnotiforms in the family Apteronotidae have electrocytes derived from modified neurons, and these are capable of generating EODs at much higher rates (exceeding 2,000 cycles per second in some species).

The EOD can be represented as a waveform, which comprises from 1 to 5 phases of alternating polarity. These waveforms vary tremendously in shape and duration from species to species, and in some cases between males and females. Typically, closely related species that co-occur ecologically have distinct EOD waveforms that permit instant recognition and the avoidance of hybridisation.

Understanding geographical variation in waveforms within and between species across the continental landmass of South America is currently the main focus of my research.