Terminology
The term shellfish is used both broadly and specifically. In common parlance, as in having "shellfish" for dinner, it can refer to anything from clams and oysters to lobster and shrimp. For regulatory purposes it is often narrowly defined as filter-feeding molluscs such as clams, mussels, and oyster to the exclusion of crustaceans and all else.
Although the term is primarily applied to marine species, edible freshwater invertebrates such as crayfish and river mussels are also sometimes grouped under the umbrella of "shellfish".
Although their shells may differ, all shellfish are invertebrates. As non-mammalian animals that spend their entire lives in water they are "fish" in the logical sense; however the denotion finfish (or finnicle fish) has been developed to distinguish fish as animals defined by having vertebrae from shellfish in modern terminology.
The word "shellfish" is both singular and plural; the rarely used "shellfishes" is sometimes employed to distinguish among various types of shellfish.
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