shellfish vs limpet

shellfish and limpet both are nouns.

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

As nouns, limpet is a hyponym of shellfish; that is, limpet 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)
limpet: mollusk with a low conical shell

shellfish (noun)	limpet (noun)
invertebrate having a soft unsegmented body usually enclosed in a shell	any of various usually marine gastropods with low conical shells; found clinging to rocks in littoral areas
meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)	mollusk with a low conical shell