Largest cephalopods
Giant squid, Architeuthis dux, Colossal squid, Mesonychoteuthis hamiltoni
13 m / 495 kg dimension(s)
Not Applicable ()

The largest living cephalopods (a group of molluscs that include squid, octopuses and cuttlefish) are two species of mega-squid. Based on length, the record holder is the giant squid (Architeuthis dux) which can grow to at least 13 m (42 ft 8 in); most of this length is in its tentacles with the main body (mantle) reaching up to 2.25 m (7 ft 5 in). The heaviest entire specimen of giant squid on record was approximately 220 kg (485 lb). Colossal squid (Mesonychoteuthis hamiltoni), on the other hand, take the record based on weight. They have a similar-sized mantle (up to 2.5 m/8 ft 2 in) but shorter tentacles with the biggest intact squid to date measuring 5.4 m (17 ft 9 in) in full; however, they are much heftier, weighing up to 495 kg (1,091 lb).

This also makes them the largest invertebrates.

Owing to their elusive nature and few intact specimens presenting themselves, very much remains unknown about the biology and habits of both of these ocean giants. Giant squid seem to be found in oceans worldwide, whereas colossal squid appear to favour the cooler waters of the Southern Ocean.

There are anecdotal historic reports of much larger squid but these are not scientifically verified. For instance, a giant squid that washed ashore in Thimble Tickle Bay, Newfoundland, Canada, on 2 November 1878 was said to have a mantle length of 6.1 m (20 ft) and one tentacle measuring 10.7 m (35 ft), giving a total length of 16.8 m (55 ft). Similarly, another female that was found in Lyall Bay, near Wellington, New Zealand, in winter 1887 was also said to have measured 55 ft 2 in in total length, but this is most likely a result of over-stretching of the elastic tentacles as its mantle length was "only" 71 in (1.8 m).

Some believe that giant squid have the potential to reach up to 20 m (66 ft) in total length (as proposed by statistical ecologist Charles Paxton of the University of St Andrews in a 2016 paper) however this view is not yet widely accepted within the scientific community.

Two of the largest colossal squid caught live to date have been captured by fishermen on board the vessel San Aspiring while in the Ross Sea, off Antarctica. In March 2003, they caught an immature female that measured 5.4 m (17 ft 9 in), while in February 2007 they hauled in an adult male weighing approximately 495 kg (1,091 lb), the largest living cephalopod ever caught. Both specimens were taken to New Zealand's Te Papa Natural History Museum for research and preservation.

Giant squid and colossal squid also share a record for having the largest eyes in the animal kingdom (most likely of all time). A 2012 study concluded that among specimens with reliable evidence that both can have eyeballs with a diameter of 27–28 cm (10.6–11 in). Again, historical reports suggest their eyes can reach even larger, up to 40 cm (15.75 in) in diameter, but once again this is unsubstantiated.