Fishermen once told tales of a giant sea monster, the kraken.

That dragged men from their boats to their death, drowning them at the bottom of the ocean.

Ever since its discovery, there has been considerable speculation as to its maximum size

We now know the kraken is real; the stories referring to species of giant squid.

And while it’s highly unlikely the squid are capable of killing fishermen, they are huge, among the largest invertebrates known.

What we don’t still know, however, is just how big giant squid grow.

Mysterious animals

Giant squid remain one of the most enigmatic large animals on the planet.

Strandings of specimens recognisable as Architeuthis dux, which is thought to be the longest of all squid, date back to 1639 in Europe, but a giant squid was only first photographed alive in its natural environment in 2004.

“Ever since its discovery, there has been considerable speculation as to its maximum size,” says biologist Dr Chris Paxton University of St Andrews, Scotland, UK.

In an attempt to discover just how long they can grow, he has reviewed known A. dux specimens, as part of an analysis published in the Journal of Zoology.

This included historical and anecdotal reports of giant squid seen from boats, remains washed up on shores, and even body parts found within the stomachs of sperm whales, which are thought to hunt and eat the giant cephalopods.

In all, some 460 giant squid specimens have been measured in some way.

Astonishing sightings

The supposed largest squid sighted was an astonishing 53 metres long, spotted off the Maldive islands during World War II from the side of an Admiralty trawler at night by a man called J.D Starkey.

But many of these measurements can be dismissed.

Anecdotal sightings tend to considerably over-estimate the size of squid. For example, according to a new book by author Darren Naish, J.D Starkey claimed the squid he sighted was longer than the boat he was on, and his measurement didn’t include the squid’s two long feeding tentacles, which normally account for more than half the squid’s length.

The longest reliably measured squid had a mantle length of 2.79m

Measuring the whole body of giant squid can also be difficult, as when removed from the water, the soft tissues can become elastic and unnaturally stretch.

So Dr Paxton eliminated unreliable records, and then used a variety of techniques to estimate the maximum size to which giant squid can grow.

These include extrapolating the overall length of squid from the size of certain body parts, such as the beak, a hard part of the squid’s mouth used to bite and eat prey, and the mantle, the large part of the squid’s body that sits just above the head and tentacles.

Biggest specimens

The longest reliably measured squid had a mantle length of 2.79m, Dr Paxton told BBC Earth.

“This was a specimen stranded in the Cook Strait, New Zealand from May 1879,” he said. “But a total length was not recorded. From this we can infer a total length of 14.28m.”

But that doesn’t mean squid reach a maximum length of 14.28m, for two reasons.

The first is that not all squid have bodies of exactly the same proportions.

Two squid of equal mantle length may have tentacles of different lengths, just as people of the same height may have longer and shorter legs than each other.

In squid, this individual variation can be quite large.

The second is that it’s highly unlikely that the largest giant squid that has lived is also one that has been caught and measured.

So bigger squid are likely to be out there.

And according to Dr Paxton’s analysis, they could be very big indeed.

Longest of them all

“Given the variation of individuals within the population, individuals of 2.79m mantle length might vary from 5.83 to 27.53m total length,” he said.

That suggests that giant squid grow to 20m-long and it is “quite plausible” that a few could reach 27m or longer.

The largest squid may be too long to be caught by female sperm whales

“I would stress though as a responsible scientist that this is based on an extrapolation,” explained Dr Paxton. “And as I say to my students ‘extrapolation can be dodgy’”.

But it’s a pretty safe extrapolation, he said, one that, even if other squid experts disagree, he hopes will be tested and vindicated by future discoveries.

The study also leads to one further conclusion.

Underwater struggles

Myths and stories of giant squid are also told of them battling sperm whales, in titanic undersea struggles.

Giant squid remains are found in sperm whale stomachs, so it’s clear that sperm whales do hunt and eat A. dux, as well as other giant squid species such as the shorter but more massive colossal squid (Mesonychoteuthis hamiltoni).

But some squid may grow too big for sperm whales to catch.

Dr Paxton’s analysis suggests the largest squid may be too long to be caught by female sperm whales, which grow to lengths of 11 to 12m.

Only the biggest bull sperm whales, which themselves can grow to 20m, may be able to hunt the most giant squid.

Matt Walker is BBC Earth's editor. He is @byMJWalker on Twitter.

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