Largest eyes on a living vertebrate

In absolute terms, the largest eyes of any extant vertebrate (i.e., animal with a backbone) are those of the blue whale (Balaenoptera musculus) with an axial diameter of approximately 10.9 cm (4.3 in), making them about the size of grapefruits. Blue whales are the largest animals to have ever lived on Earth.

With adult blue whales on average growing to 25 m (82 ft) long, this marine mammal's eyes are actually tiny relative to body size (i.e., eye diameter equating to c. 0.44% of total body length) and, as such, they are generally considered to have fairly poor vision.

A close contender for this title, and perhaps occasionally equalling the blue whale's record in particularly large specimens, is the bigeye thresher shark (Alopias superciliosus). Its eyeballs (the largest among fish) can span more than 10 cm (3.9 in). Unusually, their eyes are bigger vertically than they are horizontally and are shaped more like an upside-down pear, rather than being spherical. Notably, by contrast to the blue whale, bigeye thresher sharks typically grow to only 3–4 m (10 ft 9 in–13 ft 1.5 in) long giving them a much higher eye-to-body percentage of about 3%.

Historically, certain species of ichthyosaur, a group of extinct marine reptiles that lived from the Early Triassic through to the Late Cretaceous era, had considerably larger eyes than the blue whale. Studies of fossilized remains, particularly a bone known as the sclerotic ring that circled their eyeballs, suggest in species such as Temnodontosaurus that ichthyosaur eyes may have reached 25 cm (9.8 in) in diameter and perhaps even in excess of 30 cm (11.8 in).

The largest eyes overall in the animal kingdom today (and perhaps all time) belong to two ocean invertebrates: giant squid (Architeuthis spp.) and colossal squid (Mesonychoteuthis spp.). 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. Anecdotal reports suggest even bigger eyes for these mega-squid; one specimen that washed ashore in Thimble Tickle Bay, Newfoundland, Canada, in 1878 was claimed to have eyes measuring 40 cm (15.75 in) in diameter.

As the largest animals ever to live on Earth, blue whales hold many size-based anatomical records including the largest nose, the largest lungs and the longest penis. They also bear the largest offspring and have the slowest heart rate among mammals.