The colossal squid's giant eyes seem built to spot sperm whales coming
The colossal squid of the Southern Ocean has the biggest eyes of any animal known, up to about 30 centimetres across. A swordfish of similar weight manages with roughly 10. One idea is that such enormous pupils let the squid see a sperm whale moving more than 120 metres off, farther than the whale's sonar reaches.
It is the heaviest invertebrate alive. The biggest specimen confirmed so far, caught in 2007 and now at Te Papa, the national museum of New Zealand, weighed 495 kilograms, and beaks found in sperm whale stomachs hint at animals of 600 to 700. For comparison, a typical squid weighs a couple of hundred grams. Its body length tops out around 4.2 metres; longer figures come from counting tentacles stretched out on dead animals, whereas a live squid keeps them tucked away until it strikes.
Unlike its glassy relatives in the family Cranchiidae, it carries hooks on its arms and tentacles, some swivelling and some three-pointed. Sperm whales wear scars thought to come from those hooks, and the whales need the fight: colossal squid account for about 14 percent of the beaks found in Antarctic sperm whales' stomachs, which works out to an estimated 77 percent of the biomass they eat.
Their eyes do more than look. Light organs beside each lens glow thanks to symbiotic bacteria, and the huge pupils scan big volumes of dark water for the faint flashes of plankton disturbed by something large swimming through. Hearing is no help against whales, since the squid's balance organs register only low sounds and cannot pick up high-pitched sonar clicks.
The squid appears to be a slow ambush hunter with a remarkably slow metabolism; an adult of about 500 kilograms may need only around 30 grams of food a day. It eats lanternfish and smelt, and of 8,000 Antarctic toothfish hauled up by trawlers between 2011 and 2014, seventy-one bore marks of its attacks. Young squid live near the surface and move deeper as they age, and females may carry more than 4.2 million eggs. The species was first described in 1925, yet an adult was not found until 1981.
Source: Colossal squid
