What is a Colossal Squid?

The colossal squid is also known by its Greek name Mesonychoteuthis hamiltoni derived from mesos (middle), onycho (claw or nail), and teuthis (squid); 'Hamiltoni' is from the name of the person who discovered it. Mesonychoteuthis hamiltoni is believed to be the largest in body mass amongst all squid and is the largest known invertebrate.

Everything that's known about Mesonychoteuthis hamiltoni has been discovered from only a few specimens and studies and even fewer sightings. It measures 12-14 metres in length, including outstretched arms and tentacles, and can weigh up to 750 kilograms. The information on its maximum size has been based on marine biologists finding undigested squid beak in the stomachs of their main predator, the sperm whale. Data was then configured based on specimens that have been caught and studied to calculate these potential measurements.

Mesonychoteuthis hamiltoni (a.k.a. Colossal Squid) is a member of the cephalopod family, belonging to the class of Cephalopoda that evolved from the earliest known mollusks and their lengthy evolutionary history spans an incredible 500 million years. It's thought that the earliest forms of squid are at least 60 million years old and that they've survived through challenging and changing environments. Through fossil identification tracked by marine biologists, it's understood that the squid species generally doesn't appear to have changed much as it's evolved.

There are approximately 800 known existing species of cephalopods, although biologists estimate that there were at one time 10,000 or more, judging by the fossil record. The cephalopod group includes the octopus, cuttlefish, nautilus and the squid, all of which share certain characteristics, such as all having a crown of arms, a beak and a head attached to a sac-like body containing the organs. That's about as far as the similarities go, though, and the different strands of the cephalopod family aren't thought to be any more closely related than cats are to dogs. The evolutionary distinctions are thought to have happened in very much a similar way that birds and mammals split into distinctive groups.

The Cephalopoda divides into a subclass known as Coleoidea and an order called Teuthida. This order has a family known as Cranchiidae, which has a further 80 species including the Coleoida, or transparent 'glass squid', the group to which Mesonychoteuthis hamiltoni belongs. The giant squid, which belongs to the genus Architeuthis and isn't to be confused with Mesonychoteuthis hamiltoni, is another example of a glass squid, although it's different in many ways to Mesonychoteuthis hamiltoni, which only lives in the Antarctic.

'Cephalopod' literally translates as 'head-foot' from Greek and the name comes from the fact that the cephalopod's visceral mass (the soft, non-muscular metabolic area containing the body organs) has been stretched along the dorsoventral axis (the line through the centre of the body) above the foot, bringing the head and foot closer together on the ventral (stomach/front) side. So, the proper name Cephalopoda reflects the proximity of the head and foot.

Cephalopods are characterised by a number of special features:

Huge squid are legendary in folklore and in history. Until the late 1800s, little was known about the oceans in that most scientists thought the cold darkness would make the deep sea uninhabitable and research at that time concluded that life could not exist below about 500 metres.

During the 300 years from the 16th to the 19th centuries, the heyday for new-world discovery and colonisation, there was popular belief that the oceans harboured enormous sea creatures that were threatening to man. These ranged from alleged dragon-like creatures to huge squids such as the mythical Kraken, rumoured to dwell off the coasts of Norway and Greenland.

Mesonychoteuthis hamiltoni was discovered in the early 20th century, when a couple of tentacles were found inside a sperm whale. This ended all speculation about the existence of this ocean giant, which had not previously been firmly established as fact.

Global Regions and Habitat of the Colossal Squid

Mesonychoteuthis hamiltoni lives in the Antarctic regions of the Southern Ocean at depths of around 1,000 metres or more. Their total global habitat extends over many thousands of square kilometres from the Antarctic to South America and South Africa, as well as the southerly tip of New Zealand. The deep ocean that's their home is a mysterious and inhospitable world that takes up 95% of the living space on earth, yet more men have travelled into space than have explored the deepest depths of the ocean. It's the largest wilderness on the planet and the least explored.

By the 1960s, it was becoming more and more apparent to marine biologists that the oceans were teeming with life. In this inhospitable, murky deep-sea environment, the physical properties would seem to make it nearly impossible to support life because temperatures can be as low as freezing and underwater pressure a crushing 8 tons per square inch, which is about 1,000 times the standard atmospheric pressure at sea level. At just 650 feet below the surface, photosynthesis (the process where plants and some bacteria take up light for growth) ceases and light fades rapidly. This makes it harder for creatures to survive due to scarcity of food sources. At greater depths, the temperature drops even further and the pressure intensifies so that, at depths of 13,000 feet or more, the temperature registers below freezing and light is completely absent.

An astonishing variety of creatures have adapted to life in these deep regions of the ocean. Using sophisticated deep-sea submersibles, marine biologists have begun to explore this vast, icy cold, pitch black and crushing deep-sea environment and have discovered some amazing new phenomena, including ecosystems that thrive at very deep levels thanks to the presence of hydrothermal vents, which have a remarkable purpose in the habitat and survival of Mesonychoteuthis hamiltoni.

Hydrothermal vents occur in geologically active regions at the very foot of the ocean, where seawater penetrates down into the Earth's crust through cracks in the ocean bed. The water is heated to boiling point by magma below the surface, where it then expands and rises back up to the surface. On its journey back up via the same route by which it found its way in, the boiling water dissolves chemicals and minerals from the rock and ends up as a dark, chemical cocktail when it reaches the ocean floor again. Some of the minerals begin to harden on the rim of the vent and gradually that rim builds into a chimney-like structure, with dark water spouting out and this has given rise to the name 'black smokers'.

Although the spouting water is exceptionally hot, the biggest surprise to marine biologists was the discovery of an array of life forms, from crabs to snails, sea stars, barnacles, sea anemone, and even octopus, all thriving in the regions around these vents. These creatures derive energy not from sunlight but by the breaking down of the chemicals expelled as part of the spouting water process. This is very important for the life and survival of Mesonychoteuthis hamiltoni because the sea life sustained by these hydrothermal vents is all part of their long food chain and means that life at great depths of the ocean is very possible.

