crayfish vs shellfish

crayfish and shellfish both are nouns.

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

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

crayfish: tiny lobster-like crustaceans usually boiled briefly
shellfish: meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)

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

crayfish: warm-water lobsters without claws; those from Australia and South Africa usually marketed as frozen tails; caught also in Florida and California
shellfish: meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)

crayfish (noun)	shellfish (noun)
large edible marine crustacean having a spiny carapace but lacking the large pincers of true lobsters	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
warm-water lobsters without claws; those from Australia and South Africa usually marketed as frozen tails; caught also in Florida and California