As many as 21 species of giant squid live and die under the surface of our oceans. At least, that’s what we thought. A paper published today by my colleagues and I in Proceedings of the Royal Society B suggests there is, in fact, only one species of this enigmatic oceanic giant: Architeuthis dux.
Architeuthis made headlines earlier this year when the first live footage of the animal – which you can see in the video below – was captured. Despite its large size (up to 18 metres in total length), surprisingly little is known of this animal’s basic biology.
Almost all of what we understand about these deep-sea animals comes from studies of dead specimens, often badly damaged, that have been discovered washed up on the shore, floating on the ocean’s surface or recovered from the stomachs of dead sperm whales.
Our paper today – based on the work of our international team, led by Danish Researcher, Inger Winkelmann – provides significant insights into the creature’s biology.
What we did
We sequenced mitochondrial genomes (mitogenomes) from 43 giant squid individuals collected from around the globe, including five from Australian waters. Others originated in New Zealand, Japan, Spain, South Africa, the Falkland Islands and the USA.
As mentioned at the outset of this article, the large dataset revealed the giant squid (Architeuthis dux) appears to be only a single species distributed throughout the world’s oceans.
Our study shows the giant squid possesses remarkably low genetic diversity - 44-times lower than the Humboldt squid and markedly lower than many other comparable oceanic predators such as killer whales and big-eye tuna.
This very low level of diversity is surprising for a species that’s likely to possess a large population size and a global distribution.
Analysis of the demographic history of the mitogenome data suggests there has been a relatively recent (in historical terms) marked decline in the global population size of the giant squid – known as a bottleneck event.
This population crash would have caused a corresponding decline in genetic diversity in the species. We believe it was followed by a population expansion that was estimated to have occurred between 32,000 and 115,000 years ago.
We can’t explain this population expansion, but it’s possible that a decline in numbers of competitors or predators (such as predatory fish) may have released population size constraints on the giant squid, allowing numbers to swell.
Such a decline in competitors or predators could be associated with climatic changes driven by the last ice age.
The lack of population structure evident in Architeuthis suggests a panmictic (randomly mating) global population has existed for a long time. It also suggests this panmixia is maintained via paralarval stages drifting on the global oceanic currents (driven by thermohaline circulation).
In other words, larvae of giant squid are transported throughout the world’s oceans via large current systems, and this prevents any genetic structuring from occurring.
This may prove to be an important dispersal mechanism for the giant squid.
Although our study is based on a single marker – the mitogenome – it shines considerable light on the biology of this mysterious species.
Future analyses of genetic data from the nuclear genome are necessary to validate the findings of this study and will help uncover more secrets of this oceanic giant.
Much of its enigma, for the time being, remains intact.