 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 term "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 an informal sense; however, the term ”finfish” is sometimes used to distinguish fish, animals defined by having vertebrae, from shellfish in modern terminology.