shellfish vs lobster

shellfish and lobster both are nouns.

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

As nouns, lobster is a hyponym of shellfish; that is, lobster is a word with a more specific, narrower meaning than shellfish:

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

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