Mysterious past. A genetic study suggests giant squid may have gone through a population boom.

Mark Norman

Giant Squid Worldwide Are One Species

Fifteen years ago, biologist Thomas Gilbert walked into a store in Cambridge, U.K., and, on an impulse, picked up a book about giant squid, creatures he knew almost nothing about. Years later, sick in bed, he finally read the work—only to learn that the rest of the world knew remarkably little about them as well. That's when Gilbert, an expert on ancient DNA at the Centre for GeoGenetics at the Natural History Museum of Denmark in Copenhagen who has studied Neandertals and the evolution of maize and birds, resolved to help answer some of the questions.

Now, Gilbert and colleagues from all over the world have analyzed the genetics of giant squid for the first time. The data "corroborates what has been said earlier using morphological criteria," says Steve O'Shea, a longtime expert on giant squids who worked at Auckland University of Technology in New Zealand until 2011: that there is only one species, globally distributed, of giant squid. But the results also deepen the mystery surrounding these enigmatic creatures. The researchers found startlingly little genetic variation, raising questions about the species' recent history.

Giant squid are deep-ocean creatures that can weigh hundreds of kilograms and measure more than 10 meters from their posterior fin to the tips of their two long tentacles. Although they have been found in all of the world's oceans and feature heavily in marine lore, a live giant squid wasn't photographed until 2002.

The international team of researchers used 43 samples of giant squid tissue from around the globe -- from the coasts of Spain and South Africa to the South Pacific and the Sea of Japan. Some of the animals had been found stranded on the shore or floating on the sea; others were captured by trawlers or even discovered in the stomach of a sperm whale. "Getting this material together was actually one of the hardest parts," Gilbert says. "I'm not a cephalopod biologist, so in the beginning people were reluctant to give me these precious samples."

The team analyzed DNA from the animals' mitochondria: small energy-producing structures inside a cell. Despite the geographic range, the scientists found almost no genetic differences between individuals. The mitochondrial genome, a ring of more than 20,000 base pairs, differed in only 181 places over all 43 specimens, the researchers write in the Proceedings of the Royal Society B. "It's completely bizarre," Gilbert says. "How can something be global but have so little variation?"

One possible explanation is that the giant squid went through a genetic "bottleneck," in which the population was decimated. But it's hard to imagine what could cause a species distributed across all the world's oceans to face such a crisis, Gilbert says. Instead, he thinks a fairly recent population boom is the more likely explanation.

A possible cause for such an explosion could be the rise of the whaling industry, which removed large numbers of sperm whales, the only predator that giant squids face. But when the team used what is known about the rate of mutations to estimate the timing of the population expansion, they found that it must have happened between 30,000 and 700,000 years ago, long before whaling started.

Still, Gilbert likes the whaling hypothesis, and he says the model relies on assumptions, such as the generation time of the animals, that could be wrong. Louise Allcock, a geneticist at the National University of Ireland, Galway, who is also an author on the paper, agrees that that explanation still makes sense. "But you have to take the data that you have in front of you," she says. There are other possible causes, such as climate change, she argues. "The challenge now is to find an explanation, and one that we can back up with strong facts."

The remarkable lack of variation supports the theory that there is just one species of giant squid. In the past, researchers have proposed up to eight separate species, based largely on the morphology of the animals' beaks, and many researchers have assumed that there are at least three distinct geographic species. The results "support what has been said many times earlier by some and contradicted by others, and debated by a few," O'Shea says.

But the paper raises more questions than it answers, Gilbert admits. For instance, could there be something special going on in the squid mitochondrial genome that is misleading the researchers? Researchers hope to answer that question soon. The team has worked on sequencing the full genome of a giant squid for some time; with any luck, Gilbert says, the first results should be available by the end of the year.