mollusc vs shellfish

mollusc

noun
  • A soft-bodied invertebrate of the phylum Mollusca, typically with a hard shell of one or more pieces. 

  • A weak-willed person. 

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 mollusc and shellfish occurred in a corpus of books? (source: Google Ngram Viewer )