The footage shows the light organs flashing moments before the squid attacked baited lines that were lowered, along with undersea cameras, from the team's research ship. (See a video of the squid attacking using a flash of light [videos by T. Kubodera, Y. Koyama, and K. Mori, copyright NHK, courtesy Royal Society].)
"This emission may work as a blinding flash for the prey, as well as a means of illumination and measuring target distance in an otherwise dark environment," the team writes.
Such behavior had been hypothesized by many cephalopod experts, including Michael Vecchione of the Smithsonian's National Museum of Natural History.
"It's great to have the actual in situ observations on a large specimen," Vecchione said.
These observations back up existing theories about the creature, he added.
"It's nice to have some evidence in support of that, because mostly it's been speculation up until now," he said.
The video footage also revealed other bioluminescent signals when the squid weren't hunting, behavior that Kubodera's team believes may represent communication between the squid.
The researchers say, for instance, that a single glow lasting more than two seconds seemed to act as "a precautionary signal used when approaching unidentified objects."
A longer glow lasting up to 8.5 seconds followed by several short glows may represent courtship or mating behaviors.
The Japanese team adds their footage also shows T. danae as a powerful and agile predator, not as the sluggish creature previously suspected.
The squid reached speeds of 5.6 miles (9 kilometers) an hour when attacking the baited lures, swam both forwards and backwards, and rapidly changed direction.
"Some people have said all deep-water squid are pretty sluggish, because their muscles are not real firm when you catch them," Vecchione said. "But this particular family has got very muscular fins, and that's what it's using for swimming."
Other types of deep-sea squid, including the giant squid, propel themselves by squirting jets of water, he added.
As with Kubodera's previous giant squid footage, his latest recordings have greatly enhanced our knowledge of large, deep-water cephalopods, said Richard Ellis, a research associate at the American Museum of Natural History in New York and author of The Search for the Giant Squid.
"He is probing a new dimension in our knowledge of large squids, enabling us to see for the first time how marvelously complex and innovative their behavior actually is," Ellis added.
Although T. danae is one of the world's five biggest squid and is believed to be widespread in tropical and subtropical oceans, it has proved elusive until now.
The glowing squid has never previously been observed in the wild and is rarely caught in fishing nets.
"Perhaps their ability to illuminate their surroundings better than any other cephalopod gives them the ability to see the nets before they become entrapped," Ellis said.
The largest known intact specimen, caught in the North Atlantic in 1993, weighed 135 pounds (61 kilograms) and measured 7.6 feet (2.3 meters) long.
Most adult records have come from the stomachs of dead sperm whales. These whales, which prey on the species, have been found with hundreds of the squids' hard beaks inside their stomachs.
As with their giant squid search, the Japanese researchers used sperm whales as guides in pinpointing likely haunts of the mysterious deep-sea creatures.
"The entire series of things that they're finding are really interesting and exciting," Vecchione, the cephalopod researcher, said.
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