lobster vs shellfish

lobster and shellfish both are nouns.

Word	Noun	Adjective	Verb	Adverb
lobster	Yes	No	No	No
shellfish	Yes	No	No	No

As nouns, shellfish is a hypernym of lobster; that is, shellfish is a word with a broader meaning than lobster:

lobster: flesh of a lobster
shellfish: meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)

lobster (noun)	shellfish (noun)
any of several edible marine crustaceans of the families Homaridae and Nephropsidae and Palinuridae	invertebrate having a soft unsegmented body usually enclosed in a shell
flesh of a lobster	meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)