Some are the size of a horse, some can paralyse you with a 500-volt electric shock and some are said to have developed a taste for testicles.

Beneath the still waters of the world’s rivers lie a wealth of sharp-toothed, slithery and terrifying-looking creatures which send shivers down the spines of the locals who regale gruesome stories about their unpleasant underwater neighbours.

But intrepid British filmmaker Jeremy Wade has ignored conventional wisdom to track down the world’s most frightening freshwater fish to find out just how scary they really are.

Jaw-dropping: The Goliath Tiger Fish, known by its Latin name Hydrocynus goliath was found in the Congo River. The biggest Tiger Fish on record was nearly 5ft long and weighed 154lbs, the equivalent of a super-welterweight prizefighter

To the point: Intrepid explorer Jeremy Wade with a Payara fish. The animal is known as the Dog Tooth Characin or the Vampire Fish, due to its protruding front dentures

Watch out! Jeremy goes to extreme lengths to solve local legends about mysterious creatures of the deep, often finding the reality can be a lot more scary

Gotcha! The plucky adventurer grapples with a stingray on the banks of the Parana River in Argentina. Most stingrays have one or more barbed stings on their tail, which are used exclusively in self-defence

The adventurer has hunted out the array of bizarre-looking animals for his new series of ‘River Monsters’ in which he tackles frightening fresh water sting rays, wrestles Japanese salamanders and even swims with the huge Indian 160lb goonch catfish.

Jeremy said what kept him going was a desire not only to see the unseen but to make people aware that the creatures were more under threat from man than man was from them.

He said the perfect river monster was ‘big and outlandish – looking like it shouldn’t be anywhere near people’.

He said: ‘Finding it is the first thing, but conservation is inextricably bound up with that. People can’t be expected to care about something if they don’t even know of its existence.

‘A lot of these animals have never been seen by TV audiences, because you can’t make conventional natural history programmes about them, thanks to the poor or nonexistent visibility in most fresh water.

‘The fishing line is just a means to an end. Having seen it, I then return it to the water.

‘Although potentially dangerous to people, many of these ‘monsters’ are misunderstood; only in rare instances – such as a giant snakehead protecting its young – do they wilfully attack people.  Slippery customer: Jeremy clings onto the tail of the stingray. With its beige and black skin tones the animal is perfectly camouflaged in the sandy bed of the Parana River

Nothing but the tooth: A brave tribesman opens up the jaws of a Pacu fish in Tongwinjamb in Papua New Guinea. The pacu is referred to as the ‘ball cutter’ after 𝓀𝒾𝓁𝓁ing two men by biting off their testicles

Open wide: The African Lung Fish which is found in The Congo. They have a series of rows of teeth to form a fan-shaped surface. Lungfish are best known for retaining primitive characteristics including the ability to breathe air

Choppy waters: Left, Jeremy an appropriately named Australian Saw Fish in the Fitzroy River, Australia and right, the filmmaker holds open the jaws of the Goliath Tiger Fish in the Congo River

Fin-tastic: After a spot of nighttime fishing Jeremy gets his hands on an Australian Bull Shark in the state of Queensland

‘As apex predators, they are a good indicator of the overall health of our rivers, which is something we should all care about, out of self interest if nothing else.’

Despite the rows of teeth, poisonous spines and size of the some of the fish Jeremy has faced his choice for the scariest is one that doesn’t break the weighing scales.

He said: ‘The scariest fish for me is possibly the electric eel. They don’t look much but their 500-volt shock is enough to paralyse you, and if you fall face down in the water that’s it.

‘Anybody coming to help runs the risk of getting zapped too. I’ve heard of people dying in thigh-deep water. A creepy detail is that they sometimes wrap around the victim’s chest and keep shocking to stop the heart.’

Red alert: The Sockeye Salmon found in the Kvichak River, Lake Illiamna and Bristol Bay Alaska

Biting back: Jeremy has now filmed a fourth foray into the murky waters of the amazing beast he encounters which could be coming to the UK soon

Small but deadly: The Bullseye Snakehead is a faster growing fish than most of the other species of the genus. It is a carnivorous species. The flesh has high nutritive value and its flesh is said to have wound-healing and recuperative attributes

Bad reputation: Jeremy with a piranha by the Corantyne River in Suriname. The fish are known for their sharp teeth and a voracious appetite for meat

Former secondary school teacher Jeremy, who has a BSc in zoology from Bristol University, said there had been cases when he thought he might not find the fish behind the legend.

He said: ‘I had my doubts with the goonch catfish in India; myself and the cameraman went to extremes of sleep deprivation before we finally got a big one of these on film: 6ft long and 161lb.

‘A few years before, I had an even bigger goonch cut through my line on a rock. I’ll never know how much bigger, but when I was free-diving the same pool during the filming of River Monsters, our underwater cameraman saw a fish ‘the size of a horse’.’

As he documents the fish of the world’s rivers Jeremy said it might also be a record of fish not with us in 50 years time.

He said: ‘Many of the fish I’ve found are already much more scarce than they were even 50 years ago. And many have become extinct from parts of their range.

‘But most will probably manage to hang on somewhere in small numbers, and at smaller sizes.

‘An exception is the huge predatory kaluga sturgeon in Russia’s Amur River, which only lives here, and which is likely to become extinct in a matter of decades – thanks to the illegal caviar trade.’

Wriggly customer: Wearing protective gloves Jeremy wrestles to get hold of a salamander in the Kamo River in Japan. The fish are typically characterized by a superficially lizard-like appearance, with slender bodies, short noses, and long tails

Biting back: A fisherman helps Jeremy try to catch a slithery Arapaima in the Rio Negro near the Amazonian city of Manaus

Heads up: A Wolf Fish captured near the village of Itaipavas in southern Brazil. This creature can reach 39 inches in length and grow as big as 88lbs. It comes out at dusk and during the night to feed on other fish and small invertebrates

Predator: Jeremy with a piranha on the Parana River bank in Argentina. Piranha teeth are often used to make tools and weapons by the indigenous population. The fish are also popular as food, although if an individual piranha is caught on a hook or line, it may be attacked by other (free) piranhas

Feisty fish: Jeremy with a Pacu fish. Related to the piranha, the main difference is jaw alignment; piranha have pointed, razor-sharp teeth in a pronounced underbite whereas pacu have squarer, straighter teeth like a human in a less severe underbite, or a slight overbite

Scales of power: The giant grouper, also known as the brindlebass, brown spotted cod, or bumblebee grouper and as the Queensland groper in Australia (as pictured here). It is the largest bony fish found in coral reefs

Going swimmingly: Jeremy found this arapaima fish in a lake just off the Madeira River in the state of Amazonas, Brazil. The arapaima,or paiche is a living fossil and one of the largest freshwater fish in the world

Heavy load: Jeremy with a Wels Cat Fish in Spain. These incredible river monsters could take the bait and probably the angler as well. It is a scaleless fresh and brackish water fish recognizable by its broad, flat head and wide mouth. Wels catfish can live for at least thirty years and have very good hearing.