which animals fall under the non-apex category?
Sea urchins, mussels, and other shellfish.
Keystone species
As described by Paine in 1966, some sea stars (e.g., "Pisaster ochraceus") may prey on sea urchins, mussels, and other shellfish that have no other natural predators. 
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.
wiki:11751798
Keystone species [SEP] Sea stars and other non-apex predators
As described by Paine in 1966, some sea stars (e.g., "Pisaster ochraceus") may prey on sea urchins, mussels, and other shellfish that have no other natural predators. If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species. These creatures need not be apex predators. Sea stars are prey for sharks, rays, and sea anemones. Sea otters are prey for orca. The jaguar, whose numbers in Central and South America have been classified as near threatened, acts as a keystone predator by its widely varied diet, helping to balance the mammalian jungle ecosystem with its consumption of 87 different species of prey.