There's a threat, however, to the habitat of Mesonychoteuthis hamiltoni and all ocean life in general, from something known as 'ocean acidification'. When carbon dioxide is released into the atmosphere from man's burning of fossil fuels, it causes the ocean's pH and carbonate levels to drop and, because much marine life relies on good carbonate levels in the water, a lack of it makes it much harder to live, feed and breed successfully. The negative impact on marine life is the consequence of living in a world with high CO2 levels and this phenomenon has been entitled "the evil twin of climate change" due to its likely consequences to the survival of many species. Since southern oceans are cooler, they absorb carbon dioxide at a faster rate than warmer oceans and the effects of acidification will be felt in this region of the ocean first, with potential detrimental effects on the survival of Mesonychoteuthis hamiltoni.

Colossal Squid Anatomy

Many of the creatures dwelling in the deepest regions of the oceans appear very alien to us because they've evolved to adapt to the harsh conditions. Take Mesonychoteuthis hamiltoni as a classic example, with huge eyes equipped with light organs to help it see in the dark, a ferocious beak, arms and deadly suckered and hooked tentacles, optimised to capture whatever food comes its way. It also has techniques to conceal itself so it blends into the environment to aid its survival. Add to this other incredible phenomena such as a transparent body and use of bioluminescence to communicate and it's easy to see why the colossal squid is so fascinating.

Although there's similarities between all species of squid, Mesonychoteuthis hamiltoni stands apart from the rest in many ways. In comparison to its near cousin, the giant squid Architeuthis which can measure up to 18 metres long, a mature adult colossal squid is shorter but more bulky in mass. The length difference lies largely in a different tentacle size, with those of the colossal squid being significantly shorter but equipped with swivelling hooks unparalleled in any other species.

Mesonychoteuthis hamiltoni was first discovered in 1925 when two tentacles were found after having been eaten by a sperm whale. Then, in 1981, a Russian trawler fishing off the coast of Antarctica in the Ross Sea caught a large specimen with a total length of 4 metres and was subsequently found to be an immature Mesonychoteuthis hamiltoni female. In 2003, another whole, but immature, female specimen was found near the water's surface. It was 6 metres long in total and this proved to biologists that Mesonychoteuthis hamiltoni could grow to be even heavier than 500 kilograms and possibly up to 750 kilograms.

In 2005, a specimen was captured at a depth of 1,625 metres, whilst feeding on a toothfish, on a long line off South Georgia Island in the Southern Atlantic Ocean. Although the mantle (main body) was not brought aboard, the mantle length was estimated at over 2.5 metres and the tentacles measured 2.3 metres. The creature is thought to have weighed between 150 and 200 kilograms.

The largest specimen of Mesonychoteuthis hamiltoni that was captured was another female caught early in 2007 by fishermen on a toothfish expedition in the Antarctic waters of the Ross Sea, on a ship owned by the Sanford Seafood Company. To date, this is the largest cephalopod ever recorded and it was alive during capture. It weighed 495 kilograms and was initially estimated to measure 10 metres in length, far bigger than the previous largest specimen from 2003 and exceeding it in weight by about 195 kilograms.

The greatest potential size, based on the evidence of colossal squid beak found inside sperm whales, predicts even larger specimens still. Each squid has a beak that's unique in shape and size and is relative to the size of the living creature. Scientists have used a formula to work out how big Mesonychoteuthis hamiltoni can become, based on the measurement of the length of the lower beak's straight cutting section, known as the lower rostral length, or 'LRL'. Large numbers of beaks from colossal squid found undigested inside sperm whales have been gathered and measured, with the largest one possessing an LRL of 49 millimetres.

Proof that the beak size can be used to determine the approximate overall size of Mesonychoteuthis hamiltoni was concluded during the examination of a specimen at the marine biology unit of the New Zealand National Museum, Te Papa in Tongarewa, when a smaller colossal squid was dissected. The weight of this was about 160 kilograms with a beak LRL of 40 millimetres whereas the larger specimen weighed 495 kilograms and its beak was measured at 42.5 millimetres. It was, therefore, deduced that just a few millimetres difference in beak size corresponded to a much larger size of squid.

Unfortunately, there haven't been enough whole colossal squid specimens to prove the equation that links beak size to the overall size of the creature. Whilst it's not certain what a beak LRL of 49 millimetres would produce in terms of sheer body size, the best estimates based on these scientific calculations lead to the assumption that a colossal squid weighing as much as 700-750 kilograms could exist.

The 2007 specimen was frozen and taken to the Te Papa Museum, where a specialist team of marine biologists examined it. After being thawed out in a bath of salt water, it was re-measured coming out at a disappointing 4.2 metres in total length, and marine biologists concluded that the tentacles had shrunk significantly as a result of the freezing process. After being thoroughly examined with some body parts carefully dissected, it was put on display in the museum and thousands of visitors flocked to see it.

In August 2014, Te Papa received a second colossal squid, captured by the same Sanford fishing vessel fishing in the same area as when they caught the first one. This squid was also female, 3.5 metres long and weighing in at approximately 350 kilograms. Just like the previous specimen, it was also frozen for a period of time before being thawed for inspection and partial dissection. The biologists at Te Papa realised there was a fine line between preserving the specimen for public viewing and using it to further scientific research into this little-known creature. They decided to show the inspection and dissection through a live stream on YouTube on the 16th September 2014. Over 380,000 people tuned in to watch this exceptional footage, which showed close-up examination of various parts of the squid including the arms and tentacles, the incredible suckers, the swivelling hooks, the beak and the eyes.

