- Animals evolved huge eyes to pick out sperm whales - their predators - in the dark
- Eyes the size of basketballs - out of proportion even on creatures 27ft long
By Rob Waugh
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Two men inspect a nearly intact 9.2 meter giant squid in Norway in 1954 - the creatures have disproportionately huge eyes, the size of basketballs
Giant squids can grow to 27 feet long and half a ton in weight - but even so, their eyes are far, far too big for their bodies.
Now scientists think they might know why.
Many giant squids' eyes are the size of basketballs.
Even for a creature that can weigh as much as five adult men, the eyes are huge.
A giant squid and a big swordfish are similar in size - but the squid's eyes are 27 times bigger.
'It doesn't make sense a giant squid and swordfish are similar in size but the squid's eyes are proportionally much larger, three times the diameter and 27 times the volume,' said Duke biologist Snke Johnsen.
'The question is why. Why do giant squid need such large eyes?'
Thankfully, the answer isn't to track down and smother fishing boats full of terrified humans - instead, Johnson thinks, the eyes are adapted to seeing large bulks looming at the squid through the sea.
Johnson believes the eyes are an early-warning system - giving the squid an alert when a sperm whale approaches by the 'twinkling' of glowing plankton in the water.
The huge eyeballs are a defense mechanism.
'They're most likely using their huge eyes to spot and escape their predators, sperm whales,'
Johnsen collaborated with a group of biologists to model, both physically and biologically, how and why a squid uses such a big eye.
The team found that the design and size of the eye give squids the ability to see approaching sperm whales as they disturb bioluminescent organisms.
The study appears in the March 15 Current Biology.
John Ablett, Mollusc Curator at the Natural History Museum looks down at the eye of a giant squid
To explain the squids' eye size, Johnsen and his collaborators first measured giant and colossal squid eyes using photos and captured animals.
They also found data on the water clarity and amount of light at the ocean depths where the squid live -- typically 300 to 1000 meters. Using this information, the scientists began to mathematically model how the creatures' eyes would work and what they could see.
The team found that the squids' large eyes collect more light compared to animals of similar size but with smaller eyes.
The extra light intake improves the squid's ability to detect small contrast differences under the dim conditions of the deep ocean, they argue.
Johnsen said this ability doesn't matter much to the majority of deep-sea animals, which are looking at small objects that become too small to see before they fade away.
A New Zealand fisherman with a giant squid believed to have been caught in early February 2007 in the Ross Sea, Antarctica - the squid weighed 450kg
A 30ft long giant squid at the Natural History Museum
But the boost in being able to sense contrast, which large eyes provide, is critical for detecting the low light differences of large, distant objects, the most important one being the bioluminescence stimulated by large animals such as approaching sperm whales, Johnsen said.
The team realized that sperm whales dive and swim continuously while emitting sonar to ping the squid. The cephalopods are deaf to the sonar, but the whale's wake triggers small organisms like plankton to produce light. Based on the design of the squid's eye, the animal could see this light, though contrast is low, over 'freakishly long distances,' about 120 meters -- the length of an American football field, Johnson said.
'It's the predation by large, toothed whales that has driven the evolution of gigantism in the eyes of these squid,' Johnsen said.
'I like the idea. The paper is speculative, however,' said Michael Land, a University of Sussex zoologist who was not involved in the study.
'Big eyes are always better, and the laws of growth that tend to make large vertebrates have relatively smaller eyes may not apply to cephalopods. Maybe they just grow that big,' he said.
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