Sponges primarily use chemicals to protect themselves, and the chemicals are either toxic or just taste bad. This not only prevents predation but also competition, as the chemicals they release prevent other organisms from growing near them. Individual species have other strategies, such as burrowing into corals, rocks or molluscs to gain protection.
Another strategy many corals use to keep predators such as sea stars away is shedding tiny bits of their skeletal elements, called spicules, onto the sea floor. These can accumulate in a thick layer, deterring predators that must crawl to reach them. Not all sponges have a direct defense mechanism. Glass sponges do not produce any toxins, but they live in the very deep ocean where predators are rare.
Despite their defenses, sponges can only make slight movements, when they can move at all. They are vulnerable to any organisms that can overcome their defenses and are prey to many species of turtles, fish and invertebrates. Sponges can partially benefit from predation, however, as fragments of sponge left behind by predators can often survive and re-establish themselves as independent organisms. Their extremely simple cell-level organization means they can often survive even severe damage caused by predators or environmental effects.