A Spanish vessel operating off the Azores in the mid-Atlantic found the dead creature in July when it became entangled in fishing lines. Crew members froze it at sea and brought the squid back to this northwestern port last week.
Giant squids are so elusive there have only been 300-odd documented sightings in the past 500 years and no one's ever seen one alive, said Mario Rasero, a biologist at the Spanish Oceanographic Institute.
"They are one of the sea's last big mysteries," he said Monday.
In 1999 the world's top expert on the species, Clyde Roper of the Smithsonian Institution in Washington, D.C., attached a video camera to the head of a sperm whale the giant squid's only known predator in deep waters off New Zealand in a novel but failed bid to see a live one.
Giant squids usually live alone at depths ranging from 660 to 3,300 feet, drifting with the current. But sometimes they surface, as in this case, apparently to feed on the Spaniards' fish bait with a beak-like mouth of material similar to human toenails, Rasero added.
After being studied, the specimen will be preserved in formaldehyde and exhibited in a museum.
Its central torso - the main yardstick for comparing jumbo cephalopods is 5 feet long long, but from the tip of its head to the end of its tentacles it's about 16 feet. The creature weighs about 145 pounds.
The species can be much, much bigger, though, with eyes as big as soccer balls. The largest one ever recorded was a 60-foot, one-ton monster recovered about 100 years ago, Rasero said.
Spaniards love to eat small squid but the giant ones are toxic because the bodies contain ammonia, which is lighter than water and thus helps these weak swimmers stay afloat.
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