Decapod crustaceans (crayfish, shrimp, and crabs) are the largest invertebrates inhabiting freshwater systems. They can be found in lakes, ponds, rivers, streams, wetlands, and caves. Decapods often play important roles in the systems they inhabit. They can directly influence the abundance of algae, aquatic macrophytes, and other invertebrates. They can also indirectly influence the abundance of vertebrates, particularly fish, as a result of their effect on habitat structure and prey availability. They are an important prey item for many species of fish, mammals, and birds. They are also host to a wide variety of ectosymbionts, including bacteria, protozoa, worms, and small insects. Some ectosymbionts may be engaged in mutualisms with their decapod hosts. Decapods (primarily crayfish and shrimp) have been introduced into new environments and new continents. These introductions have had negative consequences for the aquatic communities that have been invaded.