Sparks fly when it comes to these extraordinary electrogenic fish. They’re living proof that water and electricity do mix.
Meet the extraordinary electrogenic fish who are living proof that water and electricity do mix.
The electric organ of the eel takes up 80% of its body, producing up to 600 volts – enough to knock down a horse.
ELECTRIC FRESHWATER fish are among the most fascinating animals you’ll come across. Numerous species have developed specialised electric organs that are used for navigation and communication, as well as offence and defence.
Several species are offered for sale in the hobby, and while a handful can make good aquarium subjects, others are extremely niche and fit squarely in the ‘oddball’ category.
All animals generate electricity in their muscle and nerve cells, but some fish specialise to use this electricity in unique ways. These fish are specifically referred to as ‘electrogenic’ – capable of generating an electric field – rather than simply ‘electroreceptive’ fish, which merely detect electricity.
The electricity produced in these specialised fish is known as the electric organ discharge (EOD).
Most electrogenic fish produce an EOD using specialised muscle cells known as electrocytes (or electroplaques). Interestingly, electrocytes are functionally similar across disparate groups of fish, suggesting a common evolutionary mechanism. Typically, regular muscle cells will generate a tiny fraction of a volt each, but electric fish have modified these and optimised them in an ingenious way. Electrocytes can’t contract the way regular muscle cells do, but are instead used to specifically store and discharge electricity.
Electrocytes are stacked in the electric organ like batteries in a torch and kept continuously ‘trickle charged’. When required, some species can fire them in series, multiplying the effect with each step – in a few cases, producing a discharge of several hundred volts – but they can also be used more benignly to emit regular pulses of electricity for navigation and communication.
Knifefish of the family Apteronotidae, which includes the popular Black ghost knifefish, Apteronotus
albifrons, appear to be unique in that the electric organ in larvae is derived from modified muscle before being replaced with one based on nervous tissue. This allows ghost knifefish to produce a very weak discharge of just a few millivolts, but one that emits at a very high frequency waveform of up to 2000Hz (2,000 cycles per second).
The strong and the weak
Electrogenic fish are loosely categorised into two groups, based on the strength of their EOD: Weakly electrogenic – most electrogenic fish come into this category. These fish generate a very low-voltage EOD, from an organ in the tail, of perhaps a few millivolts to around 1V or so – not enough to cause a tickle, let alone stun prey. Instead, the EOD is continuously generated in either a sinusoidal wave or a pulse, which envelops the fish in a kind of force field. Electoreceptors on the body detect the EOD generated, and the fish can interpret the environment based upon the capacitance or resistance of objects entering the EOD’S field, distinguishing between inorganic and organic objects. In short, the fish – which include Elephantnoses and Ghost knifefish – can ‘see’ the EOD
and use it to navigate, as well as hunt and detect predators, a nifty adaptation to life in turbid water.
The EOD is also used for communication. It appears to be sex-specific in many species, and some weakly electrogenic fish even avoid ‘jamming’ one another’s EODS by slightly shifting frequencies to prevent confusion. Strongly electrogenic
– these fish are capable of generating an EOD of up to several hundred volts, which they use as a hunting aid. The most notorious example is the electric eel, Electrophorus electricus (see page 39), which appears to deliver bursts of electricity to its prey. These cause involuntarily muscle spasms in the hapless victim, allowing it to be sucked in by the eel.
Of course, it’s also a wonderful defence mechanism, making strongly electric fish a challenge to keep in captivity safely.
Electrics for aquariums
There are several species of weakly electric fish that appear in the aquarium trade quite frequently. Many have quite exacting requirements, but may be worth considering if you have the time and resources to dedicate to their care.
Not all species of mormyrid have the distinctive ‘elephant snout’ (or schnauzenorgan), but all generate a pulse-like EOD that’s used for communication and navigation. The group contains a diverse range of species, ranging from tiny species a few centimetres in length, to whoppers over 1m long. For many aquarists, Peters’ elephantnose, Gnathonemus petersii, from the Congo and Niger basins, is the classic elephantnose. These are quite demanding, and fare best in dedicated shady systems with strategic planting and soft sand substrates. A group of five or so specimens will need a tank 2m in length, as the fish can reach up to 25cm and need plenty of room to spread out in a group.