Like all other squid, Mesonychoteuthis hamiltoni has 2 tentacles, 8 arms, and a paired tail fin that's attached to its tube-shaped body. The body contains the digestive and reproductive organs and is attached to the head, which has eyes, a mouth/beak and a brain, all of which are surrounded by a crown of arms and tentacles. Mesonychoteuthis hamiltoni is thought to be tiny when first born and it's believed to grow exponentially in the first few years of life, although this is only conjecture and has never been observed.

Just how and why they become so colossal is subject to several theories. Their vast size, a phenomenon known as 'deep sea gigantism', is specific to deep-sea creatures and occurs when invertebrate and deep-sea dwelling species are very much larger in size than their relatives that inhabit shallower waters. Marine biologists aren't sure but think that deep-sea gigantism arose in the evolutionary process as a result of the need to adapt due to scarcer food supplies but, to some, this may be illogical as surely a creature starved of food would be small. It would seem, though, that food scarcity results in delayed sexual maturity and all the energy of the living organism goes into the growth that results in a very large body size. Another possibility explaining deep-sea gigantism could be the greater water pressure which, at anything up to 100 bars, is an environment where a creature would need a large body mass to withstand the huge forces.

There's also a tenuous link to deep-sea gigantism and a theory known as 'Bergmann's rule', after the 1847 theory of German biologist Carl Bergmann. His theory states that the increase in size with depth occurs for the same reasons as the increase in size with geographical latitude whereby populations living in more northerly and colder regions are larger than those living in more southerly and warmer regions. Bergmann based his principle on factors relating to both ecological and geographical conditions and his theory is applied to body size as a whole, as well as to size at cellular level. It's thought that increased cell size found in creatures in colder regions leads to a larger body size and a longer life span. This theory would hold true if applied to the huge size of Mesonychoteuthis hamiltoni.

The mantle, skin, paired tail fin and gladius/pen

Measuring about 2.5 metres long, the mantle is the main body/torso of Mesonychoteuthis hamiltoni. It consists of skin and muscle mass and is designed to protect the internal organs such as the stomach, intestines, the 3 hearts and 2 lungs. The funnel and rectum are also found in the mantle and a pair of renal organs open into the mantle cavity by means of a renal pore.

The squid's top layer of skin is gelatinous and delicate, tearing easily and has a reddish/pinkish colour from small pigmented cells called chromatophores, which look like tiny red dots. These pigment-containing cells have the ability to contract or expand under the control of the squid's central nervous system, signalling a change in skin colour, usually to confuse predators or if under stress. This was observed when the latest specimen was hauled aboard the fishing vessel in 2014. The skin at the time of capture was observed to be quite a deep red because of the stress of it being caught up in the fishing lines and nets and also because it was being counter-attacked by the toothfish it was attempting to feed on!

The skin as a whole is thick and consists of 2 layers, the outer layer and then the inner layer that covers the muscles. The inner layer is the thickest, being a minimum of 2 centimetres in places and contains large chromatophores. The outer skin has smaller chromatophores is thinner and so more delicate.

All squid possess an ink sac contained inside their mantle. The ink sac sits beside the rectum and discharges via its own duct just behind the egg sac in the female and the anus. The ink sac enables the squid to rapidly discharge blueish-black 'ink' into the mantle cavity, from where it can be expelled into the water. Squid ink, which contains a dark pigment known as melanin and which is combined with thick mucus and amino acids, is expelled via the funnel if the squid encounters a predator and this inky cloud effectively provides camouflage so it can attempt to flee before it's attacked.

Observation of a colossal squid producing ink in its natural environment has never taken place, so it's not possible to ascertain with 100% accuracy what the ink looks like in the water or how the squid uses it but, based on studies of other squid, parallels can be drawn. Since there's no light in the depths of the ocean, dark ink would be pointless so it's thought that the colossal squid's ink might possess luminescent properties, but this hasn't been confirmed.

Mesonychoteuthis hamiltoni can move around at varying speeds and one of its methods is to cruise along slowly using its paired tail fins at the rear of the mantle. These tail fins, made of muscle, have a big surface area of just over 1 metre long and just under 1 metre wide and attach to the upper part of the mantle. The tail fins are much larger and more muscular in comparison to the rest of its body and also in comparison to other squid species. The colossal squid can also use its tail fins to hover in the water as well as to change direction when on the move to help it accelerate in quick bursts, essential for hunting or escaping from predators.

Another method of movement the colossal squid can deploy is to accelerate from an almost stationary position with a great burst of speed by using the muscles in its mantle to launch itself through the water. This is, however, extremely energy sapping and can only be maintained for short bursts, which is usually just enough time for the fight or flight process to take place successfully.

For the most part, Mesonychoteuthis hamiltoni gracefully combines the movement of its tail fin with smooth and slow rhythmical water pulses expelled through the mantle cavity via the funnel (or siphon as it's sometimes called) and which is situated on the surface of the mantle. This is how the colossal squid breathes and the precise action takes place as it expands the mantle cavity through the contraction of individual sets of muscles within it, thus allowing water to fill the space. A different set of muscles is then used to contract and expel the water back out through the funnel so that the mantle returns to its original state and the process repeats.

Although the mantle as a whole is very thick, it has elastic properties allowing it to contract and expand thanks to the strong muscles that compress the mantle mass and this process is greatly speeded up to allow the colossal squid to turn on bursts of speed as and when required. The mantle works a bit like a giant syringe drawing in water whereby the flaps on either side of the head close so water can escape only through the funnel as the water is drawn in and so oxygenating the gills. The colossal squid has 2 very large gills that hang down into the mantle, each possessing from 20 to 80 filaments on each side. They're striped with lines of dark pigment and this has been seen when the fins were dissected after being removed from specimens.

