Archie the giant squid in its tank alongside curator Jon Ablett

Archie the giant squid arrived at the Museum in 2004 after being caught off the Falkland Islands. Image © NHM London, All Rights Reserved.

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Giant and colossal squid: revealing the secrets of the largest invertebrates

The subject of legends for thousands of years, the largest living invertebrates remain relatively mysterious.

Living deep below the surface of the ocean, scientists are only just beginning to reveal what the lives of giant and colossal squid are like. But just how big are giant squid? And what are the differences between giant squid and colossal squid? 

Living in the darkness of the deep sea, the world's largest cephalopods lurk, waiting for their next meal.

Thought to grow longer than a London bus, the giant squid (Architeuthis dux) and the colossal squid (Mesonychoteuthis hamiltoni) tip the scales with weights of hundreds of kilogrammes. But although their enormous size should make them hard to miss, these cephalopods have continued to remain relatively unknown.

It was not until 2004 that the first photographs of a live giant squid to be seen in its natural habitat were taken, while it would take until 2016 for the first video to emerge. To date, there are still no pictures of living colossal squid in their natural habitat. This lack of observations means that even to this day, the enormous invertebrates still straddle the line between legend and reality. 

Jon Ablett, the Senior Curator in Charge of molluscs at the Museum, says, 'These squid have been part of folklore for thousands of years, with writers such as Aristotle and Pliny writing about them. 

'I think part of the reason why people love the giant squid is because they're real, but there's still a lot of mythology about them. From the Lusca in the Caribbean to the Scylla in ancient Greece, they may be responsible for our legends of sea monsters.'

Scientists are now beginning to catch up to the myths of these animals, and sort the fact from the fiction. 

The Museum's Shell Gallery in 1911 with a giant squid model suspended from the ceiling

The giant squid has fascinated people for a long time, with a model formerly hanging in the Museum's Shell Gallery (now the Jerwood Gallery). Image © NHM London, All Rights Reserved.

What are cephalopods?

Giant and colossal squid are molluscs, a group that includes everything from snails and oysters to squid and octopus. Their name is derived from the Latin for soft, describing the moist and often squishy bodies of these animals.

One group of molluscs are the cephalopods, which include the octopus, squid, cuttlefish and nautilus. While many molluscs have a shell of some form to protect themselves, living cephalopods have generally taken a different approach. Apart from nautiluses, which have an elaborate outer shell, all other cephalopod shells have become internalised.

'If you open up a squid, you might see a hard structure called the pen or a feather, which is the remains of their external shell,' Jon says. 'In octopus, the shell has been reduced so much that it's barely visible.'

Changes to the shell are one of the many ways the cephalopods have evolved very differently to other types of life, combining a mammal-like intelligence which includes tool use with a very adaptable body.

'Cephalopods have a completely bonkers body plan, and in fact the name itself translates as "head foot",' explains Jon. 'This is because their arms and tentacles are connected directly to their head while the rest of their body is behind them. 

'It's like humans having heads in the middle of our hips.'

In addition to the differences between their internal shells, squid and octopuses can be told apart based on their limbs. Octopuses generally have eight arms, which have suckers all the way along them, while squid often have an additional pair of tentacles. These tentacles are smooth, apart from a paddle-shaped structure at the end known as a tentacle club which are covered in suckers. 

In many species of squid the tentacles can shoot out to catch prey, which is then brought back towards the arms that take a firmer grip.

The two groups also differ in their size. Though there are species of octopus and squid that can measure only a few centimetres long when fully grown, others can reach enormous sizes. 

A giant Pacific octopus in an aquarium

The giant Pacific octopus is one of the largest of its kind, but is small compared to giant and colossal squid. Image © Karen Crewe/Shutterstock

What size are the biggest octopus and squid?

There are around 300 species of octopus, with the largest being the giant Pacific octopus (Enteroctopus dofleini) and the seven-armed octopus (Haliphron atlanticus). While the seven-armed octopus is heavier and can tip the scales at over 70 kilogrammes, the giant Pacific octopus is longer at between three and five metres long.

Both are blown out of the water by the colossal and giant squid, which can reach up to six times this length.

'There are a lot of arguments about how big these squid can grow,' Jon says. 'There are historical reports that they can reach as much as 30 metres long, but no specimen survives so this can't be verified. 

'Although we don't know everything that's in the deep, most scientists now believe that female giant squid reach around 13 metres long, and males around 10 metres. 

'In the case of the colossal squid, the jury is out. The biggest one found to date was around nine metres long, but it wasn't fully mature.

'Some scientists believe they don't get as big as giant squid and never reach 13 metres, while others think they could grow even bigger, to 18 metres.'

