- Who
- Bigeye thresher shark, Alopias superciliosus
- What
- 10+ centimetre(s)
- Where
- Not Applicable ()
The largest eyes on an extant species of fish are those of the aptly named bigeye thresher shark (Alopias superciliosus), whose eyeballs can span more than 10 cm (3.9 in) in larger specimens, perhaps even up to 12.5 cm (4.9 in). Unusually, the eyes are bigger vertically than they are horizontally and are shaped more like an upside-down pear, rather than being spherical. The species is distributed throughout tropical and temperate oceans worldwide.
Relative to body size, this also represents the largest known eyes on any living vertebrate. The eyes of the blue whale (Balaenoptera musculus) – the largest animal ever – are slightly larger in absolute terms at about 11 cm (4.3 in) in diameter, but proportionate to this species' 25-m (82-ft) adult body length, they are tiny (0.44%). By contrast, most bigeye thresher sharks grow to 3–4 m (10 ft 9 in–13 ft 1.5 in) long giving them a eye-to-body percentage of about 3%.
The largest eyes overall in the animal kingdom (most likely of all time) belong to two 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.
A very close contender for the fish eye title are billfish such as swordfish (Xiphias gladius), with eyeballs reaching c. 9 cm (3.5 in) wide. Both bigeye threshers and swordfish have evolved such large visual organs to help them hunt in deep waters (900–1,000 m/2,950–3,280 ft) in the ocean's "twilight zone" where light is scarce.
The whale shark (Rhincodon typus), despite being the largest living fish, has smaller eyes of approximately 6.5 cm (2.6 in) – anteroposterior diameter; this disparity is most likely because it does the majority of its food gathering near the ocean surface where light is plentiful.