The gladius or pen is the hard internal body part of the squid and is the remnants of its ancestral shell. Situated on the inside of the mantle surface, the gladius/pen runs through the upper part of the mantle between the paired tail fin and forms a protective concave structure around the soft body parts underneath it. This rigid internal shell also supports the body and is made of 'chitin', the same substance that's found in the squid's beak and hooks. Tough, protective, semi-transparent and a naturally occurring polymer, chitin is one the most abundant natural materials in the world. It's biodegradable but not easily digestible, which is why the beaks of colossal squid have been found inside sperm whales after having been eaten.

The eyes

The eyes of the colossal squid are believed to be the most highly developed and the most remarkable in the animal kingdom and are probably the largest, measuring about 27 centimetres across and being about the size of a football. Unlike any other type of squid, the eyes of the colossal squid are forward facing and have binocular vision. Their eyes are a little like the human eye with a lens and an iris that filters light back to the retina. The lens is in 2 pieces and has optic nerves to relay visual information back to the optic lobe in the brain.

What makes the eyes of cephalopods so remarkable is the position of the optic nerves. The eyes of vertebrates, such as human eyes, have optic nerves that block some of the photoreceptors detecting light, creating what we know as a blind spot. Cephalopods don't have this blind spot at all because their eyes are arranged in such as way that the nerves never block the light. This is a clever evolutionary ploy in a world that's visually challenging at greater depths. The cells in the colossal squid's eyes are most sensitive in the retina and concentrated in the lower area so that its eyesight is most acute when it's looking upwards. It's thought to probably cruise along in the water or hang motionless, searching for the silhouette of its prey from below, before attacking from out of the darkness with a short burst of speed.

Although they've extremely good vision, cephalopods are believed to be colour blind. Humans can see in colour thanks to the various kinds of receptors in our eyes, known as cone cells, which are photoreceptor cells. Squid don't have these cone cells, so it's believed they can't see colour.

Perhaps the most amazing feature of all is that the colossal squid's eyes contain what's known as 'light organs' in a vertical line on the rear of their eyeballs. These shine through bioluminescence and illuminate the depths of the ocean.

Bioluminescence is a fascinating and visually stunning process whereby light is produced by a living organism through a chemical reaction taking place inside a living creature. It's very common in marine animals that have bioluminescent light organs to contain cells known as 'photophores' in various parts of their bodies.

The colossal squid's bioluminescent light organs in its eyes act like headlamps in the pitch-black ocean. It's thought that this bioluminescent light can be used in short bursts of light flashes for hunting and these flashes can blind and confuse prey. The light can also be used as a way to estimate distances by illuminating objects in the water at various points. Additionally, the light could be used in communication with other squid, possibly for mating. It's a known fact that each species of squid has its own distinct pattern of lights so they can recognise their own species. It could also have the advantage of being able to frighten and confuse predators, much in the same way as it's used to lure prey. All these theories are, however, speculative as a colossal squid hasn't been directly observed using bioluminescence.

Bioluminescence is common amongst all species of squid, with an estimated two-thirds having bioluminescent light organs that can be found almost anywhere on the body, the most common places being the eyes, mantle, head and arms, internal organs, funnel, and tentacles. Biologists aren't sure if the colossal squid has light organs in all of these places but the eyes are a certainty.

Bioluminescent patterns generated by the eyes have been observed in a live giant squid, Architeuthis, in its natural habitat just off the Ogasawara Islands in 2006. This observation showed some distinct patterns in that there was a long glow when approaching the target/prey of approximately 4.4 to 8.5 seconds and several short emissions of light separated by intervals as the giant squid prowled around its bait, observing without attacking. These light flashes are very interesting to biologists as an insight into the ways in which squid communicate through bioluminescence and it's thought that the colossal squid might do something similar.

Bioluminescence takes place through a specific chemical reaction within light-emitting cells (photophores), triggering the generation of light. To understand how this works at cellular level, each photophore contains a lens that comprises modified muscle cells to focus the light beam and a reflector is used to intensify the light. This reflector is made from stacks of very thin plates called lamellae, spaced at a small distance apart from one another to reflect and diffract the light in such a way as to intensify it. The central chamber contains the light-emitting tissue, which is made up of canals and pockets of tissue teeming with special bacteria, which play a critical role in the generation of light.

To understand the role of bacteria, it's necessary to understand the chemical reaction that causes bioluminescence to take place. This involves two unique chemicals - luciferin and either luciferase or photoprotein. Luciferin is a light-producing compound and, in the chemical reaction that takes place, is called a substrate. All bioluminescent colour results from a particular arrangement of luciferin molecules. Some bioluminescent organisms produce luciferin on their own but some don't, instead absorbing it through other bacterial organisms either as food or in what's known as a 'symbiotic relationship'.

The colossal squid synthesises luciferin through a symbiotic relationship with bacteria known as Vibrio fischeri. It houses these bioluminescent bacteria in the light organs in its eyes so providing it, the bacteria, with a home and benefits from this by being able to reflect the luminescent quality in the bacteria to its advantage. In return, the bacteria benefits by being able to live in an environment that's nutrient-rich and free from any competition from other bacteria.

Bioluminescence produces what's known as a 'cold' light, which means that less than 20% of the light generates any heat or thermal radiation. Most bioluminescent organisms live in the ocean but some, including fireflies, live on land, although they're almost completely absent from freshwater habitats.

In a world that's dark and murky and taking into account the optical properties of water, which restrict how far away things can be seen, the colossal squid's shining eyes not only act like 2 strong headlamps but also, in a clever evolutionary trick, can mask their own presence. This is achieved by the light organs working together to create a cloaking or masking feature, so that its eyes don't give the colossal squid away if a predator approaches. The rows of light organs in the eyes match the beam of light emitted from up above and, in reality, this makes not only their eyes but also their silhouette in the water perfectly invisible. Exactly how the bioluminescent light is controlled by the colossal squid and turned on or off isn't known but it could take place under the control of the complex central nervous system.

