shellfish vs ecrevisse

shellfish and ecrevisse both are nouns.

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

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

shellfish (noun)	ecrevisse (noun)
invertebrate having a soft unsegmented body usually enclosed in a shell	small crayfish of Europe and Asia and western North America
meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)	tiny lobster-like crustaceans usually boiled briefly