Weird fish of the week: Electric eel

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When is an eel not an eel? When it's this week's weird fish, the Electric eel (Electrophorus electricus).

It's strange the way so many fish are lumped together by their common names simply due to a passing resemblance to another unrelated species.

How many 'sharks' do we see each week in our local aquatic shop, which are actually more closely related to a goldfish than a Great white? It seems that if you have a pointy dorsal fin and forked tail then you're a shark, and by the same logic if you have a long snake-like body then you're an eel.

The Electric eel is probably among the most famous of these aquatic misnomers because, despite initial appearances, they are not cousins to a Conger but are in fact Gymnotiformes – more commonly known as the knifefishes.

A closer inspection of one of these Amazonian residents quickly uncovers the shocking truth of their lack of anguilliform ancestry. Their elongated, scale-less bodies can reach over 2.5m/8.3', making them the largest of the Gymnotiformes but the first real sign they are not eels is their lack of a dorsal fin.

They move around with a rippling motion of their large anal fin which extends from just under the head to the tip of their tail, a swimming method typical of knifefish. This, in combination with a paddling action of their stubby pectorals allows them to swim both forward and backwards - a useful trick in the tangled world of the flooded forest.

They have a board, flattened head and a large, square mouth with a prognathous jaw and both head and body are pocked with hundreds of deep pores, especially along the lateral line. Some of these are a series of high-frequency sensitive tuberous receptors which are believed to be used in hunting other Gymnotiformes.

Their famous electrical abilities are produced by specially developed muscular tissue organs which generate two types of electrical discharge - low and high voltage.

The low voltage current is used in electrolocation to help these largely nocturnal fish to navigate and detect prey while the famous high voltage discharge, which can exceed 500 volts and 1 amp of current, is used to stun or kill prey, as well as in self defence. Younger, smaller fish produce lower electrical outputs.

They are obligate air breathers and will drown if denied access to the surface as they acquire around 80% of their oxygen requirements this way. This allows them to survive in the stagnant waters that can often occur during the dry season.

The breeding behaviour of the Electric eel is also unusual. During the dry season the larger males, which outnumber the females by a ratio of about three to one, build a foam bubble-nest with saliva into which the female deposits the eggs for the male to fertilise in three batches. The male then guards the nest, which can contain as many as 17,000 eggs. It is thought that the first fry may eat unhatched eggs from later spawns until the nest is washed away by the first seasonal rains, dispersing the now 10cm long juveniles into the flooded forest.

Check out the video below:

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