Marine biologists believe that the colossal squid's eyes have little or no gain for close-up vision and the purpose of their large pupil and giant retina is to give them an advantage over and above any other creature. Their eyes can collect light from other bioluminescent creatures, such as sperm whales, from up to 120 metres away and from a depth of 500 metres and deeper. This allows the colossal squid plenty of time to escape using its high-powered sprint speed. The estimated distance at which colossal squid can spot predators and food sources suggests that it's king over a vast region of the ocean and can look for movement covering a very wide radius.

One point worth noting is that Mesonychoteuthis hamiltoni has no power of hearing so its eyes make up for this sensory deficit. Like any giant organ, however, the eyes use up a lot of energy and it's thought that the majority of its energy and brain capacity are devoted to servicing this most critical of senses.

The fact that Mesonychoteuthis hamiltoni grow to be so huge doesn't necessarily explain its need for huge eyes. The swordfish, for example, is also a large creature but, relatively speaking, its eyes are not as big and, proportionally, the colossal squid's eyes are much bigger, being 3 times larger in diameter and 27 times greater in volume.

Marine biologists have come up with a theory to justify the size of such huge eyes. Their scientific calculations show that, after about 90 millimetres in diameter, the visual advantage from very large eyes doesn't justify the energy demands required to support their size and weight. Mathematical calculations show the optimal diameter for the eye of a deep-sea creature to be from 9-30 centimetres. Any smaller would mean the failure to spot other bioluminescent creatures and any larger would mean diminishing returns not worth the energy expenditure it takes to develop such big eyes and carry them around. At 27 centimetres in diameter, the eyes of the colossal squid are at the upper end of the perfect size for optimal efficiency.

Through studies, biologists have been able to conclude that colossal squid use their eyes for a purpose not shared by any other animal. They don't just have the ability to see all objects more clearly and at a greater distance but specifically creatures approaching in the water that are also very large. The extra large eyes are, therefore, a necessary survival tool to pick out gigantic creatures that might have a greater possibility of being a threat, a potential food source, or even a mating opportunity.

The beak

Like all other squid and the octopus, the colossal squid has a beak and is the largest, bigger than the giant squid's beak, and possibly more robust and strong. The beak is essentially the mouth of the squid and plays the part of being the first stage of the digestive system.

This hard chitinous structure is in 2 parts, the upper and the lower, and is rather like a parrot's beak, except the lower beak of the squid overlaps the upper beak. It's surrounded by muscular tissue that's firmly rooted in the head to give it a firm anchor to aid the process of crunching up the biggest and toughest of prey. The beak slices and chops food before passing it down through the oesophagus into the stomach for digestion and must make a good job of this because the oesophagus is narrow and has to pass through the middle of the brain, which is especially doughnut-shaped to allow for this.

Food is shredded in the beak with the help of a rasped radula (tongue), which is rather like a teeth/tongue combination due to its extreme sharpness, with further palatine teeth lining the cheeks (these are called palatine palps). The radula moves a bit like a conveyor belt, pushing the food down through the oesophagus once sufficiently processed.

The arms and tentacles

The colossal squid, like all squid, has 8 arms and 2 tentacles. Bizarrely though, measurements taken from specimens show that each arm differs in length slightly at around the 1 metre mark. The arms on the specimen captured in 2014 were just over 1 metre and the tentacles slightly more than 2.5 metres but, unfortunately, these were not completely intact as damage was incurred when the colossal squid was hauled ashore, possibly as a result of combat with a toothfish or entanglement in the fishing line.

The ends of each of the arms are modified with a membrane, which is particularly thick around the hooks, to give the squid protection from its own hooks that could otherwise catch its body and tear the fragile skin. The membrane also folds over the suckers and gives them side support.

One of the most terrifying and yet, at the same time, incredible features of the colossal squid is its swivelling hooks. These hooks on the arms differ from those on the tentacles, as the arm hooks can't swivel whereas the tentacle hooks can swivel and do so with vicious capability. The tentacles are longer than the arms, up to approximately 2.5 metres long and club shaped at the ends with two separate rows of hooks that swivel. Each tentacle hook is positioned on the end of a short stalk, which allows the hooks to rotate and which sits flush with the surface of the tentacle club. The underside of the hook rotates through a whole 360 degrees but it's not known if the hooks are controlled individually by the central nervous system once the squid's latched onto its prey or whether they operate collectively.

The middle part of the tentacle club has 2 rows of swivelling hooks, with a total of 22-25. These are smaller than the arm hooks and have only one main 'claw'. A row of marginal tiny suckers flanks each row of hooks.

The tentacles also have pairs of suckers and pads set diagonally into the distral two thirds of the tentacle stalk (i.e. the two thirds that are furthest away from the body). There are also smaller marginal suckers with corresponding bumps along the opposite side that act like press-studs, allowing the tentacles to fit together so they can function as one long snapping claw, very useful in attacking prey. Whilst other families of squid feature hooks on their arms, tentacles, or both, the colossal squid is the only species of squid in its family of approximately 20 species (the Cranchiidae) that possesses this deadly, horrifying yet fascinating feature.

The hooks on the arms are bigger on the ventral (underside) arms compared to those on the dorsal arms (the arms located on the same side as the back). This is an important feature as the ventral arms are closer to the beak and, therefore, more important for manipulating food towards the feeding zone.

The arm hooks sit in a double row running down the centre of each arm and are embedded within muscular, fleshy sheaths that are firmly attached so they can effectively hold and immobilise struggling prey. Sharp lethal serrated suckers with a calcareous inner structure sit both above and below the hooks.

Most of the arm hooks feature a strong main hook or claw, with 2 smaller claws closer to the base. This maximises the colossal squid's opportunity to really dig into its prey and firmly hold onto it to deny all means of escape.

