Ancient ‘alien' brittle stars linked to relatives across the globe by a deep-sea evolutionary superhighway
Researchers analysed DNA from 2,699 brittle star specimens collected from all of the Earth's oceans – from the equator to the poles, and the intertidal zone to the abyss (more than 3,500 metres deep) – and housed in 48 museums across the globe.
The unique dataset revealed connections stretching tens of thousands of kilometres, such as 17 species of brittle stars from Tasmania that have close relatives near Iceland, in the North Atlantic.
'You might think of the deep sea as remote and isolated, but for many animals on the seafloor, it's actually a connected superhighway,' said Dr Tim O'Hara, senior curator of marine invertebrates at Museums Victoria Research Institute and lead author of the study published in Nature.
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Like sea stars, brittle stars live on the ocean floor. These ancient, spiny animals vary in colour and size, with some only a few millimetres wide, and others with metre-long arms.
'They're slightly alien,' said O'Hara. 'They don't have a brain, they don't have eyes. They just have a round disc, with a stomach and things like reproductive organs, and five arms.'
The analysis found that over millions of years and without the help of fins or wings, brittle stars have quietly migrated across entire oceans.
'We fly over the oceans now, and the Earth doesn't seem too big to us,' he said. 'But really, it is huge.'
How can such closely related species be found on opposite sides of the planet? The logical answer was the ability of brittle star larvae to drift on ocean currents in suspended animation, even for up to a year, O'Hara said.
The study also revealed connections based on depth, with similarities between brittle stars living 500m deep off Antarctica and others at 500m off Australia, almost like 'horizontal highways of life through the ocean', O'Hara said.
The scale of the dataset also offered bigger picture insights into the workings of the deep sea – which covers 50% of the planet – beyond the individual threads illuminated by ocean expeditions.
Scientists had suspected the deep sea to be more connected than shallow water systems, said Dr Nerida Wilson, a research scientist specialising in marine invertebrates, and a molecular biologist at the CSIRO who was not involved in the research.
'This study has confirmed that in a very robust way, by such comprehensive sampling.'
Wilson said it also demonstrated the value of museum collections, which 'house not only specimens, but all the information that's locked inside their DNA that can provide insights into times long past.'
Dr Sue-Ann Watson, an associate professor in marine biology at James Cook University, who was also not involved in the research, agreed.
Natural history collections provide time capsules of past life on earth, which allow researchers to understand links 'across vast oceans, even in hard-to-reach places', Watson said.
'As scientists begin to unlock new knowledge from collections, this research highlights that it is critically important to continue to collect new biological specimens on a global scale as rapid environmental change, including climate change, continues.'
In O'Hara's view, the study also highlighted the fragility of deep-sea life, which has high extinctions as well as dispersal, and its dependence on the presence of ice at the poles.
'If there's no ice in Antarctica, in the Arctic, then there's no sinking of cold, oxygenated water down into the deep sea.
'If we don't have ice at the poles, then we won't have a deep-sea circulation. And if we don't have deep-sea circulation, we'll have a dead ecosystem over half a planet.'
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