A tweet has gone viral over the last few days claiming that throughout history fishermen have mistaken beluga whales for mermaids because of their... human-like knees.

It may be a windup, though the account generally posts facts from history and science. Whether it's a windup or not, it appears to have been swallowed whole by tens of thousands of people who have retweeted it.

So let's get the obvious out of the way – no, beluga whales do not have human-like knees. They do not have legs.

"If you look at their skeleton they don’t even have legs so how would they have knees. Dolphins are the same way. The only thing close to this are two tiny pelvic bones that don’t really serve any purpose. You should really fact check," one person tweeted.

"To me it looks like that’s a beluga whale that recently had a calf because those swollen looking sections are near the mammary glands. But I’m not a super expert on if that’s what that actually is but I know for sure it’s not knees."

The last time this image of the "beluga mermaid" went viral on Reddit, many people pointed out that from any other angle they nothing like a mermaid. For example, in this video of a beluga whale appearing to thoroughly enjoy being serenaded by a mariachi band, there's nothing about the creature that a sailor could mistake for a part-woman, part-fish.

The "legs" can only be viewed seen from below, which is not an angle you'd usually view them from, and only in very rare photos. They're actually just a coincidence of how the whale's folds of fat have fallen whilst the photo was being taken.

"Belugas that are especially well-endowed with blubber will show visible 'rails' along each side of their belly where extra fat is contained," Travel4Wildlife explain.

A lack of knees, or indeed legs, becomes rather obvious when looking at their skeletons.

Also, we don't want to be a complete killjoy here, but why on Earth would mermaids – which famously have no legs – have knees?