The circulatory and respiratory system

The colossal squid has 3 hearts, 2 of which supplement the action of the main systemic heart. Unlike other types of mollusks, the cephalopod circulatory system is what's known as a 'closed system' where the blood supply is closed off at all times within blood vessels of various sizes and differing thicknesses. In a closed system, blood is pumped by the hearts through blood vessels but doesn't fill any of the body cavities. The 3 hearts of the squid are pale green in colour, 2 of which are used to feed the gills and are known as 'paired branchial hearts'. They're located at the base of the gills and make up only about 0.5% of the colossal squid's body weight. The branchial hearts collect the blood from the body by means of the single, large vein known as the anterior vena cava, a large blood vessel running underneath the gills towards the siphon and which splits into a left and a right 'precava'. Paired posterior vena cava, the large blood vessels that run the length of the colossal squid's back, also drain into the branchial hearts together with the left and right mantle veins.

Each individual branchial heart circulates blood to the gills on the corresponding side of the body, pumping the blood through the afferent branchial artery, where it crosses the gills and becomes oxygenated. The blood then leaves through branchial veins connecting to the single systemic heart to be pumped around the body. The systemic heart consists of 3 sections, one lower ventricle chamber and 2 upper auricles. The systemic heart, triangular in shape, is larger than the two branchial hearts and sits between them.

The colossal squid's blood contains a protein called hemocyanin, commonly found in mollusks and also some invertebrates, which is rich in copper and aids the transportation of oxygen around the body in a similar way that hemoglobin transports oxygen to cells in mammals. As the blood circulates through the squid, it's clear in colour but, when exposed to air, it turns to a shade of blue. The best-known arthropod with copper based blood is the horseshoe crab and a number of other arthropods also have blue blood. This comes from an evolutionary process where hemocyanin functions better than hemoglobin in transporting oxygen through the body in the cold, oxygen depleted depths of the ocean.

The brain

The brain of the colossal squid is a mass of soft nerve tissue arranged in a doughnut shape that surrounds the oesophagus, which makes it extraordinary because every meal devoured has to travel through the middle of its brain!

Divided into 3 parts, the colossal squid brain has 2 optic lobes and a central ganglion which, as its name implies, lies in the central region of the head. This is a dense cluster of neurons that are interconnected to process sensory information to control motor output. The optic lobes are situated behind the eye sockets, are a yellowish-white colour and have a soft and fleshy texture.

While cephalopods such as the octopus are believed to be very intelligent and have been studied in their natural habitat as well as in captivity, the colossal squid's intelligence is unknown and unmeasured due to a distinct lack of study opportunity. It's easy to study the octopus since they live in dens on the bed of the ocean and spend time manipulating shells and rocks in search of food. Octopuses have even been given complex puzzles to solve and their intelligence has been measured in various studies. The domain of Mesonychoteuthis hamiltoni is, however, far-reaching, extremely deep and largely inaccessible to man, so observing them either in their natural habitat or in captivity is, as yet, an unconquered challenge.

The doughnut-shaped brain of the colossal squid weighs comparatively very little against its overall body weight and is contained inside a head capsule dominated by the large optic lobes. Its brain is wired with a central nervous system that's far more complex than most invertebrates and mollusks, although it's believed that almost all their brainpower is given over to processing visual information.

Colossal squid are able to determine their position in the water and which way up they're facing by means of 2 'statoliths', which are little bones that work like an ear. These small calcerous structures are embedded within a chamber known as a statocyst and act as the balancing or equilibrium organs of the brain. Statoliths function by moving around within the statocyst chamber as the colossal squid moves or accelerates in the water and use gravity to help the brain interpret direction, either up or down, in the darkness.

Statoliths are very interesting to marine biologists and are often referred to as the 'black box' of squid because, when they've been studied in other squid species, they've been found to record a lot of information about squid life. It's possible, for example, to reveal key details such as the estimated age and growth rates with daily precision, as well as possible migratory routes and population structure. This is achieved using what's known as a 'trace element analysis', which reveals fascinating facts such as how many spawning events a given animal has had. Statoliths are one of the best traces in fossil records and their features can be used to gather evidence about the life spans of extinct species and work on theories for current species.

Sadly, the decrease of carbonate levels and the rise in ocean acidification due to global warming could be having a negative impact on the colossal squid's statoliths, as calcium carbonate is one of the minerals present in their composition and the balance is very delicate.

The digestive system and diet

Mesonychoteuthis hamiltoni is believed to feed on prey such as Chaetognatha, which are more commonly known as arrow worms and which are a type of predatory marine worm that make up a large part of the world's ocean plankton. They also eat large fish like the Patagonian toothfish and possibly prey on other smaller squid.

New research suggests, however, that Mesonychoteuthis hamiltoni may not be the savage hunter that we've traditionally considered it to be. This new view comes from data analysis by the Rhode Island University, who looked at the similarities between metabolism (how the body converts food into energy) and the size of the body in smaller squid of the same family. They scaled up this information and used it to gain information about the metabolism of the colossal squid and their findings were somewhat surprising, indicating that this creature probably has a slower metabolism than expected, despite its size. This theory tells us a lot about the hunting habits of the colossal squid, which probably moves more slowly than expected, waiting to ambush its prey rather than chasing it down. It's now believed that the colossal squid is a mostly sedentary creature rather than an aggressive predator, consuming less rather than more and also expending less energy.

The digestive system of the colossal squid, like that of all cephalopods, is complex. The muscular stomach sits roughly in the middle of the body. From the beak, the 'bolus' (chewed up food) passes through the buccal bulb (the mouth structure), through the oesophagus and into the caecum, a pouch-like long, white organ connected to the intestines where the food gets digested. The caecum sits next to the ovary in the female or the testis in a male and connects to the liver, which is located at the siphon or funnel, after passing through the pancreas; liver and pancreatic enzymes help the colossal squid with digestion. Its intestine is connected to the stomach near the entrance of the oesophagus and the u-shaped pancreas is located underneath its heart.

