Colossal squid

The colossal squid (Mesonychoteuthis hamiltoni) is the largest member of its family Cranchiidae, the cockatoo or glass squids, with its second largest member being Megalocranchia fisheri. It is sometimes called the Antarctic cranch squid or giant squid (not to be confused with the giant squid in genus Architeuthis) and is believed to be the largest squid species in terms of mass. It is the only recognized member of the genus Mesonychoteuthis and is known from only a small number of specimens. The species is confirmed to reach a mass of at least 495 kg, though the largest specimens—known only from beaks found in sperm whale stomachs—may perhaps weigh as much as 600 –, making it the largest known invertebrate. Maximum total length has been estimated between 10 m and 14 m but the former estimate is more likely. The colossal squid has the largest eyes of any known creature ever to exist, with an estimated diameter of 27 - 30 cm to 40 cm for the largest collected specimen.

The species has similar anatomy to other members of its family, although it is the only member of Cranchiidae to display hooks on its arms, suckers and tentacles. It is known to inhabit the circumantarctic Southern Ocean. It is presumed to be an ambush predator, and is likely a key prey item of the sperm whale.

The first specimens were discovered and described in 1925. In 1981, an adult specimen was discovered; in 2003, a second specimen was collected. Captured in 2007, the largest colossal squid weighed 495 kg, and is now on display at the Museum of New Zealand Te Papa Tongarewa.

In 2022-23 there were several attempts made by scientists including an ocean exploration non-profit KOLOSSAL to find and film the colossal squid in its natural habitat for the first time to learn more about its biology and ecological behavior. The science team used a tourism vessel to survey 36 locations throughout the Southern Ocean and may have filmed for the first time a small juvenile colossal squid. Researchers have confirmed it is a species of glass squid, but due to marine snow the footage has been harder to confirm without a DNA analysis, and may be Galiteuthis glacialis or a new species of glass squid unknown to science. If confirmed, this would be the first filming of a colossal squid underwater in its natural habitat and the first footage of a juvenile.

More expeditions are being planned for 2025, the hundred year anniversary of the first discovery of the colossal squid, in attempts to find and film an adult colossal squid living freely in its natural environment.

Morphology
The colossal squid shares features common to all squids: a mantle for locomotion, one pair of gills, a beak or tooth, and certain external characteristics like eight arms and two tentacles, a head, and two fins. In general, the morphology and anatomy of the colossal squid are the same as any other squid. However, there are certain morphological characteristics that separate the colossal squid from other squids in its family: the colossal squid is the only squid in its family whose arms and tentacles are equipped with hooks, either swivelling or three-pointed. There are squids in other families that also have hooks, but no other squid in the family Cranchiidae.

Unlike most squid species, the colossal squid exhibits abyssal gigantism, as it is the heaviest living invertebrate species, reaching weights up to 495 kg. For comparison, squids typically have a mantle length of about 30 cm and weigh about 100-200 g.

The giant squid also exhibits abyssal gigantism, but the colossal squid is heavier. Although it is unclear what the maximum weight for colossal squids is, analysis of squid beak dimensions from sperm whale stomachs provided estimates that colossal squids may weigh up to 700 kg.

The colossal squid also has the largest eyes documented in the animal kingdom, with a diameter of 27 - 30 cm.

Distribution and habitat
The squid's known range extends thousands of kilometres north of Antarctica to southern South America, southern South Africa, and the southern tip of New Zealand, making it primarily an inhabitant of the entire circumantarctic Southern Ocean. Colossal squid are also sighted often near Cooperation Sea and less near Ross Sea because of its prey and competitor, the Antarctic toothfish. The region between the Weddell Sea and the western Kerguelen archipelago has been deemed a "hotspot" based on characteristics of the habitat. The squid's vertical distribution appears to correlate directly with age. Young squid are found between 0 - 500 m, adolescent squid are found 500 - 2000 m and adult squid are found primarily within the mesopelagic and bathypelagic regions of the open ocean.

