The world’s biggest squid species have developed huge eyes to give early warning of approaching sperm whales.
Colossal and giant squid both have eyes that can measure 27cm (11in) across – much bigger than any fish.
Scientists found that huge eyes offer no advantages in the murky ocean depths other than making it easier to spot enormous shapes – such as sperm whales.
Writing in Current Biology journal, they say this could explain the equally huge eyes of fossil ichthyosaurs.
Lead scientist Dan Nilsson from Lund University in Sweden was present at the unique dissection of a colossal squid performed four years ago in New Zealand.
There, he examined and handled the eyes – in particular, the hard parts of the lens.
These alone are bigger than an entire human eye.
“We were puzzled initially, because there were no other eyes in the same size range,” Prof Nilsson told BBC News.
“You can find everything up to the size of an orange, which are in large swordfish.
“So you find every small size, then there’s a huge gap, then there are these two species where the eye is three times as big – even though squid are not the largest animals.”
In general, other squid species also have eyes that are smaller in proportion to their body size.
Sense of proportion
The streamlined giant squid (various species of Architeuthis) and the much chunkier colossal squid (Mesonychoteuthis hamiltoni) can both grow to more than 10m long, as measured from the tip of the body to the end of their tentacles.
The colossal squid especially is equipped with a fearsome arsenal of weapons, including barbed swivelling hooks.
Scars on the bodies of sperm whales indicate that they regularly do battle with the colossal squid, at least in the Southern Hemisphere waters where it lives.
And the number of colossal squid beaks found in the stomachs of sperm whales indicate that the latter often win.
Though colossal squid are encountered remarkably rarely by people, they are thought to make up about three-quarters of sperm whales’ diet in the Southern Ocean.
Whereas the whales can spot squid using sonar, the squid can deploy nothing except vision – which suggests there would be a powerful evolutionary pressure towards developing effective eyes.
But structures this big and complex are “expensive” in terms of nutrients, meaning that for most animals the evolutionary pressure is against evolving them.
Prof Nilsson’s team used mathematical models of how different sizes of eye perform at depths up to 1km.
There, moving objects are most detectable through the bioluminescence they provoke from countless tiny creatures in the water.
The models showed that in general, there is no benefit to be gained from developing eyes bigger than the swordfish’s.
The exception is a really large moving object.
Here, large eyes enable better detection of a pattern of point sources of bioluminescence – light given off by tiny organisms – in low-contrast conditions.
This would give the squid warning of a sperm whale approaching at a distance of about 120m, the researchers calculate – potentially allowing it to take evasive action and avoid being eaten.
“It’s the predation by large, toothed whales that has driven the evolution of gigantism in the eyes of these squid,” commented Soenke Johnsen, a biologist at Duke University in Durham, US, who was also part of the research team.
They speculate that eyes of the same size might have enabled ichthyosaurs, large marine reptiles that died out about 90m years ago, to avoid the fearsome jaws of the even larger pliosaurs.