The ocean’s twilight zone – which stretches from 200 to 1,000 metres below the surface – is little understood. But new research from Cardiff University suggests that life may have become established there during a period of ocean cooling over the last 15 million years.

Life in the twilight zone relies on ‘marine snow’ – organic particles floating from the surface – as a major source of food. The scientists found than with cooler ocean temperatures, the marine snow was preserved for longer, allowing it to reach to greater depths. This meant that diversity could flourish in deeper waters as there was a reliable source of food raining down from above.

“During our study, we observed evidence of species migrating from the surface to progressively deeper regions of the oceans over the 15-million-year period, which was puzzling,” said palaeontologist Dr Flavia Boscolo-Galazzo, the co-lead author of the study.

“The temperature of the water turned out to be key to the mystery,” said co-lead author Dr Katherine Crichton, who developed a computer model simulation of the way the marine carbon cycle developed through time. “The interior of the ocean has cooled markedly over this period. That had a refrigeration effect, meaning that the sinking marine snow is preserved longer and sinks deeper, delivering food.”

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In order to investigate how marine life in the twilight zone has changed over time, the scientists took drill cores of mud from the bottom of the ocean to study fossilised plankton. They were able to establish the depth at which the creatures lived, but also how actively the marine snow was sinking round them.

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The scientists studied fossils of planktonic foraminifera
The scientists studied fossils of foraminifera, which are tiny, shelled planktonic creatures © Richard Bizley ( with scientific input from Paul Pearson and Flavia Boscolo-Galazzo

Today, a wide variety of weird and wonderful creatures live at this depth, including plankton, jellyfish, krill, squid and fish, and the sheer amount of diversity and biomass is key to the health of our seas. The scientists are therefore concerned that the current ongoing warming of the oceans may have an impact on this array of life, and wider implications on the marine food webs.

Reader Q&A: Is it true that the Pacific and Atlantic Oceans don't mix?

Asked by: Sonia Cooke, Northampton

While we’ve given our planet’s oceans separate names, in reality there’s no border between them, and currents continually flow between them and mix their waters. The Atlantic and Pacific oceans ‘meet’ at the southernmost tip of South America. In this region, a strong current carries water from west to east, sweeping water from the Pacific into the Atlantic.

The videos you may have seen online showing two different coloured bodies of water drifting alongside each other are actually showing light-coloured, sediment-rich freshwater from melted glaciers meeting dark, salty ocean water in the Gulf of Alaska (and over time, currents and eddies cause these to mix, too).

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Alice Lipscombe-SouthwellManaging editor, BBC Science Focus

Alice is the managing editor at BBC Science Focus Magazine. She has a BSc in zoology with marine zoology. Her interests include natural history, wildlife, the outdoors, health and fitness.