Giant squid eyes are sperm whale defence
Posted by Xeno on March 16, 2012
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.
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. …