The seas surrounding the UK - and the rivers that flow into them - are filled with animal and plant species that originate far from our shores.
While some of these non-native invaders will die out over time, others are thriving - and are becoming a huge threat to biodiversity.
Here, two experts - Jack Sewell and Philine zu Ermgassen - explain how these alien invaders are affecting our ecosystems.
Slideshow production by Paul Kerley.
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