Feeding
Little is known about their behaviour, but it is believed to feed on prey such as chaetognatha, large fish such as the Patagonian toothfish, and smaller squid in the deep ocean. A recent study by Remeslo, Yakushev and Laptikhovsky revealed that Antarctic toothfish make up a significant part of the colossal squid's diet; of the 8,000 toothfish brought aboard trawlers between 2011 and 2014, seventy-one showed clear signs of attack by colossal squid. A study in Prydz Bay region of Antarctica found squid remains in a female colossal squid's stomach, suggesting the possibility of cannibalism within this species. Studies measuring the δ15N content of the chitinous beaks of cephalopods to determine trophic ecology levels have demonstrated that the colossal squid is a top predator that is positively correlated with its increased size. This new confirmation of the colossal squid's trophic level suggests that it likely preys on large fishes and smaller squids, according to its size, and that its predators include sperm whales and sleeper sharks.

Metabolism
The colossal squid is thought to have a very slow metabolic rate, needing only around 30 g of prey daily for an adult with a mass of 500 kg. Estimates of its energy requirements suggest it is a slow-moving ambush predator, using its large eyes primarily for prey-detection rather than engaging in active hunting.

Predation
Many sperm whales have scars on their backs, believed to be caused by the hooks of colossal squid. Colossal squid are a major prey item for sperm whales in the Antarctic; 14% of the squid beaks found in the stomachs of these sperm whales are those of the colossal squid, which indicates that colossal squid make up 77% of the biomass consumed by these whales. Many other animals also feed on colossal squid, including the beaked whales, such as southern bottlenose whales, Cuvier's and Baird's beaked whales; the beaked whales essentially resemble oversized dolphins, some with a more pronounced underbite on their snout (or "beak"). They are among the deepest-diving cetaceans ever recorded, besides the sperm whale. This places the beaked whales as some of the few food competitors of the sperm whale. Other possible squid predators include the pilot whale, killer whales, larger southern elephant seals, Patagonian toothfish, southern sleeper sharks (Somniosus antarcticus), Antarctic toothfish, and albatrosses (e.g., the wandering and sooty albatrosses). However, beaks from mature adults have only been recovered from large predators (i.e. sperm whales and southern sleeper sharks), while the other predators only eat juveniles or young adults.

Reproduction
Not much is known about the colossal squid's reproductive cycle, although it does have two distinct sexes. Many species of squid, however, develop sex-specific organs as they age and develop. The adult female colossal squid has been discovered in much shallower waters, which likely implies that females spawn in shallower waters, rather than their normal depth. Additionally, the colossal squid has a high possible fecundity reaching over 4.2 million oocytes which is quite unique compared to other squids in such cold waters. Colossal squid oocytes have been observed at sizes ranging from as large as 3.2x2.1 mm to as small as 1.4x0.5 mm. Sampling of colossal squid ovaries show an average of 2175 eggs per gram. Young squid are thought to spawn near the summer time at surface temperatures of -0.9-0 C.

Vision
For pelagic organisms of similar weight to the colossal squid, such as the swordfish, the average eye diameter required for visual detection is 10 cm, but the colossal squid's are as large as 30 cm. The allowed increase in visual detection strategies, including reduced diffraction blurring and greater contrast distinction, must be extremely beneficial to the colossal squid to justify the large energetic expenses to grow, move, camouflage, and maintain these eyes. The colossal squid's increased pupil size has been mathematically proven to overcome the visual complications of the pelagic zone (the combination of downwelling daylight, bioluminescence, and light scattering with increasing distance), especially by monitoring larger volumes of water at once and by detecting long-range changes in plankton bioluminescence via the physical disruption of large moving objects (e.g., sperm whales).

The colossal squid's eyes glow in the dark via long, rectangular light-producing photophores located next to the lens on the front of both eyeballs. Symbiotic bacteria reside within these photophores and luminesce through chemical reaction.

