shellfish vs crabmeat

shellfish and crabmeat both are nouns.

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

As nouns, crabmeat is a hyponym of shellfish; that is, crabmeat 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)
crabmeat: the edible flesh of any of various crabs

shellfish (noun)	crabmeat (noun)
invertebrate having a soft unsegmented body usually enclosed in a shell	the edible flesh of any of various crabs
meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)