The colossal squid excretes solid waste materials through its rectum. The excretory process takes place as waste passes into the intestine and is then emptied into the rectum for excretion via the anus into the funnel, from where the waste is expelled. In other species of mature squid, biologists have studied that their body prioritises reproduction over digestion to such an extent that the stomach and caecum can greatly diminish as the squid ages and almost shrivel up to nothing.

Behaviour and Life Cycle of the Colossal Squid

Intelligence and communication

Although the behaviour, intelligence and communication of Mesonychoteuthis hamiltoni haven't been directly observed, biologists can look to studies of cephalopods and other squid to speculate on its behaviour.

Cephalopod intelligence is influenced by the fact that much of their evolution has been driven by the need to avoid predators. The earliest cephalopods are believed to have used their ability to escape predators by leaving the bottom of the ocean and swimming up into the water column. The 2 major groups of cephalopods, the ammonites and nautilus, became some of the most common marine animals and these 2 groups developed external shells to give them extra protection from predators. The ammonites, however, are now extinct and there's only 6 species of nautilus left in existence. The modern cephalopods, known as the coleoid cephalopods, have reduced and/or internalised shells and their ability to avoid detection by predators is seen in their ability to change colour, texture and shape. It's thought, however, that a strong visual decoy would probably not benefit the colossal squid in a deep-sea environment with absolutely no light.

By extreme contrast to using the element of disguise, some cephalopods have also been observed to use shock tactics to frighten off predators, using for example strong visible colour patterns and extending their arms to appear as huge as possible. Biologists believe that these shock tactics are designed to make predators hesitate, giving the cephalopod an opportunity to make a successful escape.

In addition to this, cephalopods use other various behaviours to hide their identity and communication seems to be most advanced in cephalopods that live in groups. Cuttlefish and squid are good examples whereby striking behaviours can be seen, such as vivid skin colour changes during courtship as well as contrasting light and dark shades and displays involving different body postures to demonstrate acts of aggression. Behaviour in males may go as far as parallel swimming with pushing and shoving against each other in the water, often accompanied by an increase in colour intensity but only rarely escalating into fighting. This behaviour pattern is very similar between different squid species and even between different orders, for example the squid and the cuttlefish, although it's not known if this is entirely relevant to Mesonychoteuthis hamiltoni.

One amazing communication method that's been seen in other squid species is a 'double signalling' technique and some video footage of this behaviour is featured in the nature documentary called 'Incredible Suckers'. In double signalling, a squid sends 2 messages at the same time to another squid. The best example of this is a male squid signalling to a female squid in courtship by using the half of his body closest to her then sending out a separate signal with the half of his body that's facing away from her. The incredible nature of this doesn't end here because, as the male squid's position relative to the female squid changes, so does the side of his body used to make these signals, so the signal is always relevant to the direction he's facing.

At present, biologists can only speculate as to how the colossal squid communicates but, based on other squid varieties, only general assumptions can be made.

Life span

Little is known about the precise life span of Mesonychoteuthis hamiltoni. One way of potentially telling the age is to look within the body at certain parts such as the lens of the eyes, the statoliths and the gladius, which have types of 'growth rings' much like those on a tree trunk. These can be counted by marine biologists to help establish age but one of the challenges they face is that, if they don't know how the particular growth rings are established and laid down in a species, it's difficult to count them. Head growth rings observed on the specimens of Mesonychoteuthis hamiltoni suggest it has a growth span of 2 years to reach maturity but the only sure way of tracking the age would be to keep one in captivity and this hasn't ever been possible. Based on studies of the giant squid, however, one of the estimates of the lifespan of Mesonychoteuthis hamiltoni is between 6 and 13 years.

Movement and swimming

Mesonychoteuthis hamiltoni can swim backwards and forwards. Nobody has ever observed a live colossal squid swimming but it's been possible to draw some assumptions together using observations on how related squid swim. Scientists have studied the family of Cranchiidae (glass squid), of which Mesonychoteuthis hamiltoni is a member. This group has forward facing eyes and binocular vision but, as already described in a previous section, one of the disadvantages of this is that these squid can't hold their arms outstretched as they swim without their field of vision being blocked by their arms. Other types of squid, like the giant squid, have eyes set into the sides of their head and this doesn't pose the same problem. So, the majority of the cranchiid squid family hold their arms either up over their heads, in what's known as the 'cockatoo' position, or down below in what's logically called the 'reverse cockatoo' position. Video evidence has been captured of cranchiid squid swimming in this way.

So, from this, it can be deduced that Mesonychoteuthis hamiltoni, like other Cranchiidae, doesn't swim with outstretched arms unless it's attacking its prey. When going in for the kill, Mesonychoteuthis hamiltoni has to strike the prey with its arms stretched out in front of it to maximise its chances of making a successful capture. Nobody is exactly sure if, when not attacking prey, Mesonychoteuthis hamiltoni swims in either the cockatoo position or the reverse cockatoo position but, since its lower arms are longer than its upper arms, if it were to swim in the cockatoo position then the lower arms would have to reach further to meet the tips of the upper arms, which is very feasible. Also, the positioning of the non-swivelling hooks on its arms suggests that Mesonychoteuthis hamiltoni uses the cockatoo position to swim but, so far to date, this hasn't been proved through direct observation.

Squid, like other marine creatures, can stay afloat either through swimming and the speed of their movement or through achieving a neutral buoyancy - that's to say they can achieve almost the same density as sea-water so they simply are able to float. To achieve perfect neutral buoyancy, however, is a difficult challenge for a sea creature as proteins contained in the muscle and tissues are dense and so make animal tissue denser than water. Active sea creatures need plenty of muscle protein, therefore have denser bodies and so use motion and swimming when they need to readjust their height in the water if they begin to sink. Some creatures, however, such as Mesonychoteuthis hamiltoni, use a very different strategy and have lower protein-content muscles containing more water and thus are less dense and heavy. This means they might not have the power to be continuously active and so have to employ different strategies to survive, such as moving in short bursts when catching prey or waiting in ambush for prey to come along.

