A new species of nudibranch nicknamed the “mystery mollusc” has been discovered by researchers in California, US, in the Pacific’s deep-sea midnight zone. Researchers at the Monterey Bay Aquarium Research Institute found it at 8,576ft deep and spent years documenting the sea slug.

Photograph: MBARI

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A sea slug floats off the coast of the city of Batroun in northern Lebanon. The colourful nudibranches are native to the Indian ocean, but are believed to have escaped through the Suez canal into the Mediterranean, where they were first spotted in 2004.

Photograph: Ibrahim Chalhoub/AFP/Getty Images

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