shellfish vs crawdad

shellfish and crawdad both are nouns.

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

As nouns, crawdad is a hyponym of shellfish; that is, crawdad 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)
crawdad: tiny lobster-like crustaceans usually boiled briefly

shellfish (noun)	crawdad (noun)
invertebrate having a soft unsegmented body usually enclosed in a shell	small freshwater decapod crustacean that resembles a lobster
meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)	tiny lobster-like crustaceans usually boiled briefly