There are different types of muscle in living organisms. White muscle is 'anaerobic', ideal for short fast bursts of speed but which tires quickly as it has a poor oxygen supply. It's used, therefore, to hunt prey and/or flee from predators but isn't for sustained performance. Red muscle, however, has a rich oxygen supply and, although much smaller and less powerful than white muscle, is able to move continuously and so is ideal for cruising along. Sea creatures, including Mesonychoteuthis hamiltoni, with low protein content and therefore more white muscle, can live in the deep sea where food supplies are scarce because they simply wait to ambush their prey using short, fast bursts of speed when they pounce and often luring their prey towards them through passive, low energy techniques such as bioluminescence. Bioluminescent flashes can be used in ever-imaginative ways, even to resemble the prey of their intended victim, fatal for the prey when it falls for this ruse.

The colossal squid uses another technique, however, that's employed by many other sea creatures to reduce their water density and help them to maintain neutral buoyancy. They have a certain ammonium chloride content in their bodies that's lighter than seawater and this differs from the flotation methods used by many other sea creatures, such as a gas-filled swim bladder to stay afloat.

By comparing Mesonychoteuthis hamiltoni to other squid species, it's believed that the tissues of the colossal squid may contain a high ammonium ion content and a low sodium ion content. Since ammonium has a lower mass than sodium, this results in body tissues that are less dense. It's not so much the mass that matters, however, but the way in which the ammonium ions interact with the surrounding water molecules, as this has the greater effect on the tissue density. Mesonychoteuthis hamiltoni is thought to have a much higher ammonium/sodium ratio (about twice the volume) in the mantle region than in the head and arms, to suggest that it could hang in the water, with its head, arms and tentacles dangling downwards, to help it be ready to catch prey passing underneath. This theory suggests that the colossal squid could be a very effective passive ambush hunter.

There's another feature that helps Mesonychoteuthis hamiltoni turn on a burst of speed and this is called the 'giant axon', which is a nerve cell that controls part of the water jet propulsion. It's called the giant axon because, in smaller squid species, it's relatively large compared to the rest of their body size. Squid use this to help them make short but very fast accelerations through the water. The giant axon is thought to initiate and control the fast contractions of body wall muscles and the drawing in of water, which is then expelled through the siphon to help the squid to swim fast when hunting or fleeing from predators.

Reproduction

Scientists don't know much at all about the reproductive cycle of Mesonychoteuthis hamiltoni, mainly because only females have been found, but it's thought that the male probably has a large penis to be able to implant sperm straight into the female.

We do know that all types of squid lay eggs, some single but others clusters in a large jelly-like floating mass. Giant squid lay eggs in this way so it's thought that colossal squid probably do the same. The eggs hatch out into tiny versions of the adult and become mature adults within 3 years.

Looking at the mating patterns and behaviour of other squid species can give an insight into the possible ways that Mesonychoteuthis hamiltoni could mate. Other species of squid have been observed through careful research and, when mating occurs, males and females collect in large schools to swim in circles before pairing off. The male squid are able to change the colour of their skin to attract a female and, once she's shown interest, the mating takes place, often involving quite aggressive behaviour. It's not known, though, if the colossal squid use this pack-like behaviour of if they're loners relying on single sexual encounters.

In other squid species, there's some known breeding zones where squid gather prior to the mating beginning and some squid swim long distances to take part in it. The female squid's ink sac, which is hidden under glands in the gills area, also serves as the protective shell for the eggs she'll produce and, during the mating process, the male sperm is placed inside this sac to fertilise the eggs.

Some female squid species can produce thousands of eggs at a time but it isn't known exactly how the eggs develop inside her body. The size of the eggs depends on the species but they're usually quite small with some species producing up to 70,000, many of which are eaten by predators.

After a female lays her eggs, the hatching process takes up to 8 weeks, so keeping them safe from predators is a challenge but, ironically, the female squid don't wait for their eggs to hatch and leave immediately after laying them. Baby squid must, therefore, fend for themselves from the start and have an inbuilt knowledge of how to swim and feed. Many are killed by predators and, in some squid species, the parents die soon after reproducing due to their short life spans. Male and female squid in many species only mate once in their lives and it's only possible, therefore, to speculate if this could apply to Mesonychoteuthis hamiltoni.

Predators

Known predators of Mesonychoteuthis hamiltoni include the sperm whale, which can dive down to a depth of 2000 metres to catch them, and sleeper shark. The toothfish is also known to be a predator of many squid species, which is a bizarre fact given that toothfish can also be the prey of the colossal squid. Smaller squid are eaten by a number of other animals such as seabirds and some beaks have been found. Seabirds don't, however, dive deep so it's known that the smaller squid live much higher up.

As already identified, sperm whales are the main colossal squid predator. The chitinous beaks of squid remain in their digestive systems for a long period of time, due to their indigestible nature. Biologists believe that colossal squid might constitute as much as 77% of a sperm whale's diet with an average daily consumption of around 1-1.5 tons.

Since Mesonychoteuthis hamiltoni is a relatively cold-blooded creature, it's not believed to be a very nutritious meal for its predators, so sperm whale and other predatory animals would have to eat a lot of squid to get any nutritional value. This is not good news for the survival of squid stocks!

Conclusion

Mesonychoteuthis hamiltoni, the colossal squid, is one of the big mysteries of our time. Marine biologists go on expeditions to further their research in the hope of finding further specimens. Scientific techniques continue to improve and, with this, the potential to observe the colossal squid in its natural environment. In the meantime, much of the knowledge base is speculative or assumed through studies of other cephalopods and squid species. Precious examples of this large complex creature are still in the Museum of New Zealand, Te Papa, Tongarewa.