There is something in the water 100 metres offshore. Its grey and grainy form is poorly defined in the gloomy light of a Scottish winter. But the trail of ripples suggest a gliding, ghost-like motion. Only as it draws closer does the compelling spectacle become clear. It's another boatload of American tourists looking for a monster.

'Nessie' may be the mother of all mythical creatures, but it's also an animal with a serious identity crisis. Thousands of alleged 'sightings' have produced little in the way of zoological consensus. A 1972 study found that there was enough food in Loch Ness to feed about 20 beasts, weighing a tonne and a half each. But what kind of beasts could they be?

One popular idea is that the monster is a plesiosaur, a giant sea-dwelling reptile with a long, slender neck and tail, small head and paddle-like flippers. But while this description may match up nicely with some eyewitness accounts, there is a small snag – plesiosaurs went extinct with the dinosaurs, about 65 million years ago. Even if a few individuals somehow managed to survive the mass extinction, the cold, dark freshwater of Loch Ness seems like the last place a large, cold-blooded marine reptile would choose to set up home.

Many sightings of the monster refer to a series of undulating humps, but this drives another nail into the coffin of the plesiosaur theory. Reptiles are good at flexing their bodies sideways, but not up and down. That's more of a mammalian feature and suggests nothing more mythical than a train of swimming otters or seals.

But all air-breathing animals – reptiles, mammals, amphibians and birds – seem unlikely candidates for the monster's true identity. For wouldn't an animal that needed regular intakes of air provide us with more frequent sightings? Nessie's apparent diffidence is more consistent with the lifestyle of some kind of fish – perhaps a giant eel, a catfish or a sturgeon.

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