Colossal squid

## Description

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 495 kilograms (1,091 lb), though the largest specimens—known only from beaks found in sperm whale stomachs—may perhaps weigh as much as 600–700 kilograms (1,300–1,500 lb), making it the largest-known invertebrate. Maximum total length has been estimated at 9–10 metres (30–33 ft).

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 495 kilograms (1,091 lb), and is now on display with a second specimen at the Museum of New Zealand Te Papa Tongarewa.

## Appearance

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 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).

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 700 kg or 1,500 lb) exist. The colossal squid also has the largest eyes documented in the animal kingdom, with an estimated diameter of 30–40 cm (12–16 in).

## Distribution

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 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 and bathypelagic regions of the open ocean.

## Diet

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 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.

## 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 −0.9–0 °C (30.4–32.0 °F).

## Population

### Population number

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.