Colossal squid
The colossal squid ("Mesonychoteuthis hamiltoni") is part of the family Cranchiidae. It is sometimes called the Antarctic squid or giant cranch squid 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 , though the largest specimens—known only from beaks found in sperm whale stomachs—may perhaps weigh as much as , making it the largest-known invertebrate. Maximum total length has been estimated at  . 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 and tentacles. It is known to inhabit the circumantarctic Southern Ocean. Although little is known about the behavior, it is known to use bioluminescence to attract prey. It is presumed to be an ambush predator, and is a major prey of the sperm whale. The first specimens were discovered and described in 1925. In 1981, an adult specimen was discovered, and in 2003 a second specimen was collected. Captured in 2007, the largest colossal squid  weighed , and is now on display with a second specimen at the Museum of New Zealand Te Papa Tongarewa. Morphology. The colossal squid shares features common to all squids: a mantle for locomotion, one pair of gills, and certain external characteristics like eight arms and two tentacles, a head, and two fins. In general, it is safe to describe the morphology and anatomy of the colossal squid the same way one would describe any other squid. However, there are certain morphological / anatomical characteristics that separate the colossal squid from other squids in its family: The colossal squid is the only  squid in its family with hooks, either swivelling or three-pointed, equipped on its arms and tentacles. There are squids in other families that also have hooks, but no other squid in the family Cranchiidae. The colossal squid is unlike most squid species, for it exhibits abyssal gigantism; it is the heaviest living invertebrate species, reaching weights up to . For comparison, squids typically have a mantle length of about and weigh about . Compared to the giant squid, which also exhibits deep-sea gigantism, the colossal squid is heavier. One evidence is that an analysis of the beaks of other specimens  from the stomach of sperm whales have shown proof that it is likely that colossal squids much heavier (up to ) exist. The colossal squid also has the largest eyes documented in the animal kingdom, with an estimated diameter of . 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 predator 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 , adolescent squid are found and adult squid are found primarily within the mesopelagic and bathypelagic regions of the open ocean. Behavior. Feeding. Little is known about their behavior, 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 using bioluminescence. 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 δN content of the chitinous beaks of cephalopods to determine trophic ecology levels have demonstrated that the colossal squid is an apex 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 of prey daily for an adult with a mass of . 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 beaked whales (such as the southern bottlenose whale), pilot whales, 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. Little is known about the colossal squid's reproductive cycle although the colossal squid 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 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 . 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 colossal squid's are as large as 27 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 one time 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  rectangularly shaped 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. Although squid can't hear sound, they can 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 (like 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. Notable discoveries. 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 , which was later identified as an immature female of "M. hamiltoni". This is a significant  discovery, as it was not until 2003 that another full individual was discovered. In 2003, a complete specimen of a subadult female was found near the surface with a total length of 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 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 . The animal is thought to have weighed between . 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 of Antarctica. The squid was close to dead when it was captured and subsequently was taken back to New Zealand for scientific study. The specimen was initially estimated to measure about 10 meters 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. 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 total length of only  4.2 m, probably because the tentacles shrank once the squid was dead. Parts of the specimen have been examined: Exhibition. The Museum of New Zealand Te Papa Tongarewa began displaying this specimen in an exhibition that opened on 13 December 2008; however the exhibition was closed in 2018 and was slated to return in 2019. The exhibition is now open again for public viewing at Te Papa. Second specimen. In August 2014, Te Papa received a second colossal squid, captured in early 2014. The specimen was also female, measuring long and weighing approximately . Conservation status and human interactions. 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.