Lobster is considered to be a bottom-feeder because it feeds on the bottom-dwelling creatures in the ocean. This is one of the reasons why lobster is also considered to be one of the more mercury-rich types of seafood out there.
A lot of bottom-dwelling organisms in the sea tend to have quite a bit of mercury in them and when lobster eats these organisms they in turn become filled with mercury as well. This is precisely why it is so important to limit the amount of lobster you eat, which isn’t a problem for most people considering how expensive it can be. Lobsters can typically be found on the bottom of bodies of water feeding on whatever small organisms happen to be down there.