Which of these is the second-largest mollusc?

The giant squid is the second-largest mollusc and one of the largest of all extant invertebrates. It is only exceeded by the colossal squid, 'Mesonychoteuthis hamiltoni', which may have a mantle nearly twice as long.

Giant squid size, particularly total length, has often been exaggerated. Reports of specimens reaching and even exceeding 20 m (66 ft) are widespread, but no specimens approaching this size have been scientifically documented. According to giant squid expert Steve O'Shea, such lengths were likely achieved by greatly stretching the two tentacles like elastic bands.

A giant squid specimen measuring over 4 m (13 ft) without its two long feeding tentacles. Based on the examination of 130 specimens and of beaks found inside sperm whales, giant squids' mantles are not known to exceed 2.25 m (7 ft 4.6 in). Including the head and arms, but excluding the tentacles, the length very rarely exceeds 5 m (16 ft).

Maximum total length, when measured relaxed post mortem, is estimated at 13 m (43 ft) for females and 10 m (33 ft) for males from the posterior fins to the tip of the two long tentacles.

Giant squid exhibit sexual dimorphism. Maximum weight is estimated at 275 kg (606 lb) for females and 150 kg (330 lb) for males.