Complex sentence: Some crayfish suffer from a disease called crayfish plague, caused by the North American water mould "Aphanomyces astaci" which was transmitted to Europe when North American species of crayfish were introduced there.
Split into simpler sentences: Some crayfish suffer from a disease called crayfish plague, caused by the North American water mould "Aphanomyces astaci". Crayfish plague was transmitted to Europe when North American species of crayfish were introduced there.