What do crustaceans eat?

The majority of crustaceans are scavengers, meaning they feed on scraps and leftovers that other sea creatures have discarded.  Some crustaceans such as certain crabs and lobsters that have very powerful claws, tend to be predatory and will actively hunt prey.  There are also crustaceans that are filter feeders; barnacles are such creatures and they simply filter out the food as they pass sea water through their systems.  A third group of crustaceans are parasites; these live on other animals and feed off of them.