what does the second animal that is mentioned above, feed on?
Plankton and other microscopic sea creatures which are free-floating in seawater.
Mussel
Both marine and freshwater mussels are filter feeders; they feed on plankton and other microscopic sea creatures which are free-floating in seawater. 
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. where is the last mentioned animal mostly found? Most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. can they be poisonous? Shellfish caught in Alaska can lead to paralytic shellfish poisoning (PSP). does it cause any reaction when consumed by some people? It can lead to death by paralyzing the breathing muscles. what is the nutrition value of the second mentioned animal? UNANSWERABLE
wiki:1114559
Mussel [SEP] Feeding
Both marine and freshwater mussels are filter feeders; they feed on plankton and other microscopic sea creatures which are free-floating in seawater. A mussel draws water in through its incurrent siphon. The water is then brought into the branchial chamber by the actions of the cilia located on the gills for ciliary-mucus feeding. The wastewater exits through the excurrent siphon. The labial palps finally funnel the food into the mouth, where digestion begins. Marine mussels are usually found clumping together on wave-washed rocks, each attached to the rock by its byssus. The clumping habit helps hold the mussels firm against the force of the waves.