Largest eye

Giant squid (Architeuthis spp.) and colossal squid (Mesonychoteuthis spp.) have the largest eyes of any living animal. One photographed specimen of giant squid caught in February 1981 off Hawaii, USA, had a 9-cm pupil with a total eye diameter of at least 27 cm (10.6 in). A similar diameter of 27–28 cm (10.6–11 in) was measured for the eyes of a colossal squid in New Zealand collected in 2007, the largest specimen ever caught and preserved. The research into giant squid eyes was published in Current Biology on 24 April 2012.

There are anecdotal historic reports of even larger eyes in these giant cephalopods. One Architeuthis specimen from Thimble Tickle Bay, Newfoundland, Canada, found in 1878 was said to have eyes measuring 40 cm (15.75 in) in diameter but this was not scientifically confirmed so it's impossible to verify for certain.

The colossal squid was caught by fishermen on board the San Aspiring in the Ross Sea of Antarctica, and taken to New Zealand's Te Papa Natural History Museum for research, where the catch was announced on 22 February 2007.

The only animals whose eyes may have been larger than those of these squid were certain ichthyosaurs, an extinct group of marine reptiles. Examining the sclerotic ring aperture's diameter in fossil ichthyosaur skulls to estimate the eyeball diameter, palaeontologists consider that the eyeball of one of the largest known ichthyosaurs, Temnodontosaurus, may have exceeded 30 cm (11.8 in), perhaps up to 35 cm (1 ft 1.8 in). It lived in the deep oceans of what is today Europe (specifically those around England, France, Germany and Belgium), during the Early Jurassic Period (200–175 million years ago).

Owing to their elusive nature, deep-sea habitat and limited observations/anatomical study, much is still unknown about these mega-squid. They're thought to be distributed in oceans worldwide, though colossal squid seem to favour waters in and around the Southern Ocean.