In the northwestern United States, this crayfish would be just a friendly bit of local fauna. But in Scotland, it's an invasive species wreaking havoc on trout streams. Ari Shapiro/NPR hide caption

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A commercially harvested crayfish from Lake Tahoe near Incline Village, Nev., in July. Max Whittaker/Prime for NPR hide caption

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The new species, Barbicambarus simmonsi (left) is more than twice the size of typical crayfish found in the same creek. L. Brian Stauffer/UI News Bureau hide caption

toggle caption L. Brian Stauffer/UI News Bureau

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