mussel vs shellfish

mussel

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

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

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

shellfish

noun
  • A fisheries and colloquial term for an aquatic invertebrate having an inner or outer shell, such as a mollusc or crustacean, especially when edible. 

  • A culinary and nutritional term for several groups of non-piscine, non-tetrapod, aquatic animals that are used as a food source. The term often exclusively refers to edible aquatic crustaceans, bivalve mollusks and cephalopod mollusks; but sometimes echinoderms may be included as well. 

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