Aquatic invertebrates are animals without a backbone that live in water environments, including oceans, rivers, and lakes. They play essential roles in aquatic ecosystems, serving as food for many fish and other animals. Common examples include crustaceans, mollusks, and cnidarians. These creatures vary widely in size and shape, from tiny zooplankton to large jellyfish. Aquatic invertebrates contribute to nutrient cycling and help maintain water quality. Their diverse adaptations allow them to thrive in different habitats, making them vital components of aquatic biodiversity.