The electric eel
The ELECTRIC EEL is not an eel, but a knifefish belonging to the order Cypriniformes. Often they are placed in the single family Gymnotidae, although other authorities recognise six families with sixty two species. All species live in South and Central America and have an electric organ. There is still relatively little known about the biology of this fish.
Of the various Ostariophysi orders extant in South America the knifefish represent the smallest number of species. To boot, they are also the class whose systematics and ecology have been least investigated so far.
One reason for this is that knifefish are often difficult to differentiate and catch.
In addition, they scarcely have any commercial fishing value. Of the six families grouped under Gymnotimormes, the ELECTRIC EEL is the sole representative of the family Electrophoridae. The ELECTRIC EEL can reach a length of 250 cm and tip the scales at over 20 kg. It can produce electric shocks of up to 800 V at 2 A.
Normally the ELECTRIC EEL discharges brief electric pulses to find its bearings and its prey. Yet for defence purposes the electric shock can be far higher and sustained. How can the ELECTRIC EEL generate electric shocks at all? Unlike other knifefish whose electric organ evolved only from the one muscle the ELECTRIC EEL has no fewer than three electric organs. These are the main organ consisting of up to six thousand stacked electroplaques for catching prey and defending itself, Hunter’s organ, and Sach’s organ. The last of these emits twenty to thirty bursts a second for orientation purposes.
The ELECTRIC EEL has underdeveloped eyes, and its home waters do not permit deeper fields of vision. Similar to an echo sounder its electric pulses are reflected off rock and similar structures and their “echoes” are registered and analysed by receptors on the head. The powerful electric shock from the main organ is of great importance for catching prey, which is temporarily paralysed before it is swallowed without risk of injury. At ZOO-AQUARIUM, though, the ELECTRIC EELS are given only dead rudd and bream to feed on. These electric shocks also provide effective protection against attack. Direct contact with the skin of the ELECTRIC EEL is enough to drive off even a crocodile.
Even though the ELECTRIC EEL is the best known electric fish with these capabilities, it is by far not the only one. Also well known is the sea-dwelling electric ray that has evolved electric organs in parallel.
The ELECTRIC EEL is a true powerhouse, generating a veritable barrage of electric shocks that make it much feared among the natives.
A number of knifefish have adapted to the low oxygen content of the water.
The ELECTRIC EEL takes in air at the water’s surface through its modified mouth. Its gill slits are underdeveloped and too small to take up enough oxygen from the water. No aquarium has as yet succeeded in rebreeding ELECTRIC EELs. Also, there are no known observations in the wild. There is still not even a reliable method for differentiating between the sexes.