Uncertainty over the size of the giant cephalopods is in part due to the difficulty in studying them. Until recently, the majority of scientific knowledge was based on carcasses and fragments of the squid either found floating at the ocean's surface or in the stomach of sperm whales, with few intact specimens being found. 

Museum scientists discuss the preparation of Archie the giant squid after it was received at the Museum.

What do we know about giant squid?

Giant squid almost exclusively live in the depths of the ocean between 200-1,000 metres beneath the surface, and perhaps deeper. Shallower than this, it is thought that the sea temperature is too high for them to survive due to the very low oxygen-carrying capacity of the hemocyanin in their blood.

This means they are unable to take the dissolved oxygen out of the water to breathe, and so any individuals found at the surface are probably dead or dying. However, recent efforts have started to illuminate the mysteries of this species in its natural environment.

'The first confirmed report of a giant squid in the wild is from Dr Tsunemi Kubodera in 2004,' Jon explains. 'There had been a few dead and dying individuals seen before then but this was the first to be photographed in its natural habitat.

'He put a baited camera where he thought there could be giant squid and waited for them to take the bait. 

'One individual appeared and attacked, so we have assumed that the giant squid is a solitary hunter. It seems to be quite aggressive, and its streamlined shape had previously suggested it could be a fast-moving predator, but these photos have helped confirm it.'

Six years later, the first confirmed video footage of a giant squid in its natural habitat was taken by a team comprised of Dr Kubodera, Dr Edith Widder and others. They made a jellyfish-like lure which gave off light to attract the squid towards it.

Giant squid are thought to eat deep sea fish and other species of squid, although there are reports from Spain of dying individuals washing up that show signs of being attacked by other giant squid. Whether this is a result of an attempted cannibalism or fighting over prey is not entirely known, although giant squid beaks found in the stomachs of other giant squid suggest it is the former

Despite slowly increasing our knowledge of the giant squid, the colossal squid remains elusive. While the giant squid is found all over the world (although rare in the tropics and polar regions), colossal squid live solely in Antarctic waters, which makes it difficult to find and film them. 

A group of people surround the carcass of a colossal squid on a beach

The colossal squid has, to date, never been pictured alive in its natural habitat. Image © Throast, licensed under CC BY-SA 3.0 via Wikimedia Commons

What is the difference between giant and colossal squid?

Though the giant and colossal squid are both large cephalopods, they are not closely related and belong to different families of squid. 

Previously, up to 21 separate species of giant squid had been described, but genetic evidence taken from specimens including the Museum's Archie has suggested there is just one. This is also likely true for the colossal squid. 

'Lots of things in cold water grow slowly and become large through a process known as polar gigantism,' Jon says. 'It's not entirely certain why, although it may relate to the constantly cold temperatures and high amount of oxygen available for their metabolism.

'Colossal squid have a huge body but relatively short arms and tentacles, which means their body plan is in some ways opposite to a giant squid. It could mean these animals hang in darkness and wait for prey to pass before striking, adapting to the lower amounts of food in the deep Southern Ocean.' 

A close up of the head of Archie the giant squid

Specimens such as Archie continue to help scientists to answer questions about these enormous invertebrates today. Image © NHM London, All Rights Reserved.

Why are giant and colossal squid so big?

Getting big not only increases the range of prey available to eat, but also limits the number of animals which can eat them. This is particularly important when the squid are likely born at a few centimetres long.

'None of these animals start off at 13 meters long,' Jon says. 'Until they get massive, there are lots of things which eat them and their beaks are found in the stomachs of seabirds and ocean-going fish.'

Once they reach their full size, the only animal likely large enough to take them on is a sperm whale. Some remains have also been found in the stomachs of sleeper sharks, but these fish are known to be scavengers and so may feed on dead individuals.

This growth probably takes place over many years, unlike other squid which mature more rapidly.

'Most squid live fast and die young,' Jon says, but in the deep water, we think that life slows down. 

'Inside cephalopods, there is a structure known as a statocyst which tells them their 3D position in the water, and studies in cuttlefish and other cephalopods have shown that the statocyst gains a ring for every day they are alive. 

'However, we don't know if that corresponds to deep water species and it's likely it doesn't. That's not too surprising, as it's hard to imagine something 13 meters long growing in a year or two.'

While there have been suggestions that the giant squid lives anywhere between two and 12 years, there is no clear consensus on how long these majestic monsters of the deep can live for. It is another mystery that remains to be solved about these elusive animals.

'Questions like these are one of the reasons I love my job,' Jon says. 'There is still so much about the natural world we haven't solved.

'Hopefully, someone will come along soon and resolve this question. I look forward to finding out the answer!'