2 April 2012

"I spy with my HUGE eye, something beginning with w..."

One of last months posts, 'Hunting in the depths', featured sperm whales and talked about how they dove down into the depths of the oceans to hunt their main source of food, giant squid.

Obviously, giant squid want to avoid being eaten by sperm whales and are extremely vulnerable to attack due to the whales' fearsome arsenal. One of the main defences that they have evolved against the whales is developing incredibly large eyes that can measure more than 11 inches across! The large size of these eyes has long confused scientists because they are very expensive to make and are virtually completely useless in the deep oceans, where there is no light. This means that their eyes should have disappeared over the course of their evolution, like in many other species of deep cave-dwelling organism.

A sperm whale hunting a giant squid. Note the giant squids enormous eye, which works via a lens refracting light in the same way as a human eye does.

The fact that they possess such highly developed eyes, which can have lenses that are bigger than the entire human eye, means that this can't be the case. New research into giant squid by Professor Nilsson from Lund University, Sweden  has helped to 'shed light' onto the function of these eyes and explain why evolution has favoured their development rather than their loss.

The depths of the ocean are filled with bioluminescent organisms, which when disturbed, flash and glow. Thus, diving sperm whales agitate these organisms as they swim past and giant squid are able to see the light from this disturbance. The squid are able to gage the size of the organism and the direction that it is travelling in from this light and consequently, can determine whether or not the shape is a sperm whale to either ready itself against an attack or flee.

Therefore, the large eyes of giant squid have direct implications to the animal's survival so that in the past, individuals with biggest and best eyes survived for the longest and could breed the most. This meant that large eyes were under a strong selection pressure in giant squid, despite being useless for seeing anything else, so that today the species has abnormally large eyes.

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