Korean mussel vs mussel

Korean mussel

noun
  • A species of mussel, Mytilus coruscus, heavily used as a food item via mariculture in Korea and China. 

mussel

noun
  • A saltwater mussel, usually edible, of the order Mytilida in subclass Pteriomorphia. 

  • A freshwater mussel, usually edible, of the order Unionida in subclass Palaeoheterodonta. 

  • Any of certain other bivalves of somewhat similar appearance, such as the zebra mussel and quagga mussel of the family Dreissenidae in subclass Heterodonta. 

How often have the words Korean mussel and mussel occurred in a corpus of books? (source: Google Ngram Viewer )