It can also be used for navigation and communication. They can use electricity to 'feel' the environment, and they can 'talk' to each other using electrical signals. All of these electric fishes produce electricity from an organ in the tail called an 'electric organ'.

The Elephant Nose fish, or more properly, Gnathonemus petersii, a member of the Mormyrid family, is an example of an electric fish. The electric organ contains electrically-excitable cells called 'electrocytes', which re-ceive simultaneous command signals from the brain to 'fire'. At the moment of 'firing', the electrocytes are asymmetrically polarized acting as serially-connected batteries, like a car battery. The simultaneous firing of electrocytes results in electric organ discharges (EODs) which are emitted into the surrounding water.

In strongly electric fishes, such as the Electric Eel, Electric Catfish and Electric Rays, where the electric organ is used for navigation and communication, the discharge voltage is much smaller; often less than a volt. There are two types of EODs; pulse type and wave type. Those fish that are capable of high voltage discharge, and a few weakly electric fishes, are called pulse-type electric fishes, and they discharge short electrical pulses intermittently. The remaining weak voltage emitting electric fishes are called wavetype. They produce waves of current, like continuous A.C. electricity.

A pulse-type electric fish is the Elephant Nose Catfish. It sends out spurts of electricity less than a millisecond in length. All the fish mentioned so far cannot only produce electricity but also sense it with a very sensitive sensory organs called 'electro-receptors' which are embedded in their skin.

Electroreceptors are used to detect a slight change of electric field caused by nearby objects. Electric fishes can thus electrically 'see' objects in an environment where vision is useless (at night, or in murky water). This process is called 'active electro-location' because the source of electricity that they use for electro-location is their own electric organ. This is similar to the “sonar” technique used by submarines to detect nearby objects by the speed at which sounds are bounced back.

By the combination of electro-generic and electro-ceptive capabilities, some electric fishes are known to communicate each other by electric signals. In a study at the Department of Biology, Trinity College, Hartford, Connecticut, USA, Brown Ghost Knifefish, Apteronotus leptorhynchus, were seen to produce sexually dimorphic, androgen-sensitive electrocommunication signals termed “chirps”. Basically, by exposing male and female fish to these androgens and measuring the “chirps” it was found that males and females differed in both their production of chirps and their ability to elicit chirps from other fish: males chirped about 20-30 times more often than females and elicited 2-4 times as many chirps as females.

So there you go, contrary to popular belief, men do talk more than women….

Some fish can only sense electricity but cannot produce electricity. These fishes are also categorized as electric fishes. They are Sharks, Rays, Skates, Catfish and Paddlefish. These fish can sense very weak electricity generated by prey animals.

So, Sharks can find a small fish buried in sand by weak electricity given off by the prey. This type of electro-location is called 'passive electro-location'.

Electric fishes are divided into the three main categories:

i) Strongly electric fish

• Electric Eel
• Electric Catfish
• Electric Rays

ii) Weakly electric fish

• Knifefishes
• Elephant-nose

iii) Fishes that can only sense electricity

• Sharks
• Rays
• Skate
• Catfish
• Paddlefish
• Platypus (though not a fish, they are electro-receptive)

The jamming avoidance response

The wave-type of electric fish normally discharge at a fixed frequency, and each individual has its own frequency of discharge just like every radio station has its own broadcasting frequency. However, if two individuals meeting up have identical or very close discharge frequencies, their EODs will interfere with each other causing problems in electro-location. To prevent their identical waves cancelling each other out or confusing communication, they shift their frequencies until their frequencies are separated enough for normal operation of electro-location.

Since its discovery by Watanabe and Takeda in 1963, this alteration in frequency has provided neuroscientists with much to think of in terms of temporal and spatial pattern recognition, feature detection, and distributed computation of sensory information.

Listen to your own fish!

As a first, I thought I'd provide you all with the basic information you need so that you can listen to your very own Elephant-Nose! It really is that easy!

All you need is a piezoelectric earphone, or a small amplifier. You can get them at Maplins. They are very sensitive to small electrical signals, and so they are perfect for detecting the electrical signals from the Elephant-Nose fish, and converting them into sounds you can hear.

So, in order to listen in on the fish, simply put one wire from the earphone into the water at one side of the fish tank, and the other wire into the water at the other side of the fish tank, and put the earphone up to your ear. If the fish is sitting still you probably won't be able to hear anything, or if you do it is likely to be very infrequent. As the fish begins to move around it will start to make more electricity as it needs more information about its surroundings in order to navigate. It uses the pulses like radar, to avoid obstacles, to find food, to avoid predators, and to locate other members of its species.

If you want to hear the fish on loudspeakers it is not as complicated as you might think! Simply put two wires in to the tank at each end of the tank and connect the other ends of the wires to an input on your amplifier. A phono (record player) input will provide more noise gain than an aux/radio/tape input. But watch your speakers; keep the level low on the amplifier to start with! You will easily hear the clicks and buzzes of the fish as it reacts to the environment. Not that I suggest it, if you reversed the process you could similarly talk to your fish by connecting the output from a Walkman into the water. Wonder if fish like Handel's Water Music?

With an oscilloscope or a computer sound card, we can also capture the signal and look at a picture of it. The graph below is a snapshot of a bit of fast burst from the fish as it darted around the tank. The electric organ of the Elephant-Nose fish is clearly visible. It is the narrow reddish area connecting the "skirt" of the fish to the tail fins. This is the organ that generates the signal. It is a modified muscle.

To receive electric signals, either from other fish or from echoes of its own discharge, the Elephant-Nose fish has three different types of receptors. These are the mormyromasts, the knollenorgans and the ampullary receptors.

The mormyromasts are the organs that detect the echoes of the electric organ discharge. They are what enable the fish to navigate in murky water and find prey.

The knollenorgans detect the electric organ discharge of other Elephant-Nose fish, and therefore aid in communication and finding mates.

The ampullary receptors measure the low frequency electric fields emitted by other aquatic animals.

Fish are very sensitive to tiny amounts of pollutants in their environment, and are able to detect very small amounts of pollutants such as lead and trichloroethylene in the city's water supply. Using fish and “listening to them” is a cheaper and safer way to determine chemical levels in water than by using other chemical tests, and it can be done continuously.

Other Electric Fish

• The South American Electric Eel, Electrophorus electricus
• The African Electric Catfish, Malopterurus electricus
• The Mediteranean Electric Ray, Torpedo torpedo
• The Electric Catfish from China, Parasilurus asota

These are large fish with powerful electric discharge organs. They use the electric discharge to stun their prey and deter predators.