Live food, such as bloodworm and glassworm, will be needed at first, but frozen foods should be accepted once the fish have settled in.
The Gymnotiform knifefish of South America (not to be confused with the non-electric Notopterid knifefish of Africa) are ‘wave type’ fish and include the most commonly kept electric fish in the hobby, Apteronotus
albifrons, the Black ghost knifefish. This species is also the most highly recommended, providing its needs can be met.
While not exactly a stunner, the Black ghost is certainly a distinctive and unusual species, with its undulating fins and tendency to swim backwards. A. albifrons has been reported to reach 50cm in length, but 30cm is a more realistic
The EOD is continuously generated in either a sinusoidal wave or a pulse, which envelops the fish in a kind of force field
maximum. Aim for a tank length of around 2m for a group of five.
Black ghosts are found in the Amazon basin of Peru and the Paraguay and Paraná rivers of Venezuela, where they are nocturnal hunters. The aquarium environment should reflect this, with extensive aquascaping, plus dim lighting and/or tannin staining in the water, allowing them to settle and be seen at their best (or seen at all, for that matter).
Black ghosts are fairly easy to care for – they’re micropredators and accept frozen foods such as bloodworm and Mysis, although live foods may need to be offered initially. It’s possible to house them with smaller heterospecifics, with medium to large South American tetras being ideal. The Brown ghost knifefish,
Apteronotus leptorhynchus, is widespread in many freshwater systems of Brazil, Colombia, Peru and Venezuela. Reaching 20cm in length, this species is slightly smaller than the Black ghost. Colouration is a drab brown, with a white dorsal stripe. Brown ghosts tend to be more finicky than Black ghosts – they can be kept in a peaceful community tank, but they don’t mix well with conspecifics or other knifefish species.
Quite frequently, the Glass knifefish, Eigenmannia virescens, is seen for sale. This sociable species, which has a wide distribution across South America, is best kept in a small group of conspecifics, but also does well in a mixed community of peaceful, non-electric heterospecifics. Expect a maximum adult length of 35cm or so, with males being larger than females, and provide the same conditions as for the Black ghost knifefish.
When it comes to species of strongly electrogenic fish available in the hobby, we’re probably talking electric catfish.
While many catfish are known to
Typically, regular muscle cells will generate a tiny fraction of a volt each, but electric fish have modified these and optimised them in an ingenious way
have effective electroreceptive capabilities, one order, Malapteruridae, has specialised to generate electricity as an offensive and defensive strategy. There are 21 known species of electric catfish in Malapteruridae, spread across two genera – Paradoxoglanis, whose largest species, P. parvus, reaches only 15cm, and Malapterurus, which includes species over a metre in length.
Electric catfish use specially-adapted pectoral muscles capable delivering a shock of up to 450V for stunning prey or as a defensive mechanism. While not enough to kill a human, it’s still quite a zap and these fish should therefore be treated with great care. It goes without saying that they should never be handled directly.
In the hobby, the most commonlyseen species is Malapterurus
electricus, the Nile electric catfish, which occurs in Africa’s Nile and Niger systems, Lake Chad and various other regions, usually in sluggish, turbid water. Usually offered as juveniles less than 10cm long, these fish have a real oddball charm, although lacking dorsal fins and with their prominent barbels, they’re bizarre-looking fish. Cute or ugly? You decide!
Growing quickly, M. electricus can reach 50cm or more within a few years (specimens well over 1m have been reported, but this appears to be exceptional). Either way, they require plenty of room – around 1000 l for an adult – and are best kept as a single specimen in a dedicated species tank.
They appear to spawn in caves or burrows, but breeding in captivity has so far been elusive. They are generally easy to care for, being undemanding in terms of water parameters, and willing to accept a range of meaty foods as well as vegetable fare.
They may not be pretty, but they’re real stunners.
Elephantnoses use electric signals to map their world.
ABOVE: Diagrams show how the EOD reacts with its surroundings and other fish.