where is the last mentioned animal mostly found?
Most kinds of shellfish are harvested from saltwater environments, some are found in freshwater.
Shellfish
Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. 
why are certain traits adaptations in one ecosystem but not in another apex UNANSWERABLE which species are present in an apex? Gray wolf why is it present under the apex list? They often kept animals from grazing in riparian areas, which protected beavers from having their food sources encroached upon. which animals fall under the non-apex category? Sea urchins, mussels, and other shellfish.
wiki:5378695
Shellfish [SEP] Introduction
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. In addition, a few species of land crabs are eaten, for example "Cardisoma guanhumi" in the Caribbean. Shellfish are among the most common food allergens. Despite the name, "shellfish" are not actually fish, but are simply water-dwelling animals. Most shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. Many varieties of shellfish, and crustaceans in particular, are actually closely related to insects and arachnids; crustaceans make up one of the main subphyla of the phylum Arthropoda.