As nouns the difference between shellfish and finfish
is that shellfish is an aquatic invertebrate, such as a mollusc or crustacean, that has a shell, especially as food while finfish is any fish, but especially fish other than flatfish.
shellfish
English
Noun
An aquatic invertebrate, such as a mollusc or crustacean, that has a shell, especially as food.
finfish
English
Noun
(en-noun)
Any fish, but especially fish other than flatfish
Usage notes
*Used industrially to denote ordinary fish, rather than flatfish, eels and other seafood.