The creatures behind repeated sightings of the fabled Loch Ness Monster may be giant eels, according to scientists.
Researchers from New Zealand have tried to catalogue all living species in the loch by extracting DNA from water samples.
Following analysis, the scientists have ruled out the presence of large animals said to be behind reports of a monster.
No evidence of a prehistoric marine reptile called a plesiosaur or a large fish such as a sturgeon were found.
Catfish and suggestions that a wandering Greenland shark were behind the sightings were also discounted.
The aim of the research was not to find Nessie, but to improve knowledge of what plants and animals live in Loch Ness.
European eels are among the creatures in the loch, and whose DNA was picked up by the new research.
Juvenile eels, known as elvers, arrive in Scottish rivers and lochs after migrating more than 3,100 miles (5,000 km) from the Sargasso Sea near the Bahamas, where the animals spawn and lay eggs.
Prof Neil Gemmell, a geneticist from New Zealand's University of Otago. said: "People love a mystery, we've used science to add another chapter to Loch Ness' mystique.
"We can't find any evidence of a creature that's remotely related to that in our environmental-DNA sequence data. So, sorry, I don't think the plesiosaur idea holds up based on the data that we have obtained."
He added: "So there's no shark DNA in Loch Ness based on our sampling. There is also no catfish DNA in Loch Ness based on our sampling. We can't find any evidence of sturgeon either,
"There is a very significant amount of eel DNA. Eels are very plentiful in Loch Ness, with eel DNA found at pretty much every location sampled - there are a lot of them. So - are they giant eels?
"Well, our data doesn't reveal their size, but the sheer quantity of the material says that we can't discount the possibility that there may be giant eels in Loch Ness. Therefore we can't discount the possibility that what people see and believe is the Loch Ness Monster might be a giant eel."
DNA from humans, dogs, sheep, cattle, deer, badgers, rabbits, voles and birds were also identified by the researchers.
How Nessie came to grip the public imagination
The Loch Ness Monster is one of Scotland's oldest and most enduring myths. It inspires books, TV shows and films, and sustains a major tourism industry around its home.
The story of the monster can be traced back 1,500 years when Irish missionary St Columba is said to have encountered a beast in the River Ness in 565AD.
Later, in the 1930s, The Inverness Courier reported the first modern sighting of Nessie.
In 1933, the newspaper's Fort Augustus correspondent, Alec Campbell, reported a sighting by Aldie Mackay of what she believed to be Nessie.
Mr Campbell's report described a whale-like creature and the loch's water "cascading and churning".
The editor at the time, Evan Barron, suggested the beast be described as a "monster", kick starting the modern myth of the Loch Ness Monster.
In 1934, highly respected British surgeon, Colonel Robert Wilson, claimed he took a photograph of the monster while driving along the northern shore of Loch Ness.
Known as the "Surgeon's Photograph", 60 years later it was confirmed as a hoax hatched in revenge after a newspaper ridiculed journalist Marmaduke Wetherell for finding "Nessie footprints" on the shore.
The "monster" caught on camera was apparently a toy submarine bought from Woolworths, with a head fashioned from wood putty.
The hoaxers then gave the photo to Wilson, a friend who enjoyed a good practical joke.
Explanations for the monster offered in the past include it being swimming circus elephants.
In his research of Nessie, Glasgow-based palaeontologist Neil Clark found fairs and circuses were a common occurrence in the Inverness area, particularly from the early 1930s.
He said elephants may have been allowed to swim in the loch while the travelling carnivals stopped to give the animals a rest.
Another theory is that large fallen branches floating in the loch are the cause of monster sightings.
Steve Feltham, who is recognised by the Guinness Book of Records for the longest continuous monster hunting vigil of Loch Ness, is not convinced the scientists have yet identified the creature behind the sightings.
Mr Feltham, who made childhood visits to the Highlands and moved from Dorset almost 30 years ago to look for Nessie, said the research had not ruled out other animals such as seals being mistaken for the monster.
The presence of eels in the the loch was no big surprise, he added.
He added: "A 12-year-old boy could tell you there are eels in Loch Ness. I caught eels in the loch when I was a 12-year-old boy."
Gary Campbell, keeper of a register of Nessie sightings, receives on average 10 reports a year of something unexplained being spotted in the loch's waters.
He welcomed the latest research and hoped more scientists will examine what lives in Loch Ness.
Mr Campbell said tourism that has developed around the story of the monster would be unaffected by the new study. He said: "The Loch Ness Monster has evolved into a world-wide icon."
Chris Taylor, of VisitScotland, said he expected the myth of the monster would continue to bring tourists to the loch.
He said: "This scientific investigation, led by Professor Gemmell, into the inhabitants of one of Scotland's largest lochs has once again shone a spotlight on the Highlands.
"Its findings will provide further insight into what lies beneath but questions still remain, and visitors will, no doubt, continue to be drawn to the loch to seek the answers for themselves."
Loch Ness expert Adrian Shine said the new study had provided researchers with a new list of species to compare against records going back 40 years.