The colossal squid (Mesonychoteuthis hamiltoni) is a species of very large squid belonging to the family Cranchiidae, that of the cockatoo squids, or glass squids. 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. The species is confirmed to have a mass of at least 495 kilograms (1,091 lb), although some specimens, known only from beaks found in sperm whale stomachs, may have weighed as much as 600–700 kilograms (1,300–1,500 lb), making the squid the largest extant invertebrate. The maximum total length is ~4.2 metres (14 ft). Larger estimates exist; however, these include the feeding tentacles measured on dead specimens. In life the squid's tentacles are hidden, only released when capturing prey. If tentacles are considered, lengths of 10 metres (33 ft) and 14 metres (46 ft) exist, 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 (11–12 in) to 40 cm (16 in) for the largest collected specimen. The species has anatomy similar to that of 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, with a diet including various fish, and is likely a key prey item of the sperm whale.
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 swiveling 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 (1,091 lb). For comparison, squids typically have a mantle length of about 30 cm (12 in) and weigh about 100–200 g (3+1⁄2–7 oz). The colossal squid also has the largest eyes documented in the animal kingdom, with a diameter of 27–30 cm (11–12 in).
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 circumantarctic Southern Ocean. Colossal squid are also often sighted near Cooperation Sea and 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 (0–1,640 ft), adolescent squid are found 500–2,000 m (1,600–6,600 ft) and adult squid are found primarily within the mesopelagic (approximately 200 to 1,000 meters below the surface) and bathypelagic regions (1,000 to 4,000 m below the surface) of the open ocean.
Behavior
Feeding
While little is known about their behavior, colossal squid are thought to feed primarily on small fish, such as lanternfish and deep-sea smelt, which have been found as stomach contents in adult specimens. They also attack larger fish; of 8,000 Antarctic toothfish brought aboard by trawlers between 2011 and 2014, seventy-one showed clear signs of attack by colossal squid. A study in the 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, such as the Patagonian toothfish, 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 grams (1 oz) of prey daily for an adult with a mass of 500 kilograms (1,100 lb). 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 that are 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 likely 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 and Cuvier's and Baird's beaked whales. Other possible 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). Beaks from mature adults have only been recovered from large predators (i.e. sperm whales and southern sleeper sharks), while other predators only eat juveniles or young adults.
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