The Global Kraken
Earlier this year, the Discovery Channel released the first ever footage of the legendary giant squid in its natural environment. But did the crew manage to film the giant squid, or one of the giant squids?
The giant was first described by the Danish naturalist Japetus Steenstrup in 1857, who named it Architeuthis dux. As Craig McClain writes, the name “translates to ‘most important squid leader’. That is scientifically an awesome name.” Since then, biologists have slapped the Architeuthisbrand on no fewer than 21 potential species. Some of these over-eager taxonomists were just going on fragments of flesh, like beaks or arms that sperm whales had coughed up. Others reasoned that squid remains in far-off corners of the world must belong to different species. After all, giant squids are everywhere—they’ve turned up in all oceans except the waters around Antarctica.
How many of these species are valid? “All of them” seems unlikely as an answer. Some people think that there are just three, which live in the North Atlantic, North Pacific, and Southern Ocean respectively. Others insist that there’s just the single globe-spanning species, caressing the world in its tentacles.
Now, a team of scientists from eight countries, led by Inger Winkelmannfrom the University of Copenhagen, has tried to settle the debate by looking at the kraken’s genes. Together, they amassed tissue samples from 43 giant squids caught all over the animal’s range, from Florida to South Africa to New Zealand. They sequenced each sample to piece together its mitochondrial genome—a small secondary set of DNA, which sits outside the main genome in tiny bean-shaped batteries.
The team found that the giant squid’s genetic diversity is incredibly low. Even though the individuals hailed from opposite corners of the world, they differed at less than 1 in every 100 DNA letters. For comparison, that’s 44 times less diverse than the Humboldt squid, which only lives in the eastern Pacific. In fact, the giant squid seems to be genetically narrower than any other sea-going species that scientists have tested, with the sole exception of the basking shark.
This strongly suggests that the 21 proposed species of giant squid can indeed be collapsed into one. There’s just the one global kraken—Architeuthis dux, the one-and-only original. What’s more, the population seems to have very little structure—in other words, squids that hail from nearby waters aren’t going to be genetically closer than distant individuals. The mitochondrial DNA of a Japanese squid is basically the same as that of a Floridian squid.
Why? It’s possible that the adults are wandering nomads that swim over large areas, but that seems unlikely. Chemical analyses of their beakssuggest that they stick within a relatively contained patch of ocean. The alternative is that they go a-wanderin’ as larvae and youngsters. Young marine animals are certainly capable of passively drifting over tens of thousands of kilometres on ocean currents, so it’s entirely possible that the squids do the same. These young nomads would feed on plankton and other small creatures until they became big, whereupon they’d settle down and sink to the nutrient-rich waters of the deep ocean.
“I am not in the least surprised by their findings,” says giant squid expertSteve O’Shea. “They support what has been said many times earlier by some, contradicted by others and debated by a few, to what end I will never know.” O’Shea himself has suggested that larval giants drift over considerable distances and, on another Discovery Channel-sponsored research trip, he has captured 17 of these larvae at the surface of the ocean.
Craig McClain from the National Evolutionary Synthesis Center is more enthusiastic. “This is the research project I dreamed of conducting, and the results are even more interesting than I would have imagined,” he says. “The finding of just one species is not unexpected, but the study finally provides the molecular evidence that was so sorely missing. What is amazing is the total lack of genetic structure among ocean basins. I know of few animals that have the long range dispersal ability or behavior to ensure genetic exchange over such great distances.”
But why does the squid have such low genetic diversity? Winkelmann couldn’t find any signs that its mitochondrial DNA evolves at a slower pace than that of other animals. Instead, it’s possible that the giant squid—like the basking shark—went through a population bottleneck at some time in its past, and today’s individuals descended from that narrow stock of ancestors. Maybe they used to be restricted to a specific part of the world but were released by some ancient event, like a change in climate, or the death of a competitor.
This is all just guesswork. As Winkelmann writes, “We cannot offer a satisfactory explanation for the low diversity.”
And there’s one last, important caveat—the team’s conclusions are based on the mitochondrial genome alone. That’s useful for looking at things like diversity and ancestry, but the team still need to analyse the giant squid’s nuclear genome, which contains the vast majority of its DNA. Nuclear genomes have a habit of complicating the stories told by mitochondrial ones. Who knows what they will do for the giant squid?
“It is clear that there is much that remains an enigma about these ocean giants,” says McClain.
Reference: Winkelmann, Campos, Strugnell, Cherel, Smith, Kubodera, Allcock, Kampmann, Schroeder, Guerra, Norman, Finn, Ingrao, Clarke & Gilbert. 2013. Mitochondrial genome diversity and population structure of the giant squid Architeuthis: genetics sheds new light on one of the most enigmatic marine species. Proc Roy Soc Bhttp://dx.doi.org/10.1098/rspb.2013.0273