Fish
Elephantnose Fish
Gnathonemus petersii · also called elephantnose fish, peters's elephantnose fish, long-nosed elephant fish
The elephantnose fish lives in African rivers so murky that eyes are almost useless, and it has solved the problem in a remarkable way: it generates a weak electric field around its body and navigates and hunts by feeling the distortions that objects make in it. Doing this well takes serious processing power, and the elephantnose fish has one of the largest brains, for its size, of any animal.
Overview
Elephantnose fish live in slow, turbid, weedy rivers and floodplains across West and Central Africa. They are usually about 23–25 cm long, dark and laterally flattened, with a deeply forked tail and a flexible, finger-like extension of the lower jaw — not a nose, but a mobile probe packed with sensors.
Seeing with electricity
An organ in the tail fires weak electrical pulses many times a second, and receptors across the skin detect how nearby objects distort the field. From this the fish works out the location, size, and even the material of things around it — rock, wood, or living tissue — in total darkness. It sweeps its sensitive chin over the bottom to find buried insect larvae and worms.
A language of pulses
The timing and pattern of the discharges also carry information about a fish’s species, sex, size, and mood. Elephantnose fish change their rhythm, speed up, or fall silent during disputes and courtship.
An extraordinary brain
Relative to its body, the elephantnose fish’s brain is among the largest of any fish and rivals some primates. It consumes an exceptionally high share of the body’s oxygen, and its cerebellum is hugely enlarged to process the electric sense. In aquaria the fish is often described as curious or playful, nudging objects and responding to individual people.
Status
The elephantnose fish is Least Concern, widespread and common, though sensitive to water pollution and collected for the aquarium trade.
Elephantnose Fish: common questions
How does the elephantnose fish use electricity?
It has two separate electrical systems. A specialised organ near the tail, made of modified muscle cells, fires brief, weak electrical pulses many times a second, creating an electric field in the water around the fish. Spread across the fish's skin are thousands of sensory receptors that detect tiny changes in that field. When an object such as a rock, a plant, another fish, or a buried insect larva enters the field, it distorts the pattern of voltage over the skin, and the fish's brain reads those distortions to work out the object's location, size, distance, and even whether it is living tissue, rotten wood, or stone. This lets the fish operate in complete darkness and in water too muddy to see through.
Why does the elephantnose fish have such a big brain?
Its brain is unusually large for a fish, close to the brain-to-body ratio seen in some primates, and it burns through oxygen at a rate far above that of most fish, using a large fraction of the body's total supply. The reason appears to be the electric sense. Interpreting a constantly changing three-dimensional electric field, and separating a faint signal from a buried worm out of all the background clutter, is computationally demanding, and a specific part of the brain, the cerebellum, is enlarged to an extreme degree to handle it. Essentially the fish has traded a large, expensive brain for the ability to hunt and navigate where vision fails.
Is the trunk a nose?
No. Despite the name and the resemblance to an elephant's trunk, the appendage is an extension of the fish's lower jaw and chin, sometimes called a schnauzenorgan. It is flexible and muscular and the fish moves it around actively, but it is not used for breathing or smelling. Instead it is covered in a high density of electroreceptors and touch-sensitive cells, effectively concentrating the fish's sensing power at the tip of a probe that it can poke into mud, gravel, and leaf litter to find food that it cannot see.
Can you keep an elephantnose fish in an aquarium?
Yes, but it is considered a fish for experienced keepers. It needs soft, clean, well-filtered water, a fine sand bottom it can dig in, dim lighting, plenty of hiding places, and a peaceful tank, and it can be shy and a slow, fussy feeder that does poorly in competition with boisterous tankmates. Its electric sense also makes it sensitive to water quality and to electrical interference. Kept well, it can be long-lived and interactive, and some keepers regard it as one of the more personable freshwater fish, but it punishes neglect and casual care.