Giant squid around the world are "basically identical" despite looking very different, scientists say.
The super-sized cephalopods live deep in the oceans and are little-known by the scientific community.
An international team of researchers investigated rare samples of the elusive animals' DNA to reveal their family secrets.
They discovered that there is just a single species of squid with no population structure.
The findings are published in the journal Proceedings of the Royal Society B.
The giant squid has been a source of fascination both before and beyond its formal description in 1857 by Danish biologist Japetus Steenstrup.
Its deep-dwelling lifestyle is largely unknown but specimens have been found globally, with the exception of Arctic and Antarctic waters.
Inspiring tales of sea monsters such as the ship-destroying Kraken, giant squid are rumoured to reach 50m in length but scientists say an 18m estimate is more appropriate according to studies.
A further debate about the mysterious animals relates to how many species there are, with researchers suggesting there could be as many as eight based on differences in appearance and where they have been found.
"Your general [giant] squid is a long, scrawny beast: it's got a long thin body and long thin arms," said Professor M. Thomas P. Gilbert, from the Natural History Museum of Denmark.
"But off [the coast of] Japan for example, they're much shorter and stubbier. Their arms are fatter and much shorter."
Prof Gilbert worked with colleagues from the University of Copenhagen and researchers from Australia, Japan, France, Ireland and Portugal to understand how the seemingly diverse squid are related.
The team took 43 tissue samples from a variety of sources: stranded animals, remains found in the stomachs of beached sperm whales and accidental by-catch. They then used DNA sequencing techniques to understand the genetic makeup of the squid.
'Very weird'Results revealed that the squid are all one species.
Genetic diversity was also found to be very low, meaning that the squid are genetically very similar despite being found all over the world and varying greatly in appearance.
"There's normally local distinction between [animals] genetically," Prof Gilbert told BBC Nature.
"Things that live in one area eventually become different from things in other areas but [giant squid] are basically identical everywhere."
Prof Gilbert described the findings as "very weird" but suggested that migration could be the key reason specimens from as far apart as Japan and Florida, US are genetically so similar.
"We speculate the larval stage must drift globally in the currents then dive to the nearest dark, deep spot when they are large enough, thus stopping any [population] structure appearing," he explained.
"Instead of the adults and their young living in the same place, the young distribute to a completely new place on the Earth every time."
Judging by their size and remarkable adaptations for the environment, scientists believe the giant squid have a substantial population. Another theory to explain the large numbers of identical animals is the possibility of a rapid and recent population boom.
According to Prof Gilbert, this expansion could have been caused by either a decrease in predators or an increase in prey numbers of the giant squid.
"This year is the 200th anniversary of Steenstrup... So on his 200th birthday we can say we know more about it!" he told BBC Nature.
The evolutionary biologist commented that although the team had succeeded in answering one question about the mysterious squid, they have uncovered many more.
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Source: http://www.bbc.co.uk/nature/21829711
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