Giant squid Architeuthis dux

Giant squid Architeuthis dux are found in deep oceans worldwide and can grow to a size of 13 m (43 ft) for females and 10 m (33 ft) for males, measured from the caudal fin to the tip of the two long tentacles. The mantle is only about 2 m long, and the length of the squid excluding its tentacles is about 5 m.  Although numerous species have been described in the family (Architeuthidae), it is now thought only one species exists, and the differences between specimens are caused by post-mortem changes.

The colossal squid Mesonychoteuthis hamiltoni belongs to a different family of squid (Cranchiidae), and it is believed to be the largest squid species. It is the only member of the genus Mesonychoteuthis. Though it is known from only a few specimens, current estimates put its maximum size at 12–14 metres total length, based on analysis of smaller and immature specimens, making it the largest known invertebrate.

The giant squid Architeuthis dux is frequently reported to attain a lotal length of 60 feet. The largest specimen known washed ashore on a New Zealand beach, Lyall Bay (Wellington, New Zealand) in the winter of 1887. It was a female and "although the body was in all ways smaller than any of the hitherto-described New Zealand species" (Kirk 1887); it measured 55 feet 2 inches in total length, exaggerated by great lengthening (stretching like rubber bands) of the slender tentacular arms; its mantle length was 71 inches (1.8 m). A comparable-sized female (ML 1.8 m) measured post mortem and relaxed (by modern standards) would have a total length of ~ 32 feet.

Because of the elastic nature of the long tentacles, mantle length (or body length), as opposed to total length, is the usual measurement used for cephalopods. Architeuthis dux is not known to attain a mantle length in excess of 2.25 m. Standard Length (SL) is the length of a squid body and head, excluding the tentacles; in Architeuthis this measure very rarely exceeds 5 m. The rest of the animal's length, to a total length of 13 m, is made up of the two long tentacles. Of  more than 100 specimens examined, none has exceeded these figures.

Architeuthis beaks recovered from the stomachs of sperm whales (Physeter catadon) are smaller or equivalent to Architeuthis beaks recovered from specimens trawled in New Zealand waters. That is to say that no evidence exists for recognising larger specimens than those known from New Zealand. Moreover, it is most likely that a single species, Architeuthis dux, exists worldwide, so 'larger species' of Architeuthis do not occur. Thus reputed lengths to 60 feet and weights to a ton are incorrect.

Reproduction of giant squid is poorly known. Sexual maturity is attained at about three years of age; males are smaller than females. Females have a single median ovary near the rear of the mantle cavity and paired convoluted oviducts where mature eggs pass, exiting through glands (nidamental glands), which produce a gelatinous material used to keep the eggs together once they are laid. Eggs, averaging 0.5-1.4 mm long and 0.3-0.7 mm wide, are produced in large quantities of more than 5 kg.

In males, as with most other cephalopods, the single, posterior testis produces sperm that move into a complex system of glands that manufacture the spermatophores. These are stored in an elongate sac (Needham's sac), that terminates in the penis from which they are expelled during mating. The penis is prehensile, over 90 cm in length, and extends from inside the mantle.

How the sperm is transferred to the egg mass is unknown. Giant squid lack a hectocotylus – a specially modified arm used to transfer spermatophores to the female. It may be transferred in sacs of spermatophores, called spermatangia, which the male injects into the female's arms.

Post-larval juveniles have been discovered in surface waters off New Zealand.