It is hypothesized that the colossal squid's eyes can detect predator movement beyond 120 m, which is the upper limit of the sperm whale's sonar range.

Hearing
Squid have been found to detect the movement of sound waves via organs called statocysts (similar to the human cochlea). Squid statocysts likely respond to low frequency sounds less than 500 Hz, similar to pelagic fish. Colossal squid are essentially deaf to high frequencies, such as whale sonar, so they rely largely on visual detection mechanisms to avoid predation.

Taxonomy and history
The colossal squid, species Mesonychoteuthis hamiltoni, was discovered in 1925. This species belongs to the class Cephalopoda and family Cranchiidae.

Most of the time, full colossal squid specimens are not collected; as of 2015, only 12 complete colossal squids had ever been recorded, with only half of these being full adults. Commonly, beak remnants of the colossal squid are collected; 55 beaks of colossal squids have been recorded in total. Less commonly (four times), a fin, mantle, arm or tentacle of a colossal squid was collected.

First specimens
The species was first discovered in the form of two arm crowns found in the stomach of a sperm whale in the winter of 1924–1925. This species, then named Mesonychoteuthis hamiltoni after E. Hamilton who made the initial discovery, was formally described by Guy Coburn Robson in 1925.

Entire specimens
In 1981, a Soviet Russian trawler in the Ross Sea, off the coast of Antarctica, caught a large squid with a total length of over 4 m, which was later identified as an immature female of M. hamiltoni. In 2003, a complete specimen of a subadult female was found near the surface with a total length of 6 m and a mantle length of 2.5 m (8 feet 3 inches). In 2005, the first full living specimen was captured at a depth of 1,625 m while taking a toothfish from a longline off South Georgia Island. Although the mantle was not brought aboard, its length was estimated at over 2.5 m (8 feet 3 inches), and the tentacles measured 2.3 m. The animal is thought to have weighed between 150 and 200 kg.

Largest known specimen
The largest recorded specimen was a female, which are thought to be larger than males, captured in February 2007 by a New Zealand fishing boat in the Ross Sea off Antarctica. The squid was close to death when it was captured and subsequently was taken back to New Zealand for scientific study. The specimen was initially estimated to measure about 10 metres in total length and weigh about 450 kg.

Defrosting and dissection, April–May 2008
Thawing and dissection of the specimen took place at the Museum of New Zealand Te Papa Tongarewa. AUT biologist Steve O'Shea, Tsunemi Kubodera, and AUT biologist Kat Bolstad were invited to the museum to aid in the process, joined by Marine Ecologist Mark Fenwick and Dutch scientist Olaf Blaauw. Media reports suggested scientists at the museum were considering using a giant microwave to defrost the squid because thawing it at room temperature would take several days and it would likely begin to decompose on the outside while the core remained frozen. However, they later opted for the more conventional approach of thawing the specimen in a bath of salt water. After thawing, it was found that the specimen was 495 kg with a mantle length of 2.5 m and a total length of only 4.2 m, probably because the tentacles shrank once the squid was dead.

Parts of the specimen have been examined:


 * The beak is considerably smaller than some found in the stomachs of sperm whales, suggesting other colossal squid are much larger than this one.
 * The eye is 27 cm wide, with a lens 12 cm across. This is the largest eye of any known animal. These measurements are of the partly collapsed specimen; alive, the eye was probably 30 to 40 cm (12 to 16 in) across.
 * Inspection of the specimen with an endoscope revealed ovaries containing thousands of eggs.

Exhibition
The Museum of New Zealand Te Papa Tongarewa began displaying this specimen from 13 December 2008. The exhibition was closed between 2018 and 2019, but is currently open again for public viewing at Te Papa.

Conservation status
The colossal squid has been assessed as "least concern" on the IUCN Red List. Furthermore, colossal squid are not targeted by fishermen; rather, they are only caught when they attempt to feed on fish caught on hooks. Additionally, due to their habitat, interactions between humans and colossal squid are considered rare.